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Planning 4
| Question | Answer |
|---|---|
| Define 'stereotactic' in a medical context | Describe use of 3D co-ordinate systems to locate small targets inside the body for ablation, biopsies, injections, stimulations, implantation, radiation therapy etc |
| List characteristics of stereotactic treatments | High precision + image guidance (1mm and degree) Fewer treatments <5 Highly conformal dose with steep gradients Intra-fraction motion mgmt Thinner CT slice (1-2mm) Dose grid res <2mm Strict adherence to protocol |
| Define SBRT and list sites | Stereotactic BODY radiation therapy Dose escalation with proximal OARs Spine, prostate |
| Define SABR and list sites | Stereotactic ablative BODY radiation therapy Dose escalation with NO proximal OARS Lung, liver, kidney |
| Define SRS and list sites | Stereotactic radiosurgery - single fraction (can be body) Very small targets - benign or malignant intracranial, kidney, lung |
| List SABR protocols/regulatory documents | RANZCR Guidelines for Safe Practice of Stereotactic Body (Ablative) Radiation Therapy (2023) AAPM 101 ICRU 83 |
| Define SRT and list sites | Stereotactic radiation therapy. Fractionated intracranial Larger benign and malignant intracranial targets |
| Describe Hexapod (elekta) | Frame w/ high sensitivity infrared balls for optical tracking Optimal frame-iso dist 30-50cm Difficulties: arm pos, patient height + BMI, VB Vectors for iso cor. =1m long (small corrs = big movements) Patient/equip must fit inside without touching |
| What SABR sites use hexapod | Ideally ALL if avail YES: spine, pancreas, prostate, intracranial lesions (esp BOS), abutting lesions (2cm to critical normal tissue), multiple lesions NO: round targets that can be corrected w/ trans only e.g. kidney, lungs (round - gantry collisions) |
| State dose fractionation for conventional RT and describe rationale | 1.8-2.4Gy per day, 15-40 # over 3-8 weeks Repair, repop, redis, reox, radiosensitivity |
| Describe the 5Rs (conventional RT fx) | Repair: normal cells repair sublethal DNA damage b/w fxns Repop: healthy tissues repop (tumour cells might) Redis: move to radiosensitive phases Reox: blood brings oxygen to hypoxic cells Radiosens: different tissues react diff to rad |
| State dose/fractionation for SBRT and explain rationale | 8-30+ Gy/fx, 1-5 fx, 1-2 weeks Anti-tumour effects not predicted by classic radiobiol Enhanced anti-tumour immunity, injured vasculature Re-ox: smaller tumours may not be hypoxic Disrupted vasculature, stimulation of immunity -> abscopal effect |
| Define the abscopal effect | Tumour regression of non-irradiated tumours |
| Define oligometastasis | A middle stage of spread where tumor cells move from the main site to only a small number of distant places. -> treated as curable |
| List the types of oligometastasis | Synchronous - @ diagnosis Metachronous - after 1 treatment Oligoprogression - some, not all mets progressing Oligopersistence - few mets persist on systemic txt Induced - held by systemic therapy Serial -small numbers of new mets appearing in waves |
| What do we treat with SABR | Primary+secondary - oligometastatic state <5cm dimension (larger for liver) Non-malignant spinal cord AVM, meningioma Radioresistant tumours (renal cell, melanoma) For ablation (lung, liver) Dose esc (spine, pros, pancreas) Low vol LN or bone mets |
| Patient performance criteria for SABR | (Site specific) Performance status (KPS) 0-2 Life exp > 6 months (3 months liver) Low met burden (≤ 5 mets, < 5 cm in dimension) |
| Contraindications for SABR | • Prior RT (may inc fx slightly) • Unable to lie flat • No chemo 1–4 weeks pre/post SBRT (depending on site) - toxicity++, tissue repair • Severe connective tissue disease or scleroderma • Claustrophobia • Mental status prohibitive of compliance |
| Discuss image fusion for SABR | MRI, PET, 4DCT Matched to txt pos, be aware of bore size and equip Registration errors can be largest source of error Be aware of artefacts - motion, metal implants -> myelogram for spine pts with post op hardware Image distortion - for PET and MRI |
| SABR sim reqs | 1. Comfortable/repro – long txt 2. Enable delivery to achieve goals • Good target access • Min NTT dose • Min angle restrictions • Consider arm/head pos, pos of tubing/loc bars • Accommodate equip e.g. frame, comp belt • Avoid build-up equip |
| Why are test runs on Linac rec for SABR | • Small rot corrections can req large trans moves • Poorly constructed equip can result in gantry collisions • Check breath hold reproducibility • Check tumour excursion • Check visibility of lesion & surrounding anatomy on CBCT imaging (limited FoV) |
| Patient cons for SABR sim | 1. understanding - greater change of success 2. pain control/mgmt 3. mobility/comorbidities 4. anxiety / claus 4. breath hold 5. prev/future RT |
| Education process for SABR | 1. Info sheets & pre-CT checklist 2. Long RO consult 3. Pre-CT RT consult (motion mgmt, fasting, organ filling/voiding) 4. Post-CT/Pre-Treatment nursing/RT consult 5. Videos 6. Opportunity to ask qs 7. Involve family members/team (w pt consent) |
| Conventional vs SABR dosimetric characteristics | Con: PTV covered by 95% Dose 95-105% Fall off 95-0% Up to 10mm margin Homogenous Less dose inside, more outside Stereo: PTV covered by 100% Higher max dose (covering isodose is a % of max dose) Little/no PTV margin Fall off 60/80-0% Hetero |
| How many beams for SABR | • Increased number of beams/arcs & non-coplanar beam arrangements to create sharp isotropic dose fall off (trade off: dose v delivery time) |
| When will SABR distributions NOT be heterogenous | Prostate where urethra is through the middle |
| SABR iso placement for single target | Ideally in centre - best for modulated techniques +image correction (rotational) Be mindful of lateral lesions -> PSQA reqs May alter iso to maximise MLC pos (but colly might suffice) If critical structure off centre in target, place iso at structure |
| Two options for SABR dose px | Prescribe to covering IDL: Prescribe 28Gy at 80%, max = 100% = 35Gy Prescribe to covering dose with acceptable maximum: Prescribe 28Gy to 99% with an acceptable max of 125% = 35Gy |
| SABR iso placement for multiple targets | If targets >5cm, consider sep isos if fractionations diff between isos, cannot be treated concurrently due to cross contribution of dose which cannot be fully accounted for may need sep isos if imaging correction cannot translate to both lesions |
| SABR plan eval | OAR all met (very few exceptions) Check PTV cover including min dose GTV and or CTV - check min dose, esp where NTT abuts Consider surrounding tissue - hetero (lung) difficult Dose fall off Visual assessment, not just scorecard Comms with RO |
| SABR dose metrics | D2, D50, D98 (be cautious - less sig for small vols) Conformity index Gradient index R50 D2cm |
| SABR margins | usually isotropic. 3-5mm (smaller in brain) no CTV patient specific, depends on motion mgmt |
| Define conformity index | measures how well the px dose conforms to target vol |
| Define GI | ratio of vol of half px isodose to volume of px isodose differentiates plans with similar conformity but with different dose gradients |
| Define R50 | ratio of the volume covered by the isodose representing 50% of the px dose to the vol of the PTV (function of the size of the PTV, smaller for large PTVs) - volumetric dose fall off |
| Define D2cm | the dose at any point 2cm from the PTV is recorded and expected to meet set criteria |
| List types of motion and mgmt | 1. Skeletal/muscular -> stabilisation 2. Resp motion -> 4DCT., BH, gating, comp 3. Cardiac motion -> remains 4. Peristalsis -> compression, meds 5. Bladder/bowel -> procotols, enema, meds, catheter |
| List non-patient errors in RT pathway | Imaging res, accuracy of fusion and delineation (based on user experience), accuracy of mechanical isocentre and txt isocentre, resolution of couch pos, res of infrared camera for movement verification |
| When is SABR used for early stage NSCLC | • Patients unfit for/refuse surgery • Multiple phase l/ll trials showing outcomes comparable to surgery & superior to conv RT • ASTRO guidelines for operable pts (stage 1 NSCLC) should see a surgeon. Long-term outcomes not established for SBRT (> 4 yrs) |
| When is SABR used for lung | Early stage NSCLC and pulmonary mets |
| SABR lung indications | • Central tumours (fractionate!), > 5cm diameter, no tissue diagnosis, T3 with CW invasion, salvage after prior RT, synchronous/multi-focal, post pneumonectomy with primary in remaining lung, combination SBRT and conventional where nodes |
| SABR Lung fractionations and covering isodose | GTV/ITV >1.5cm from ribs 54/3 GTV/ITV <1.5cm from ribs 48/4 Lesions approaching central/larger 50/5 Small favourable tumours 26-33/1 Convering isodose 59-80%, max 125-170% |
| PTV goals for SABR lung | • D100 ITV ≥ PD • D95 PTV ≥ PD • D99 PTV ≥ 90% PD |
| SABR lung CT acquisition | Max 2mm (1mm slices ideal but not feasuble) Include OARs 10-15cm sup if NCP beams used 4DCT Contrast if target near subclavian vessels DIBH 3 times Coaching session prior to CT - multiple breath holds |
| What is MLC leaf interplay | • Is the interaction between the movement of the MLC shapes for segments in an IMRT beam or VMAT arc and the motion of a tumour with the respiration cycle Can use DCAT or gated/BH treatment |
| IGRT reqs for SABR | – All txts require pre-fx CBCT – RO presence for all tx is a department specific mandate, ROs are responsible for approving all CBCT – RT credentialing common and recommended • Pre-fx CBCT trans > 1mm actioned* • Pre-fx CBCT rotations < 3˚ accepted* |
| What influences whether an intra/post fx SABR CBCT is done | equipment, technique, treatment duration, OAR prox, exptected tumour motion, patient stability performed during and after CP treatment beams triggered imaging or IFI images during delivery |
| SABR for spine | Used for oligometastatic disease and palliation Very steep dose gradients Most commonly 1 vertebrae, more difficult IGRT if both Different dose schedules based on pathology, patient factors and experience |
| SABR spine fractionations | 16-18/1 18-20/1 (11-12 Gy cord) 24-28/2 (14-15Gy cord) 24-30/3 (17Gy cord) |
| Patient immobilisation/prep for SABR spine sim | Myelogram (coordinate with MI, CT ASAP following) Cannot be upright after myelogram -> sim challenges C1-T4: vacbag, knee bolster, arms down, shell T5-sacrum: vacbag, knee support, arm support - full length (L/S spine) or t-shaped (low T-spine) |
| Why is a myelogram needed | Spinal cord requires precise delineation PRV 1.5-2mm • If doubt, thecal sac is used for safe practice however – Larger cord structures or thecal sac will impact PTV cover when the PTV volume is small (< 30 cc) |
| Plan evaluation of SABR spine | % target covered by PD Check min to PTV Check min to GTV/CTV and assess based on relo to cord Dose gradient at cord, conformity of cord tol ID (10-13% PD per mm) Assess other OAR - must be under tol Will it pass QA - plan parameters, segments, MU |
| Hardware considerations SABR spine (when planning) | • Contour metal, artefact in NTT & artefact in bone (cortical and spongy) • Use different window/levels • iEMR – don’t assume materials • Assign material/HU density • Different algorithms = different results |
| Liver SABR | • For patients unsuitable for surgery • Can treat up to 3 lesions if HCC simultaneously • Can treat up to 5 metastatic lesions simultaneously • Must have 800 cc uninvolved • Prefer EEBH -> otherwise pushing up towards diaphragm and lungs |
| Liver SABR fractionations | 48/3, 40-50/5, 60/6 Lower doses if underlying liver dysfunction 24-54/3-6 Px dose adjusted based on achievable liver doses OARs have volumetric constraints due to mobility |
| Liver SABR prep/immobilisation | fast for 2 hours pre CT vacbag Contrast BH, otherwise compression FB or BH planning dataset 2mm Contrast 4dct |
| Contrast phases for liver SABR sim | HCC – arterial – 30 sec after injection Mets – venous phase (60 sec after injection). Delayed - some mets are best visualised with further delay > 1 min from injection • Oral contrast may be used to help delineate stomach/small bowel |
| Arc arrangement for SABR liver | shorter arcs give better low dose wash whereas longer arcs give better intermediate dose co-planar VMAT to avoid inc low dose wash in liver Full arcs only if target is very central |
| Prostate SABR uses | Int/high-risk Less invasive and more cost-effective than brachy Non-inferiority/superiority of hypofrac Monotherapy (35-40 Gy/5) or boost (46/23 +19–21/2) Homo dose painting to dominant nodes (determined using MRI DWI), avoid heat on bladder/urethra |
| Prostate SABR contraindications | – Unable to have an MRI – Hip prostheses (compromise dose distribution) – Unable to have fiducial markers implanted – Inflammatory bowel disease or obstructive urinary symptoms |
| Pancreas SABR | 40/5 • Australian clinical trial (MASTERPLAN) - SBRT with chemo |
| Pancreas SABR cons | Inconvenient location -> surrounded by dose limiting OARs OAR must be respected, primary dose gradient will be at the duodenal interface TVI = tumour vessel interface, vessels <5mm GTV included in PTV Oral contrast may be used to distend the duodenum |
| Cardiac SABR | • Last resort for refractory ventricular tachycardia (VT) and to a lesser degree, atrial fibrillation (AF) • 25Gy single fraction • Requires significant input from Cardiology, an ECG vest, and angiogram/cardiac MRI to identify damaged ventricular wall |
| Head and Neck SABR | Limited evidence, suggest improved LC w acceptable toxicity Standalone - palliation, primary mucosal/cutaneous HNC, salvage re-irad, def olgimetastases When high-dose pall req but protracted txt not feasible Vagal Schwannomas, Paragangliomas 5-10fx |
| Reirradiation of h/n with SABR | Increasingly significant Organs change and move Dose summations need to consider each organ individually - multiple image registrations and multiple calculations to determine equivalent dose Much more needs to be done |
| How does radiation interact with DNA | May interact directly via a single strand or double strand break (hard to repair) Most interactions of radiation are indirect actions |
| Describe the indirect action of cell damage by irradiation | 1. Primary photon interaction produces high energy electrons 2. High energy electrons move through tissue + produce free radicals [H+, OH- ] 3. Free radicals ->break chemical bonds -> change DNA 4. Changes in chemical bonds = biological effects |
| What is a cell survival curve | Describes the relo between the surviving fraction of cells i.e. fraction of cells that maintain their reproductive integrity (clonogenic cells) and the absorbed dose (shape of curve depends on type of radiation) |
| Contrast cell survival curves of high LET vs low LET (diagrams) | – High LET densely ionising radiation is an exponential function of dose – Low LET sparsely ionising radiation has a shoulder region before becoming exponential |
| List factors that make cells LESS RADIOSENSITIVE | – Low oxygen or hypoxic state – Low dose rates – Fractionation – Cells synchronised in the late S phase of the cell cycle |
| What is ED50/ED90 | The dose required to achieve 50%/90% cell kill |
| Compare physical and biological planning | Planning generally involves physical quantities: dose/dose-volume parameters that are assumed to correlate with biological outcome. Surrogates for biological response However, biological metrics more directly reflect the clinical goals of RT |
| Compare biologically guided RT vs biological based planning | BGRT=use of info of individual patient bio response of tumour/NTT to design dose distributions, tumour/normal rad sensitivity, ox status, prol rate BBTP=use of feedback from bio response models in planning, automated (inverse) or manual (forward) |
| What are the two types of biological models in RT treatment planning | – Mechanistic (from first principles): preferable but more complex and difficult – Phenomenonological (models that fit available empirical data): simpler but only relevant in the data space in which they were validated. Extrapolation may be dangerous |
| Describe the levels of radiobiological optimisation | 1 - individualisation of Dpresc - isotoxic 2- individualisation of Dpres and # no - istoxic 3 -radbio functions (EUD/NCP) in inverse planning 4- ps info from func imaging in inverse planning (dose-painting) 5-pt biology added to above (genomics) |
| What is dose painting and potential issues | Higher doses given to regions of tumour with denser cellularity or more clonogens Requires very consistent and accurate set-up -> opportunity for MRI guidance |
| What are dose response models | Biological cell survival models are required for tumours and normal tissues The models should predict observations seen in clinical dose response data |
| List dose response models | gEUD, linear quadratic model, BED |
| what is the gEUD | Generalised equivalent uniform dose The uniform dose that would yield the same radiobiological effect as the non-uniform dose • a —»-used for tumours a —> +(serial organs e.g. spinal chord) a = 1 is the arithmetic mean dose |
| What is the linear quadratic model | describes cell survival, assumes 2 components a represents cell death from single-track damage proportional to dose beta represents cell death from two-track damage (dose squared) assumes each successive txt reduces cell survival in equal increments |
| explain alpha and beta | • α =initial slope of curve – Represents intrinsic radiosensitivity – Non-repairable cell damage – Linearly dependent on dose • β =curvature of curve – Repairable cell damage with time – Responsible for dose/fraction variations |
| what is BED | biologically effective dose Fractionation schemes for which BED’s are equal will be equally effective biologically |
| CONFUSED ABOUT MODELS | GO BACK AND REVIEW |
| describe early vs late responding tissues | early: immediately/during, cell depletion w/in rapidly dividing cells e.g. skin, mucosal layer, tumours, pain, discomfort late: 6-12 mo, cell depletion in slowly dividing cells e.g. spinal cord, kidney, progressive and irreversible, pot life threat |
| describe a/b of early vs late responding tissues | Early effects (Most tumours) – Large α/β (~ 10 Gy) and α dominates at low doses Late effects (Normal tissue) – Small α/β (~ 2 Gy) and β has influence at low doses |
| Define the therapeutic window(+ diagram) | the range of total doses for which acceptable treatment regimes can be designed |
| diagram of 'window of opportunity' between late and early responding tissues and explain | Window of opportunity = more normal cells surviving than tumour Also aided by conformality of dose |
| what is the oxygen effect | • The presence or absence of molecular oxygen within a cell influences the biological effect of ionising radiation (more pronounced for low LET) • Oxygen enhancement ratio (OER) = Dose to produce effect w/out oxygen /dose to produce same effect w oxygen |
| What are the 5rs of radiotherapy | – Radiosensitivity – Repair sub-lethal radiation damage – Repopulation – Redistribution – increases the cell kill in fractionated therapy relative to a single session – Reoxygenation – of hypoxic cells makes them more sensitive |
| What is the radiobiological rationale for fractionation? | Dividing dose into multiple fractions spares NTT through repop + repair of sub-lethal damage bw fxs • Former is greater for late-reacting tissues, the latter for early-reacting Fractionation inc tumour damage through reox and redist of tumour cells |
| Describe sensitivity of high a/b tissues to fraction size | • Rapidly proliferating cells, with high α/β – Not very sensitive to changes in fraction size (or dose rate) – True for most types of tumour – Evidence now that this is not true for prostate tumours |
| Describe sensitivity of low a/b tissues to fraction size | • Slowly proliferating cells with low α/β – Plenty of repair capability – Very sensitive to dose/fraction – Late responding normal tissues are therefore sensitive to large doses/fraction |
| Why do prostate tumours benefit from fewer, larger fxs | • Prostate tumours have possibly been found to have low proliferation rate • Low α/β (~ 1.5 Gy) • Lower than rectum and bladder |
| Why do we fractionate with larger number of small dose/fraction | • Most tumours are rapidly proliferating • Therefore have high α/β (10 Gy and above) • Therefore use a large number of small dose/fraction • Limit damage to late reacting normal tissue (lower α/β) |
| What are TCP and NTCP useful for | Tumour control probability and normal tissue complication probability are useful in comparing plans but not accurate enough for definitive use as patient prediction -> limited follow-up in patients to assess/confirm |
| Rationale of hyperfractionation | • Most Tumours (High α/β) • Hyper-frac = more than one fraction/day with smaller dose per fraction (<1.8 Gy) to reduce long-term complications and allow higher total tumour dose (e.g. head and neck but not practical and benefits not overly significant) |
| Rationale of hypofractionation | • Prostate (Low a/ß compared to normal tissue a/ß) • Hvpofractionation increases dose per fraction minimizing tumour cell proliferation during the course of treatment |
| why would BED be used instead of LQ | LQ requires knowledge of two parameters α andβ for each tissue Obtaining these from clinical data is virtually impossible |
| How was brachytherapy treated historically and limitations | 45/25 to whole pelvis +- PA nodes +- PSW boost 2x20Gy LDR at point A within bladder (1 week part) simulator image to assess position - manual after-loading. Lengthy inpt admissions Limited adjustments for anatomical variations (fixed points A/B) |
| Why was brachytherapy changes to HDR | Better technology –to improve ratio of tumour control/toxicity Safer and more acceptable to patients Safer for staff LDR machines no longer supported for gynaecological treatment |
| Gy/h for LDR, MDR and HDR | LDR - 0.4-2Gy/h MDR 2-12Gy/h HDR >12Gy/h |
| List disadvantages of HDR brachytherapy | • More complex treatment and planning techniques • Limited planning time • Greater potential for error due to higher dose being delivered over a short timeframe • Potential for high radiation dose to staff and patient with source failure |
| List advantages of HDR brachytherapy | • Outpatient treatment • Dose optimisation –adaptive B/T planning • Reduced radiation exposure for staff under normal situations • More stable positioning • Smaller applicators • High dose rate= short treatment time |
| Why is both EBRT and BT necessary for cervical cancer | Largely controllable if enough dose delivered - not possible to deliver necessary amount by EBRT alone due to patient morbidity & high toxicity BT and EBRT ensure tumouricidal dose imity of organs at risk (OAR) and toxicity for the patient |
| Half life of radium | 1600 |
| Half life of caesium | 30 years |
| Half life of cobalt | 5.26 years |
| Half life of iridium | 74 days |
| Half life of iodine | 59.5 days |
| How often are the sources in brachytherapy replaced? | After ONE half life |
| List workflow steps in brachytherapy | 1. Implantation (technique and applicator depends on tumour tompgraphy, size, OARs and preplanning) 2. Image acquisition (US, CT, MRI) 3. Catheters 3D digitisation 4. Target/OAR delineation 5. Planning 6. Quality control 7. Treatment delivery |
| Describe the afterloader in brachytherapy | Source is added after attachment to patient Each of the channels is hooked up to the applicator |
| What is GEC-ESTRO | Provided common language to describe the concepts/terms used in brachytherapy - necessary when using a 3D image based approach Valid for HDR, PDR and LDR (limited), different dose fractionation schemes and different applicator types |
| What topics does GEC ESTRO provide common concepts and terms for | - volumes, particularly initial/residual GTV - initial/adaptive CTV and OAR - radiobiological variations - dose volume parameters - process from planning aims to px - dose point parameters - different levels of clinical practice (level 1, 2, 3) |
| In brachy, what is the high risk CTV | - major risk of local recurrence - residual macroscopic tumour at time of BT (smaller than at time of diagnosis) |
| In brachy, what is the int risk CTV | major risk of local recurrence - initial macroscopic tumour at time of diagnosis |
| In brachy, what is the low risk CTV | potential microscopic tumour spread - treated with surgery and/or external beam RT but not brachy |
| Role of ring and tandem gynae applicator | Pushes rectum away (anterior rectal wall pushed anteriorly) and pushes superior vaginal mucosa away from high dose region |
| MDT involved in brachytherapy | Anaesthetist and tech, RO, scrub nurse, gynae oncologist, RT, medical physics, recovery nurse |
| integrated scheduling of BT (inc fractionations) | Week 1-4: EBRT Week 5: Mon, wed, thurs EBRT, tue and fri HDR BT Week 6: Mon EBRT, Tues HDR BT, Wed last EBRT, Fri HDR Week 7: last HDR BT Tues EBRT: 45/25 +- 5.4Gy/3 to PSW, PA HDR: 30/5 (2 per week) OR 28/4 (2 per week) |
| Role of pre-treatment (diagnostic) imaging in brachytherapy | PET, MRI, CT evaluate tumour determine treatment modality determine optimum treatment vol and dose |
| Role of imaging during each brachytherapy insertion | MRI, CT, US Evaluate tumour response Verify applicator position Define HRCTV, IRCTV and OAR Adaptive radiotherapy |
| Role of post-txt imaging in brachytherapy | Evaluate tumour response and toxicity |
| Why is U/s used in brachytherapy | non-invasive, portable and inexpensive real-time imaging during insertion confirm applicator pos and check intauterine tandem is centred measurements can be made of uterine dimensions |
| What is the tandem loading pattern (diagram) | Pear |
| What is the ring loading pattern (diagram) | Apple |
| How can patient customisation be applied in brachytherapy | Patient customisation - 3D printed applicator or interstitial needles, adaptive RT |
| Why is MRI used in brachytherapy | Excellent soft tissue differentiation — CTV can be located Use to define target structures and OAR Smaller aperture size, which may not accomodate all patient sizes Small FOV- external body of the patient is not visualised |
| What are the effects of inter and intrafraction variation in brachytherapy? | may cause deviation from prescribed dose - this is more significant for HDR as the number of fx is higher and higher dose per fx with steep dose gradient around applicators Tumour shrinkage and normal tissue fibrosis may occur over HDR delivery |
| idk dose limiting structures | Rectum not usually dose limiting - applicator with rectal retractor Bladder dose limiting, espec w acutely angled tandems Doses to ICRU points (point A, rectum, bladder) do not correlate with dose vol parameters (D90) - underestimate dose to bladder |
| What is the role of a dome in brachytherapy | To treat uterus or upper vagina Stops vaginal wall from collapsing onto applicator -> high dose Even dose drop-off Need largest possible diameter of applicator -> minimal space for collapse of vaginal wall onto applicator |
| Describe advantages of prostate brachy | • Image guided needle placement • Optimised dose distribution • Organ motion minimised • Radiobiological advantage • Remote afterloading • Single reusable source |
| How are needles arranged in prostate brachy | No standard arrangement due to shape/pos of prostate, urethra, rectum Often need more needles on post edge - modulation Must account for pos contour changes after insertion Needle reconstruction important - determines loc of dwell pos |
| Prostate brachy workflow | Contour, needle reconstruction, dwell times -> step size, dwell time, distance, source activity -> transfer tubes for delivery |
| Acute clinical issues in brachytherapy | Template/catheter movement -> -> bed rest Haematuria/clot retention -> continuous bladder irrigation Perineal/back discomfort -> analgesics Infection -> propylactic antibiotics DVT prophylaxis -> stockings, heparin Defaecation -> low residue diet |
| What are the considerations in imaging a tumour that is moving | Motion: physiological, resp/cardiac, volitional/reflexional Imaging Artifacts: lengthen/shorten targets, target displacement Dose-Delivery Artifacts: increasing target vol sizes , increased portal sizes, increased OAR/NTT irradiation |
| Why do we want to control for motion (quick answer) | If motion can be controlled: tumour margins could be reduced, OAR / Normal tissue sparing can be improved, potentially dose escalate, improve clinical outcomes |
| How is 4DCT acquired? | - Respiratory cycle correlated CT Scan - Acquisition of multiple images at slice positions, each image tagged with breathing signal - Images are sorted based on breathing signal - DICOM exported to TPS Slower acquisition |
| 4d imaging vs 3D CT amplitude imaging | • 3D CT at Amplitude: Max expiration & inspiration, partial tumour trajectory • 4DCT: Full tumour trajectory tracking |
| Define the ITV | CTV + internal margin (uncertainties in internal motion) = ITV (envelope of motion) |
| What is a specialty image set | a feature that combines 2 or more image sets to make a specialty artifical/synthetic image. use these pixel values: - MIP Maximum Intensity Projection - MinIP Minimum Intensity Projection - AvgIP Average Intensity Projection |
| What is the MIP | - Displays the maximum CT number of all the pixels at the same spatial location over the respiratory cycle - Basically shows entire extent of tumour motion/trajectory - Use with caution for tumours close to the diaphragm or chest wall |
| What is the MinIP | - Displays minimum CT number of all pixels at same spatial location over respiratory cycle - Basically shows where some part of the tumour is always present throughout respiration - Useful for liver tumours that present low density Good loc for iso |
| What is the Avg IP | - Displays average CT number of all pixels at the same spatial location over respiratory cycle. - Considered to be appropriate for use as the treatment planning CT (better than FBCT) • Reducing need for an additional planning scan -> rad exp to pt |
| How does optimised DCAT differ from traditional DCAT | • Traditionally DCAT has been used with constant dose rate • Modern Linacs that are capable of modulating the dose rate - (Variable MU/degree) • Effectively VMAT with limited MLC modulation - Plan quality = VMAT - Interplay issues largely negated |
| What is DCAT vs VMAT suitable for | DCAT = simple targets, limited dose shaping but steep fall-off VMAT = suitable for targets including complex concave |
| DCAT vs VMAT plan quality | • In practice, DCAT plan quality < VMAT • Lung SBRT clinically effective - limit dose to healthy lung as likelihood of patient returning is high DCAT = quicker to plan and deliver, less MU More robust - small VMAT flds - calculation/delivery accuracy |
| CT vs MRI as primary dataset for Mr-Linac | Ct=full range of electron densities, contour deformation onto daily MR not as good MRI=needs bulk density corrections (reduced accuracy), contour deformation onto daily MR is great |
| Contours for MR-Linac and cons | Oars, target volumes, planning vols and density vols (enough to calc electron scatter conditions with less calc time) More contours = more time to manually edit contours |
| Describe density volumes on Mr-Linac | bone, air, lungs or gas pockets will need to be contoured. adding additional structures such as fat muscle vessels and differentiating between hard and soft bone will potentially provide a more accurate bulk density data set -> but more time |
| Where can ED values be obtained for MR-Linac planning? | Ideally from CT scan But in house values may be provided from patient cohortz ICRU report 46 |
| What is structure layering in MR-Linac | • Structure higher in list will control density • Overlapping structures: smaller structures above larger structures • Patient contour towards or at bottom of list. • Planning contours (e.g. PTV) under patient |
| Types of contour adaptation on MR-Linac | Deformable = best for two scans of same type Rigid = copies, no adjustments, usefully for stable structures Margin = for structures created with margins (e.g. CTV, PTV, PRV) None = for those not necessary for daily plan adaption |
| MR-Linac couch/coil cons | The couch is indexed and its position effects isocentre location The coil is centred on isocentre with an adjustable height. The centre of the coil can be treated through so it must be positioned properly in relation to the isocentre |
| Isocentre placement on Elekta Unity | Iso 14cm from couch on MIDLINE. In conventional txt, iso is in centre of PTV - unity couch cannot be shifted l/r or a/p, so iso is in middle of pt at slice that corresponds to centre of PTV (sup/inf). Beam segments are offset to ensure PTV coverage |
| Beam arrangement on MR-Linac | Limited to static or IMRT 9vs18 beams - more beams resembles VMAT but more txt time Avoid cryostat pipe (G13), entry allowed but not exit. Be wary of areas around as daily shifts may cause beam to be blocked by pipe. Avoid couch edges |
| Plan parameters on MR-Linac | Min seg area depends on PTV (smaller val = more, inc delivery). Inc seg size = dec no of segs = still optimal Lowest min seg width (machine fixed) Low min mu/seg for small PTV (4), inc = dec plan segs w/out dec plan qual Max seg - set at higher end |
| IMRT constraints for MR-Linac | Develop a robust template for each site Suitable for most patients and calc with minimal adjustment Complex prescription = long calculation Consistent constraints can aid with daily plan adaption - consistency, ease and familiarity for staff |
| Optimisation goals for MR-Linac | • Long treatment delivery = longer patient is on the treatment couch • Simple prescriptions • Minimise the MU deliverable • Minimise number of segments • Minimise treatment beams |
| What is the electron return effect | Occurs on tissue-air boundaries, electrons returning to pt patient due to mag field. Inc surface dose at beam exit/ entry and air cavities like lungs Lorentz force: As electrons leave HDR and enter LDR, they spiral along B0 |
| How to manage ERE on MR-Linac | Effect of ERE is reduced with MORE BEAMS - accurately modelled by TPS. EXCEPT for unaccounted for density changes e.g. gas in rectum on daily fraction |
| What is the electron streaming effect | - Same origin as ERE - Unobstructed spiralling electrons move along magnetic paths - More significant with an inclined or curved patient surface - Parallel and antiparallel to B0 - Head and neck, chest, extremities |
| How to manage ESE | Minimised by: beam angle selection and bolus on affected sites Bolus on patient surface or anterior edge of coil Sim cons -> turn head away Ensure relevant area involved in scan region Do not angle coil - creates more ESE |
| Define adaptive radiotherapy | changing the radiation treatment plan delivered to a patient during a course of radiotherapy to account for: - Temporal changes in anatomy (e.g. tumor shrinkage, weight loss or internal motion) - Changes in tumor biology/function (e.g. hypoxia) |
| What is IGRT | - Images taken just prior to treatment delivery - Assess changes in patient position relative to treatment plan - Adapting positioning to account for variation increased treatment precision Adaptive radiation therapy not possible without IGRT |
| Types of IGRT | - 4DCT capabilities (e.g. Elekta Symmetry) - Surface/anatomical changes (e.g. SGRT) - Video-based systems (e.g. Varian RPM) - Ultrasound (e.g. Clarity Autoscan) - RF tracking (e.g. Calypso) - kV tracking (e.g. Synchrony) MRI |
| Limitations and advantages of IGRT | Limitations: image guidance in isolation cannot correct for non-rigid changes Advantage: provides rich 3D information which can be used as the basis for adaptive planning intervention – modification of the initial plan |
| When can adaptive radiotherapy occur | - Offline - between fractions - Online – immediately before a fraction - Realtime - during a fraction (relates to organ and patient motion) Some adaptive radiotherapy techniques can be a combination of online and offline |
| Describe real-time adaptive radiotherapy | - Technology to locate the target during real-time - Radiofrequency waves: Calypso, RayPilot - Dose calculation during real-time - Technology to hit the target in real-time MLCs adjusted based on imaging feedback |
| Describe deformable imahge registration | Online/real time approaches require def reg Finding correspondences between data sets that differ in time, space, modality Efficiently adapts contours required for replanning However still ill-defined (QA) Important for practical implem of ART |
| How does def reg support ART | 1. Propagation of planning contours to daily images: reduce time required for re-contouring 2. Dose deformation and accumulation: use of deformable vector field to propagate dose distribution |
| Elekta unity characteristics | 7mV FFF 1.5T 70cm bore, SAD 143.5 Sup/inf limited field size (22cm) Collimator fixed at 0 degrees Iso 0.5cm radius >400MU/min |
| What is the displacement threshold vs gating envelope | Displacement threshold: position checked against independent thresholds in all three cardinal directions OR Gating envelope: percentage overlap between the target and gating envelope in 3D |
| Types of motion management delivery | Free breathing average position Free breathing exhale Breath hold Exception gating -> beam paused if target moves unexpectedly outside tolerance |
| Discuss bladder ART | Organ motion, can change size, shape, pos Generous margins = irradiating NTT and dose limiting toxicity to small bowel/bladder IGRT reduces margins but bladder size can still change b/w fractions |
| When is bladder ART used | 1. Online ART: - Daily pre-txt CBCT - Staff select ‘plan of the day’ - Currently most popular method 2. Offline ART: - Adaptive PTV delineated based on information from first 5 fractions CBCTs Utilisation of patient-specific margins |
| Discuss need for prostate ART | size, shape, pos highly variable -> underdosage or overdosage -> inc side effects |
| DDescribe offline ART for prostate | Delivered dose and variation of organs is accounted for in planning -> dose accomplished in adaptive plan is optimised for remaining treatments - Studies have found decreased urinary and rectal side effects, inc therapeutic ratio |
| Describe online ART for prostate | kilovoltage intrafraction monitoring ++ MLC tracking = no additional hardware CBCT suboptimal due to poor soft tissue contrast ->MR-Linac |
| Discuss need for lung ART | Poor prognosis Dose escelation restricted by surrounding structures Respiratory motion affects tumour delineation ART can assist in tumour delineation and margin selection, txt delivery, adaptation due to bio/func response, dose esc and NTT sparing |
| How is respiratory motion controlled in treatment | - Active motion compensation techniques: Gating - Tumour tracking Techniques adapt the treatment to maintain constant target position in the beam’s eye view when the beam is on |
| Describe resp gating | Real time monitoring or breath hold Varian RPM or ABC |
| Describe lung tumour tracking | Dynamically shifting dose in space to follow tumour's changing position during FB 1. Identify tumour pos in real time 2. Anticipate motion to allow for time delays in beam response 3. Reposition beam 4. Adapt dosimetry |
| Describe radixact synchrony for lung | Real-time, markerless motion management - Correlates LED-based external resp signals w internal tumor pos captured via kV - Continuously adjusts jaw/MLC pos - Suitable pt selection Extended treatment times due to beam interruptions |
| DAQUF LUNG ART??? | |
| Discuss need for H/N art | H&N pts undergo considerable anatomic and tumour change (weight change, tumour prog, post-op change) -> inad coverage of tumour, OAR tols exceeding, loco-regional recurrence, side effects ART to correct pos errors, anat change and bio response |
| Two types of variation in h/n | KMS! Positioning and anatomic |
| What is biological adaptation | Modification of treatment based on changes in tumor resistance factors and normal tissue function. Adaptation based on assessment of early response. Define vol that would benefit from dose escalation |
| What is varian ethos | Uses AI machine learning to create contours/generate adapted plans within mins Def reg between pCT and CBCT - creates og plan recalc on pCT with dose metrics from edited structures AND adapted plan optimised using edited structures - one chosen and QA |
| What is radixact synchrony | - Radixact (Accuray) has integrated intrafraction motion management based on the Synchrony to predict motion based on implanted fiducials or the tumour itself Synchrony radiotherapy treatment planning technology actively tracks and corrects for it |
| Limitations of ART | Time and resource intensive, financial burden, more clinical outcome studies needed, appropriate pt selection |
| Reflexion PET-Linac | ????? |
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