ESTRO 37 Abstract book

S1064

ESTRO 37

Conclusion The knowledge-based optimisation process for the planning of anal canal cancer resulted in a significantly improved plan quality compared to the experienced planner approach. This may also lead to large efficiency gains in therapy planning. EP-1956 Prostrating ourselves to the model: A comparison of anatomy specific prostate DVH estimation models M. Anderson 1 , S. Currie 1 , L. Girvan 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective DVH estimation models have already been found to lead to improvement in plan quality for prostate EBRT [1]. It has previously been found that model outliers have minimal effect on plan quality [2], however, the effect from the range of plans included has not yet been explored. A key OAR in prostate EBRT is the bladder, an organ capable of significant size variation. The aim of this study was to investigate the effect of using different DVH estimation models based on bladder size on plan quality. Material and Methods Three RapidPlan™ (Varian, Palo Alto, USA) DVH estimation models were created, a non-specific bladder size model (A) (n = 149), followed by a small bladder model from which study-sets with bladders over 150 cm 3 had been removed (B) (n = 73), and a large bladder model from which study-sets with bladders smaller than 200 cm 3 had been removed (C) (n = 61). One EBRT plan was produced for each model using the Eclipse TPS (Varian, Palo Alto, USA) for twenty patient study-sets unknown to the models, which had been sorted into a large bladder (>200 cm 3 ) group and a small bladder (<150 cm 3 ) group. The plans were compared for target coverage, OAR dose, plan modulation, and calculation time. Statistical significance was tested for using a t-test with a p value ≤ 0.05 indicating significance. Results No significant difference was found for any recorded metric except for total calculation time, with the bladder size specific models taking significantly longer than the opposing bladder size model (p = 0.002 for both cases). Primary target coverage was investigated in relation to mean bladder and rectum doses with no significant relationship found. While not significant, a general trend was observed in that using Model C on the small bladder test patients tended to produce plans with lower target coverage, and vice versa. Conclusion These results indicate that there is no clinical benefit in using a bladder size specific DVH estimation model for the planning of prostate EBRT treatments. The results also suggest the possibility of using more time to produce a plan of the same quality when the right model is used, or a lesser quality plan if the wrong model is used. As the bladder size specific models were based on the original, non bladder size specific model, the results also suggest that plan quality for prostate plans does not increase with increasing size of DVH estimation model. [1] Fogliata A, et al. On the pre-clinical validation of a commercial model-based optimisation engine: Application to volumetric modulated arc therapy for patients with lung or prostate cancer. Radiother Oncol 2014; 113: 385-391 [2] Delaney A, et al. Effect of dosimetric outliers on the performance of a commercial knowledge-based planning solution. Int J Radiat Oncol Biol Phys 2016; 94 (3): 469- 477

volumetric intensity-modulated arc treatment (VMAT, Varian); dose was delivered with two/three arcs. Plan- specific QA check (using ion-chamber and ArcCheck) was done. Results Volume of the entire PTV structure was 300cc±80cc. For the dose-painting techniques volumes of PTV 25, 20, 15, and 10 were 30cc±15cc, 120cc±70, 150cc±100 and 30cc±12 respectively. The duodenum touched the celiac- plexus PTV in all cases. Dose coverage of the celiac-plexus (GTV) was significantly higher for the uniform-dose technique, D95 20Gy±4 vs 14Gy±4 (p<0.0012). Regarding normal tissue constraints: The small bowel volume that received above 11Gy was significantly higher for the uniform-dose plans 103cc±61 vs 4.2cc±3.2 (p<0.0086). The kidneys mean dose were 6Gy±0.6 vs 5.2Gy±0.4 (p<0.0005) respectively. The max dose to the spine was similar in both techniques: 10.6Gy±1.5 vs 10.4Gy±0.4 (p<0.778) respectively. The dose to 700cc of the liver was 3.97Gy±1.2 vs 3.5Gy±1.2 (p<0.0081)respectively. Clinical treatments were planned and delivered using the dose-painting technique. Pain relief was excellent and side-effects minimal. Conclusion For single-fraction radiosurgery to the celiac plexus, the dose-painting technique ensures rapid high-quality plans that meet normal-tissue constraints. Conversely, the uniform-dose technique ensured good GTV coverage but was unable to generate plans that respected normal tissue tolerance constraints, especially of the bowel. EP-1955 Evaluation of a knowledge-based model for VMAT planning of anal canal cancer S. Puccini 1 , D. Völzke 2 1 Strahlentherapie Bonn-Rhein-Sieg, Medical Physics, Bonn, Germany 2 Strahlentherapie Bonn-Rhein-Sieg, Radiation Oncology, Bonn, Germany Purpose or Objective To compare the performance of a knowledge-based optimisation process with the treatment planning process driven by experienced planners. Material and Methods A commercial system for knowledge-based radiotherapy planning, RapidPlan (RP), was employed to generate a DVH estimation model for the treatment of anal canal cancer. Sixty previously clinical accepted cases delivering 50,4 Gy in 28 fractions were used to train the model. The target volume was delineated according to the RTOG Consensus Panel Contouring Atlas for Anorectal Cancer, organs at risk included in the model were bladder, bowel, genitals and hip joints. Thirty new cases were included in the comparison. For each new patient an experienced planner (EP) individually chose the field geometry and the optimisation constraints. The planner was free to use as many dummy structures and as many iterations as needed to achieve a good plan. With the same field geometry a RP-plan was calculated, no intervention was allowed during the RP-optimisation process. Results Both EP-plans and RP-plans provided a good dose coverage of the target volume. Little difference was observed in the homogeneity index (between 0.07 and 0.10 for both optimisation techniques) and in maximum as well as in the near-to-maximum dose. The RP-plans allowed a significantly better sparing of the organ at risks, particularly of bladder, bowel and genitals, compared to the EP-plans. The reduction of the mean dose was of 3 Gy for both bladder and bowel and of 4 Gy for the genitals.