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Treatment 4
| Question | Answer |
|---|---|
| Define IGRT | Involves use of any imaging to aid decision making to enhance confidence in accuracy of treatment delivery |
| Explain the rationale of IGRT | To increase precision and accuracy in planning + delivery -> reduced toxicity with potential dose escalation and improved tumour control Guides whether to treat, how and when |
| List challenges in modern RT approaches | ✓Random and systematic errors ✓Inter-fraction and Intra-fraction errors -> We rely on IGRT to improve treatment outcome |
| List the pros/cons of kV imaging | •Better resolution/contrast than MV. •Lower imaging dose. •Requires additional LINAC hardware. •Similar workflow to traditional MV methods. •Widely available and utilised. |
| List sites where mV may still be used | whole brain, breast, MLC shape verification |
| Describe fiducial markers | Routinely used for prostate , gold seeds 0.9x3mm implanted under transrectal US. Surrogate to overcome factors such as bladder, rectal filling and patient motion |
| List strengths of FMs | • Surrogate of prostate motion • Very fast method of localisation • Staff have great confidence • very low intra- and inter-observer variability |
| List weaknesses of FMs | • Expense • Invasive procedure = risk of infection • Some patients ineligible, e.g. Warfarin dependency • Rely on three discrete points to localise prostate • Evaluation of nearby organs and target deformation diffcult (if not impossible) |
| Describe volumetric MVCBCT imaging | •Uses treatment beam and EPID •Standard hardware – no additional arms •Low contrast (high energy) ->high-density implants without artefact (e.g. prosthetic hips) •Same isocentre of imaging and treatment. •Dose several times greater than kV. |
| Describe volumetric KVCBCT imaging | •Flat panel detector •Daily isocentre localisation + able to monitor patient throughout treatment. •Volumetric rather than few discrete points. •Learning when to replan (adapt) treatment: disease progression, disease response, weight loss |
| Elekta source/detector orientation | Elekta = source on right (patient's left) Varian = source on left (patient's right) |
| Describe the two modes of CBCT acquisition | Full fan or half fan mode using two bowtie filters Full - head (narrow FOV) Half - chest, abdo, pelvis (detector is moved to capture larger area) |
| Dose to prostate from CT Elekta vs Varian | Elekta = 2.3-4cGy to surface, 1.6-2.2cGy to centre Varian = 3-8cGy peripheral, 3-4cGy to centre |
| Describe varian halcyon | •Multi-arc VMAT, IMRT •Dose rate – 800MU/min •6FFF, 4 Revolutions per min •100cm bore •15 sec for kV-CBCT •7 sec for 2D image –MV imager |
| Describe varian ethos | • Uses AI/machine learning to create contours and adapted plans for RO review within minutes Diagnostic CBCT + ability to register images to PET/MR in adapt. workflow (need an RO/physicist present when doing adaptive RT) -> can complicate workflow |
| Describe cyberknife | Two ceiling-mounted kV sources, two floor detectors Continuous - optical beams monitor external markers Non coplanar, non isocentric Hypofraction-> reduced margins/toxicities Sedation needed sometimes -> anxiety due to swift motion robotic arm |
| Describe ExacTrac | •two floor-mounted kV sources •two ceiling-mounted detectors •add-on for Varian, Elekta and Siemens Linacs •Allows frameless radiosurgery Good for NCP -> image after moving floor to confirm isocentre alignment |
| List CBCT based technologies | Fan beam: CT on rails, tomotherapy Cone beam: kilovoltage CBCT, MVCT |
| Describe VERO SBRT | •gantry similar to Tomotherapy •MV source (+EPID) •Stereoscopic kV imaging •Unrestricted imaging angles |
| Describe CT on rails | •Diagnostic CT opposite/orthogonal •Single couch for both gantries •Assumes fixed relationship between the isocentres of the two systems (CT and Linac). |
| List limitations of CT on rails | Considerable differences b/w in in room CT and EPI Motion between imaging and treatment Couch sag cannot be fully accounted for Associated time lag EPI image quality and outlining uncertainties |
| Describe helical tomo | •MV beam source placed on ring gantry •Xenon ion chamber array allows CT reconstruction from treatment beam •MV fan beam •Provides less tissue contrast than kV CT, but less artefacts from high-Z material (such as metal hips) |
| List ionising image guidance tools | CT on rails, ExacTrac, CBCT, MVCT |
| List non ionising image guidance tools | MR guided, US guided, surface tracking |
| List challenges in development phases of MR guided technology | Radiofrequency interference bw linac and magnetic field -> affects radiation dose distribution and MR image quality Increased complexity of imaging will impact clinical decision making Safety/QA implications |
| List MRI guidance machines | MRIdian (cobalt sources), MRIDian Linac, Elekta Unity |
| Describe Calypso | • Originally limited to prostate (implanted markers) • FDA approved for any site (surface markers) • Continuous, real-time monitoring of the prostate • Non-ionizing = no additional radiation dose |
| Describe Calypso workflow (prostate) | Three EG transponders permanently implanted Electromagnetic array in tracking system is positioned above the treatment site Array sends a signal that excites transponders System detects tumour movement and if req, txt can be paused to adjust |
| Rationale and outcome of Calypso | Motion doesn't stop once arc begins, target pos at setup doesnt always translate to target position during treatment -> reduced GI, GU and some sexual symptoms |
| Discuss US guided technologies | • Not common, prev suffered from systematic errors → inaccurate localisation →probe pressure → inter-user variability. • Traditional 2D – hard to interpret, operator dep. • New 3D US (± robotic arm): inexpensive, non-IR guidance |
| Describe Klarity | Employed at CT to aid soft-tissue vis. Probe calibrated to iso, enables ultrasound to be fused with planning CT Treatment couch is tracked using the same system as probe, highly accurate alignment. • Breast (post-surgery cavity def) • Prostate, Gynae |
| Describe SGRT | a rapidly growing technique to track patients’ surface for setup and motion mgmt 1. Setup: SGRT speeds and improves setup accuracy. 2. Txt: SGRT can reduce need for immob 3. 4DCT: SGRT enables contactless, non-invasive reconstruction of 4DCT data |
| Describe SGRT workflow | • Reference surface model is produced from: • acquiring a 3D surface at simulation. • At each fraction, the system images the current patient pos • Couch shifts then calculated to correct inconsistencies |
| Describe Varian RPM | • non-invasive, video-based system • uses infrared tracking camera and reflective marker array (surrogate) • measures respiratory pattern; displays as waveform • useful for monitoring breath hold • Indicated for lungs, liver, pancreas, breast |
| List radiosurgery characteristics | • Single high dose of radiation (12-150Gy) • Tissue ablation/destruction • Requires defined ‘target’ - SMALLER AREAS (targets less than 3-4cm) |
| List standard radiotherapy characteristics | • Multiple small doses (1.8-3.0Gy) • Relies on sensitivity differences between target + NTT • Mitotic cell death (DNA damage) in dividing cells • Often used with margins to treat regions • Can be used for large volumes |
| Define stereotactic radiosurgery | "The use of external radiation in conjunction with a stereotactic guidance device to very precisely deliver a therapeutic dose to a tissue volume" |
| What is the therapeutic ratio (DO DIAGRAMS) | Correlation of tumour control probability and normal tissue complication probability at different doses of radiation — Methods to expand the ratio "gap": e.g. radiosensitizers, fractionation etc. |
| How does gamma knife / manipulate the therapeutic ratio (DO DIAGRAMS) | Adds brute force geometry - reduce to the point of removing uncertainties associated with localisation, patient motion and treatment delivery = ZERO MARGIN Enhance gap in therapeutic ratio |
| List characteristics of modern gamma machine | 192x 60CO sources 3Gy/min max dose rate 1.17MeV and 1.33MeV y-rays Half life = 5.271 years 0.1mm iso (compared to 1mm for linac) |
| Describe collimation of gamma machine | Fixed tungsten beam collimation with 8 sectors 24 sources per sector, arranged in 5 rings 4mm, 8mm, 16mm, blocked The collimators don’t move - the sources slide over = accurate Can slide at different rates Different sizes of collimator holes |
| Describe the radiation sources in gamma machine | - Generated in Nuclear Reactor 2 years - Triple concealed, welded, stainless steel cylinders in aluminium bushing - 20 pieces of 60Co pellets/source - 192 sources - Dummy sources - Activity at loading = 222 TBq Dose rate at focal point> 3 Gy/min |
| Explain 'shots' in gamma knife | • Plan generated through summation of dose delivered to individual coords with varying coll sizes • May contain 1 to unlimited (overlapping) shots with varying collimation • 4, 8, 16mm circular coll focused to single point, NCP • Shots = Isocentres |
| gamma knife tmr vs conv algorithms | TMR10 assumes the patient's entire head has uniform, water-equivalent density. The newer convolution algorithm uses patient-specific CT data to account for tissue and bone heterogeneities (inhomogeneities) for higher dosimetric accuracy |
| Advantages of gamma knife planning/txt system | Unified system, unified database Speedy calc Minimal plan QA simple dose calc and optimised workflow Forced functions and automation |
| List characteristics of gamma knife treatment and the associated benefits | Fixed collimation, no moving parts, fixed radiation output, fixed patient pos -> reduced uncertainty, zero margin Inhomogenous dose and high max dose (-220%) - requires zero or small margins Sharp fall off - maximise benefits of above |
| Describe gamma knife frame | • Aluminium Frame (MR con), detachable feet • Insulated aluminium fixation posts (MR con), reusable titanium locking pieces/screws • Plastic disposable inserts, insulates fixation screws • Screwdiver Can be put in/out of machine <0.3mm accuracy |
| Discuss gamma frame attachment workflow | Neurosurgeon, local anaesthetic and sedative Tightening of diagonal opposed screws pairs 1/4 at a time Denting of skull be aware of craniotomy site Tolerated differently - some experience pain/terror, others talk throughout 10-15 mins |
| How are RTs involved in gamma frame attachment | Involved in checking for machine/equipment clearance and compatibility and feedback of frame (well implanted) |
| Discuss gamma frame removal workflow | Frame supported by RT Hold base of frame, instruct pt to remain still and loosen diagonal opposed screws 1/4 at a time |
| How are coordinates calibrated in gamma knife CT session | stereotactic coordinate system fiducials visible 9x fiducials visible on axial slices, the central fiducial moving in a z pattern (visible on sagittal) |
| gamma knife planning -> treatment workflow | immobilise -> MRI/CT -> plan -> treat all steps within one day (a few hours) minimal QA (consistent output and beam profiles) pre-txt CBCT not nessecary |
| Planning process for gamma knife (DIAGRAMS) | generated by RT, RO, neurosurgeon No heterogeneity correction Plans range from single shots (small mets) to more complex multi-shots (irregularly shaped lesions) 45to90% px isodose - sharpest gradient occurs further down shoulder of the beam profile |
| Treatment process for gamma knife | Shots delivered sequentially in a "run" No IGRT required (although possible) No user input required Monitor the patient, option to pause/resume Time depends on dose, PI, shot number and weightings, target size, rate of decay 15 mins -5.5 hours |
| clinical indications of gamma knife | Intracranial only : Arteriovenous malformation (vascular) Meningioma Pituitary (benign or malignant) Vestivular schwannoma Mets (single or multiple) Trigeminal neuralgia or essential tremor |
| Masks vs frame for gamma knife workflow | Frame: immobilise, scan, plan, treat Mask: scan, plan, immobilise, CBCT, dose evaluation, treat |
| when would a mask be used over frame in gamma knife | Frame = Gold standard Mask might be a better option if prev craniotomies, if pt late; neurosurg unavail, lesion too large — fractionated (1st fraction with frame), multiple lesions in same area |
| When MUST a frame be used for gamma knife | Functional cases e.g. Trigeminal neuralgia, Tremor Vascular cases e.g. Arteriovenous malformation (AVM) Treatment time too long for a Mask > 45mins (Patient tolerance of immobilisation) |
| Brain mets dose GK | Avoids complication with WBRT, improved QOL, small targets impractical for Linac 15-22Gy @ 45-85% in 1# >8cc 24Gy @50% 3# or 25-30Gy @ 50% 5# |
| Benign meningioma dose GK | <4cc 12-15Gy @ 50% (if near abutting optics Dmax <22Gy) >4cc or near optics 25Gy @ 50% |
| Pituitary dose GK | Functionising dose 18-24 @ 50% 1# Non func <4cc 12-15@50% 1# Non func >4cc 15-27@50% 5# |
| Vestibular schwannoma dose GK | <5cc 12-13Gy @ 50% 1# >5cc 25Gy @ 50% 5# |
| AVM dose GK | Goal is complete obliteration <4cc 20-22Gy @50% 4-6cc 18-20Gy @ 50% >10cc 18Gy @ 50% >10cc 16-18Gy 1# (each stage 6 months apart) |
| Trigeminal neuralgia GK dose | 75-90Gy@100% 1# No volume contoured, target is nerve |
| Essential tremor/parkinson's disease | 120-130@100% 1# |
| Adaptive planning for GK | Adaptive Planning Position of shots are preserved within the patient anatomy Variation more obvious at lower isodoses where impact of rotation increases |
| Pre-op SRS (GK) | improved target delineation, sterilisation of surgical bed and less NTT irradiated reduced radionecrosis and shorter overall treatment however: no path confirmation and wound healing surgery must occur within 7 days of SRS |
| SRS thalamotomy | for those unfit for DBS and refractory to medical treatment formal motor and QOL assessments |
| List the treatment principles of stereotactic treatment | - Uses multiple NCP beams/arcs - Steep dose gradient + conformity - Ablative doses to small targets, reduced toxicity of surrounding tissue |
| SRS vs SRT | SRS = single high dose of radiation delivered intracranially SRT = treatment > 1 fraction, intracranial or extracranial |
| Indications for SRS | Small tumour size, location (e.g. inaccessable by surgery), tumour path and recurrence, patient characteristics |
| Acute and late side effects of SRS | ACUTE SIDE EFFECTS: headache, nausea, vomiting, visual disturbances, swelling (managed with steroids) LATE SIDE EFFECTS: facial weakness, numbness, hearing loss |
| Advantages of stereo over VMAT | - Minimal damage to surrounding tissue - Treat to higher doses Treatment can be given multiple times |
| List the most frequently treated cranial tumours with SRS | - Acoustic neuroma - Meningioma - Cranial metastases GBM |
| Describe acoustic neuroma | Benign tumour of the 8th cranial nerve Presentation: hearing loss, tinnitus & balance disturbances SRS utilised when surgery risks damage to facial nerves and hearing SRS 12/12.5Gy/l # |
| Describe meningioma | Usually, benign tumour arising from meningeal tissue Stereotactic for inoperable or small tumours (usually in hard to access areas) SRS: 18-20Gy SRT: 50-60Gy @ 1.8-2Gy/# |
| Brain mets dose SRS | 16-20Gy/1 per met |
| Describe GBM | Highly malignant and fast growing — average survival 12 mo Symptoms: headaches, nausea, vomiting, seizures, memory loss, changes in speech or personality, walking difficulties Stereotactic used at time of recurrence SRS: 6 -16Gy/1# |
| Simulation for SRS | Mask made - ensure eyebrows, nose, forehead and cheeks visible Laterally outer canthus to anterior of tragus 1mm CT slices, primary image set |
| Planning for SRS | Fuse CT and MRI (for voluming) Contouring dose in multiple scans and planes with assistance of specialists RO approval and export |
| Smallest field size for SRS truebeam | 6x6mm (compared to VMAT 3x3cm jaw size). 2mm wide leaves |
| SRS tumour doses per GTV diameter/vol | GTV di <2cm, vol 1-10cc = 18-24Gy GTV 2-2.5cm di, vol 10-20cc = 18-20Gy GTV 2.5-3cm di, vol >20cc = 15-18Gy |
| SRT tumour doses per GTV diameter/vol | GTV <3cm di = 27/3 or 30/5 GTV 3-3.9cm = 30/5 or 25/5 GTV 4-5cm = 25/5, 33/10 |
| Treatment process for standard SRS on linac | Obs taken by nurse and ativan / dex administered ExacTrac system -> infrared markers ->shifts 2 stereostopic x-rays taken and fused DRRs to determine shifts X-rays retaken to confirm pos Imaging is repeated for each floor angle 20-90 mins |
| SRS linac imaging tolerances | • >3Gy/# tol is <0.5mm • <3Gy/# tol is <0.9mm • VMAT brain tol is 3mm |
| Describe cyberknife | SRS 6MV, 1000MU/min • fully robotic radiation delivery system with real-time image guidance • used alone or in conjunction with other forms of therapy Images every 15 secs, lengthy txt times but can sim, plan and treat all on same day |
| Cyberknife indications | • Patients with inoperable or surgically complex tumours, OR • Patients seeking an alternative to surgery |
| List sites treated with CK | brain tumours - benign and malignant mets meningioma trigeminal neuralgia acoustic neuromas AVMs some functional disorders |
| MDT for acoustic neuroma | — Radiation Oncologists, Neuroradiologists, Radiologists, Medical Oncologists, RTs, Physicists |
| Patient selection for acoustic neuroma SABR | — Typically <3cm diameter — Residual AN after subtotal resection or recurrent tumour — Patients not fit for surgery because of age or Other risk factors |
| Side effects of SABR to acoustic neuroma | SIDE EFFECTS = tinnitus, balance problems, numbness, pain or weakness in face muscles, vertigo |
| Planning workflow for acoustic neuroma | CT contrast, MRI (T1 and GAD) 1mm slices 12.5Gy prescribed 70-80% |
| Lung SABR indications | • lung metastases • early stage, inoperable non-small cell lung cancer • tumours in challenging anatomical locations central lung tumours surrounded by sensitive structures lung tumours near the chest wall, inoperable peripheral lung tumours |
| Common lung SABR px | 54/3 |
| Common spine sabr px | 21/3 |
| Common liver sabr px | 54/3 |
| Common kidney sabr px | 39/3 |
| Common panc sabr px | 40/5 |
| Describe SABR prostate virtual brachy boost | • High Grade • Fiducial Insertion by Urologist – 4 (5mm x 1mm) Gold seeds • IGRT VMAT 50.4Gy/25 • 3 week break with SpaceOAR insertion by Urologist • Virtual brachy with CyberKnife 19.5Gy/3 treatments • Follow up care 2 – 3 months post CyberKnife |
| Discuss the LET vs dose advantage of protons and carbon ions (graph) | Protons = dose distribution advantage but minimal LET advantage Carbon = same dose distribution advantage but also higher LET |
| Describe boron neutron captute therapy | Designed to target high LET particle radiation to tumours at cellular level Boron has high neutron absorption, immediately after capturing a neutron, boron-IO briefly becomes boron-11 before disintegrating to an energetic a-particle and a recoil Li ion |
| Issues with boron neutron captute therapy | However - need to get high energy neutrons from a nuclear reaction (not common in most hospitals) and boron 10 needs to be introduced and selectively up taken in disease tissue - NOT NTT. |
| Dose distribution cross-section of x-rays vs protons and carbon ions | Protons offer uniform high dose with quick dose fall-off Fragmentation carbon ions cause isodose drag beyond target |
| Proton characteristics | - +1.6 x 10-19C charge (1.000… amu) 1.6 x 10-27 kg, around 1800 x electron mass |
| what is the bragg peak | Bragg peak depends on proton energy Lower energy = shallower bragg peak |
| List the benefits of proton interactions in radiotherapy | The dose of a proton beam diminishes sharply downstream of BP Multiple scattering in pt dominates dose fall off laterally, penumbra excellent for low en, >ideal for higher energy Penetration controlled by energy or attenuating material upstream |
| Electrons vs protons | Electrons quickly scatter, lose energy and sharp definition - broadening and bremsstrahlung interactions Protons heavier, travel in straight lines with continual energy loss (coloumb interactions with orbital electrons and scattering from nucleus) |
| List the types of proton interactions | coloumb interactions with orbital electrons - ionisations and excitations coloumb interactions with nucleus (repulsive force - scattering) nuclear interactions (absorption) |
| what is the range of protons | • Mono-energetic beam of protons • Depth at which half of them come to rest |
| what is proton range straggling | • Individual protons with same initial energy in same material will have slightly diff range (diff number of collisions) • Fluctuation in number of interactions and energy loss per interaction broadens the Bragg peak in depth |
| why does the bragg peak occur | Most of the proton path is a continuous slowing down - i.e. collision interactions with small energy losses End of range - proton fluence reduces significantly in a short distance as proton energy becomes low and protons stopping power increases |
| describe the range of the bragg peak | bragg peak is a few cm wide in depth, larger vols in depth direction are treated to a uniform dose through delivery of multiple pencil beams with different energy |
| what does reducing proton energy do | broadens energy distribution - energy straggling |
| describe multiple coulumb scattering | multile c scattering from nuclei = negligible energy transfer proton beam spreads out in the transverse direction -> lateral broadening of penumbra increases at depth |
| how do protons generally deposit energy | through collision interactions with orbital electrons |
| what does the depth of the bragg peak depend on | proton incident energy |
| what is proton energy loss characterised by | linear energy transfer and stopping power |
| describe active scanning | Charged particles are deflected in a magnetic field If we add deflecting magnets in gantry head and control the currents in magnets we can control deflection of beam and scan it around Added challenge - accounting for diff densities which effect range |
| describe the spot scanning technique | 7mm beam regulated by computer controlled magnets to precisely position high-dose spot for specific period of time at desired location >9000 spots superimposed to deliver desired dose uniformly Energies, intervals, weightings arranged for flat BP |
| clinical implementation of protons | Cyclotrons or synchrotrons Costly and technicallv complex component = rotating beam 100 tonnes with sub-mm mechanical precision beams are narrow and deposit their energy in a BP only 6 mm wide beam needs to be spread out in width and depth |
| challenges of spot scanning techniaue | Sensitive to motion of target volume Gated spot scanning Trigger the beam delivery only within a given phase interval of the breathing cycle Track the tumour applying position corrections in real-time |
| challenges for proton therapy | reduce cost and use optimally. more patients must be treated to reduce cost per pt quantify RBEs for specific tumours and NTT, optimise biological dose to improve txt outcomes |
| Describe heavy ions | Carbon, Neon, Argon, Helium Strong inc in LET at BP BP dose to entrance dose is larger than protons Improved distribution + sparing. Less variation in BP range (thinner) Heavier mass means stronger magnets needed in gantry - high cost |
| stopping power of carbon ions | energy loss for carbon ions much higher than photons electron ints dominate most of track near end of range, charge pickup occurs (reduction in ion charge and interactions), at end of range, nuclear scattering becomes more dominant |
| lateral beam spread of carbon ions | less lateral spread than lighter protons due to heavier mass (smaller lateral deflections from nuclei) |
| Why to use protons | o Fewer radiation side effects o Minimise dose to healthy tissue, less integral dose o Potential reduction of inducing secondary cancers, particularly for paediatric pts |
| explain spot scanning again | each energy layer is delivered in sequence. within each layer, steering system moves beam to each xy spot position and delivers req dose (quantified in MU) to each spot. TPS defines the locations of all spots |
| clinical uncertainties affecting BP location | beam energy, pt positioning / shift, inherent ct uncertainties, ct artefacts, variations in pt anatomy, distal end RBE enhancements |
| advantages of protons | ✓ No dose past the target ✓ Highly conformal ✓ Charged particles ✓ Dose peaks at desired depth ✓ No exit dose ✓ Low Entry dose |
| what is the range shifter in protons | preabsorber of tissue equivalent energy, like bolus |
| single field optimisation in protons | when the spot pos and weights of each field are optimised individually |
| MFO/IMPT | where spots from all fields are optimised together -> highly conformal distribution |
| deliverable photon energy and depth | 70-230meV (4-25cm) |
| what is tomotherapy | CT guided helical IMRT (couch moving while gantry rotates) Slip ring gantry = 360 degree rotation FFF 6MV, dose rate 1180mu/min 85cm bore and 40cm diameter FOV MLC window width (sup/inf) = 1, 2.5 or 5cm max txt width = 40cm, length = 135cm |
| Effect of tomotherapy | 1. in a narrow rotating beam 2. with high-speed MLCs 3. from multiple angles around the Target We are able to “bend” the dose to conform tightly to the PTV or avoid nearby critical structures. |
| MLCs in tomotheraphy | MLCs can change every 7 degrees = 51 dynamic arc segments/ projections in one arc Maximum 64 beamlets per projection, therefore up to 3,648 beamlets available per rotation Leaves act alone - 10cm thick, open/close time = 20ms Width = 6.25mm at iso |
| Bunker cons in tomotherapy | Noisy - 78dB Cold room No applicators/heavy equipment No gantry collision hazards Max couch weight = 200kg |
| Machine cons of radixact | Larger machine, taller couch (pt mobility), touch screens, shorter useable couch length Couch catcher reduces sag and increases weight limit across couch travel |
| dose of kvct per mvct | MVCT: Mean Energy = 1MV kVCT: 120-140Kv 192-480mAs MVCT: ~2.5 cGy per scan kVCT: Approx 1cGy (~20 slices) |
| differentiate kvct and mvct | mvct loses distinction b/w soft tissue and bone but improved imaging on high atomic number materials kv = quicker acquisiton |
| how is roll managed in tomo | roll needs to be consistent as entire treatment delivery is offset starting gantry angle ie adjusted |
| shift limits on tomo | 3 degrees and +-25mm (lateral) |
| image matching process on tomo | 1. Sagittal: Check for pitch Sup-inf and ant-post 2. Coronal: Check for yaw Left-right Don’t focus on LR too much here as pitch can make it look worse 3. Transverse: Check for roll Make final adjustments CHECKERBOARD |
| Discuss weight loss/vol changes on tomo | weight loss common - nursing, dietician and speech path support (PEG) Foam can be added Plan adaptive requested Increase in side effect uptake and tumour size reduction due to immuno/chemo combos |
| Tomo contouring | RO: GTV, CTV, PTV Optic Chiasm, Brainstem, Pituitary, Cochlea, Lacrimal, Pharyngeal Constrictors, Larynx and/or Trachea, Salivary Glands, Brachial Plexus Shoulder blocks avoid entry through as not most efficient path length and variable daily pos |
| What are beamlets | (Individual no of MLCs that open/close) • Set number of beamlets available Tomo assigns no of beamlets to each structure When constraints achieved, it will divide remaining beamlets over any remaining structures that are not achieving constraints |
| How do beamlets act on structures close togwether | Decreased control over dose as beamlets from one structure can be affected by another |
| Tomo planning aims | • Achieve PTV coverage • Reduce OAR dose • Maintain reasonable txt time • aAdjustments to improve plan quality will increase your treatment time – these need to be balanced by the planner • H&N = ~4-6mins TNI/CNS = ~14mins |
| Field width in tomoo | Defined by primary collimator jaws - 1, 2.5, 5. Selected according txt volume length and variation in target sup/inf Smaller widths improve dose modulation in long direction, increase txt time Larger = extended penumbra, less precision of isodose |
| Pitch on tomo | Couch travel distance per rotation of the gantry divided by the field width Tighter pitch = inc number of rotations that treat over a target length = more beamlets avail < .100 means sup/inf edges of beam overlap Pitch = 0.86/integer to avoid threading |
| What is accelerated treatment on tomo | Limit of range of leaf intensity values ie: amount MLCs remain open Parameter improves efficiency by limiting longest/shortest leaf opening times -> reduces txt time Reducing =potentially better coverage, slower gantry rot, potentially inc hotspot |
| What are blocks in tomo | Complete blocks allow primary beams to pass through if they pass through a target first (e.g. shoulders) Complete blocks stop all primary beams passing through (e.g. eyes, contra lung) -> INC treatment time |
| What is the gantry period in tomo | Indicates the quality of the plan/gantry rotation speed Optimal gantry period - 20 sec Higher dose per frac - slower gantry period to deliver more dose |
| What can't you change once planning begins on tomo | Field width and pitch |
| Discuss final calc in tomo | Final calc process will ‘dump’ illegal beamlets – differences between dose distribution between optimised and final calc There is a setting to limit 'illegal - ness' of beamlets -> slightly longer calcs but means final calc is much closer |
| DQA for tomo | Ion Chamber measured in homo dose region (≤ 0.5Gy tol) Film evaluates geometric conformity Ion chamber/film in phantom (ideally in highest dose PTV) Phantom receives fx Optical density of film measured Failed may =replan – reasons not always clear |
| What is a tomo plan adaptive and options | Evaluating changes in external contour Requested by treatment staff Rigid reg = recalc on daily CBCT Proj dose rev assuming no further changes |
| Challenges of plan adaptive | Metal rtefacts and If FOV too small, we are not getting accurate representation in plan adaptive |
| Describe synchrony | Combines MLC and JAW tracking The field opening does not change size, just adjusts location Adapt to motion, not shape |
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jrichardss