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093 Planning Refined
| Question | Answer |
|---|---|
| Define biological optimisation | a treatment planning approach that uses radiobiological models to maximize tumor kill while minimizing damage to healthy tissues. It tailors radiation delivery based on cell survival metrics, rather than relying solely on physical dose limits |
| List items in a common plan evaluation checklist | correct pt, machine, site, px (total dose, fractionation, daily dose), beam type and energy OARs met, iso shifts, MLC shielding, density overrides, bolus, target volume and field size correlate, DRR generated to correct iso |
| Describe the ways that plan evaluation / checking is mandated. | departmental protocol (appropriate CT data, protocol for contouring/beams/dose, score cards and goal sheet) + literature/evidence ICRU, RTOG, QUANTEC |
| Define modulation | The process of varying one or more properties of a beam. (MAINLY MLCs). VMAT = MLCs synchronised with gantry rotation |
| What is modulation factor | INDEX THAT EXPRESSES COMPLEXICITY OF MLC MOTION a small value = delivery time shortens; however, a small MF value results in poorer dose distribution. necessary to set MF with a balance of delivery time and dose distribution Depends on site etc |
| What is modulation index | ▪ In Helical Tomo Planning, the user sets a value (1.0–5.0) as MF in the design of a plan ▪ MI (Modulation Index): The modulation of the beam fluence, a low MI value is associated with a beam with low complexity. |
| What is modulation complexity score | ▪ Take into account the relative variability on leaf positions, the area of the beam opening and the numbers of MU (1). ▪ A high value of the MCS is associated with a beam with low complexity |
| When might VMAT be used in breast | - Breast/chest wall with multiple dose levels, OR - Breast/chest wall plus nodes, OR - Large breast patients not suitable for IMRT Phase 2 boost |
| Discuss why historical approaches to whole pelvis planning were limited. Why is VMAT SIB preferred? | 4 field box. small bowel may fall into empty space in the true pelvis -> increased risk of acute and late GI complications we also need to deliver dose to paracervical/nodal tissues which are at highest risk of recurrence - unfeasible with 3DCRT |
| List the SIB fractionation for prostate + nodes. | 74-78Gy to prostate, 45-50.4 to LNs |
| State the IMRT beam arrangement for WPRT. | 7-9 beams with gantry of 45, 90, 115, 180, 245, 280,. 320 |
| Discuss collimator and field size considerations in WPRT. | 10-35, can use sup-inf leaf travel to shield bowel and exploit sup-inf modulation space - however be weary of increased irradiation of the bowel USE JAWS AS MAX |
| State the role of EXP Ext 1cm structure. | Used to assess weight gain with CBCT |
| List match points for SABR brain image fusion. | Clivus, internal carotid arteries, craniotomies, temporal lobes, sinuses, sulci |
| List dose fractionations for brain SABR. | 18-24Gy/1, 24/3, 27/3, 25/5, 27.5/5, 30/5 |
| Explain RMS in a quadratic overdose function | rms = dose tolerance . structure is divided into voxels and excess of each voxel is averaged - if this is greater than RMS, the optimiser must try harder to reduce |
| MF formula | max open time/average open time |
| target EUD cell sensitivity | improves uniformity cell sensitivity default is 0.5 -> higher CS increases penalty for cold spots, increase pressure to deliver |
| describe serial CF | preferred for serial OARs, applies large penalties for hot spots (even small vol) biological equivalent of a max dose - isoconstraint is EUD k 1-20 (20 working hard at tail) |
| formula for serial k value | 0.15 x d50 |
| parallel CF | used for dose volume constraints reference dose, isoconstraint = mean organ damage (biological equivalent of how much of the vol can be sacrificed) k 1-4 (4 works hard at ROI on DVH, 1 applies penalty at low dose and over most of the curve) |
| conformality CF | physical CF shape HDV around target vol/s works well fopr single volumes but not complex/overlapping targets 0-1 (0.01 increments). 1=relaxed distance 4-8cm from edge of target does not stop dose dumping in outer NTT (quad OD needed) |
| How are shrink margins used | Extremely useful when multiple targets are being optimized to transition between a high dose and a low dose. Allows a transition zone between a high dose target and an overlapping or adjacent OAR. -> can be used in place of rings |
| Fluence smoothing | controls smoothing of fluence in stage 1. higher smoothing decreases plan quality and control points off = many segments, low = more segments (complex plans), medium = average number os segments high = few |
| describe segment shape optimisation | enabled by default. improves plan quality and deliverablity. allows for areas of high and low modulation to better meet IMRT constraints by smoothing, sequencing and optimising weights/shapes range 1-20 |
| how does SSO affect optimisation time, plan quality, delivery time, segment number, MU | inc opt time and plan qual. dec delivery time and segment number. may inc MU |
| sweep sequencing | designed to inc opt space. leaves move from start to end pos in a continuous, unidirectional manner length = determined by sector first sector left to right of BEV, second sector back to right side of BEV min width of end segments = 5mm |
| Intact vs post-op brain features | intact: spherical + smaller, deep-seated, dose 130-135%, larger dose per fx post-op: bulkier and irregular, peripheries, 110-115%, smaller dose per fx |
| intact vs post-op voluming | intact: GTV + 1.5mm = PTV post-op: HRTV + GTV. GTV +2mm=CTV. CTV + 1.5mm = PTV. For intact lesions, CTV is not as important -> less risk of microscopic disease remaining and spread of microscopic disease |
| Pareto vs constrained | pareto = prioritises underdoses on targets, relaxes NTT/OAR constraints -> navigates the trade-off space to optimize NTT without compromising the target constrained = constraints on NTT/OAR met, then remaining effort to covering targets |
| what are pareto optimal /MCO plans | plans in which no objective can be improved without worsening another. plans that satisfy this = pareto optimal. MCO eliminates non-pareto optimal plans and generates a discrete representation of pareto surface - multiple plans created and selected |
| MCO explained | optimiser tries to achieve an even lower dose to OAR whilst still meeting target objective. works best when target and OAR are separated and target coverage is not significantly affected by tighter OAR constraints. can affect homogeneity |
| Explain a bias plan | two PTVs/isos/beam sets values / objectives for one plan involved in optimisation of the other |
| SABR brain evaluation | Hotspot in GTV/HRTV or CTV if intact Uniform dose drop-off Max dose relative to prescribed isodose (24/0.8=30) If ROIs far and under, leave out of optimiser Drive brain, temp lobes and hippocampus ALARA |
| SABR brain arrangement | always start with non-cp (usually full arc) -> allows more control of low dose mohawk arc can irradiate a lot of vital central structures if we dont need another arc, dont use it -> trade off for NTT sparing and patient cons NCP REDUCES R50 |
| SABR lung arrangement | Aim to minimise the amount of lung in both entry and exit paths of arcs, avoid full arcs to reduce combined lung mean and low dose wash. 200 usually required to achieve dose fall off to meet R50/D2cm constraints (longer arcs allow more isotropic fall off) |
| When might a PTV expansion be used in SABR brain | for irregular or overlapping PTVs, an A/P and L/R expansion may be useful |
| SABR lung px goals | D100 of ITV>px dose D95 PTV > px dose D99 PTV >90% px dose |
| flash troubleshooting | if close: - monitor daily on pre txt imaging - review pt set-up, iso shifts - liase with planning RT if outside: - do not txt - re set up? - consult RO + planning RT (rescan, cease until replan?) |
| explain dose fall off for single targets | works from target edge penalises voxels within ROI where there is no overlap with target voxels met when all voxel doses within the specified ROI are less than/equal to respective max dose levels |
| explain dose fall off for multiple targets | 'adapt to target dose levels' high dose level is reduced around targets with lower dose and adjusted relative to the highest dose level of each target lower dose distance is rescaled so that slope of fall off region is maintained |
| examples of dose fall off | 95% -> 50% over 1.5-2cm 95% -> 0% over 4-5cm |
| Advantages and pitfalls of dose fall-off function | Dose fall-off function can be used for parallel structures + good for spinal cord dose The more irregular the target volume gets, the less effective - still need standard ring structures |
| dose fall off vs ring | ring: penalises close to target, retain unness. dose outside ring, req multiple struc for flexibility, easier to select weights FO: no new ROIs, penalises entire outline, non-constant dose levels=flexibility, hard to understand penalty/select weights |
| opt_robust structure |
Created by:
jrichardss