ESTRO 2020 Abstract book

S53 ESTRO 2020

Pareto-fronts. The non-coplanar technique resulted in better plans in all cases (Wilcoxon signed rank test p < 0.001), with a lower secondary cancer risk and also a lower fibrosis risk. Incidentally, the mean heart dose was also reduced for the plans prioritizing lung sparing.

OC-0103 Clinical implementation of Knowledge-based (KB) planning optimization for Helical-Tomotherapy R. Castriconi 1 , C. Fiorino 1 , C. Cozzarini 2 , S. Broggi 1 , N. Di Muzio 2 , G.M. Cattaneo 1 , R. Calandrino 1 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milano, Italy ; 2 IRCCS San Raffaele Scientific Institute, Radiotherapy, Milano, Italy Purpose or Objective To implement KB-based automatic planning for Helical- Tomotherapy (Accuray Inc., HTT) by using a commercial software available out of the HTT planning station (TPS). Focus of the first clinical implementation was the case of high-risk prostate cancer, including pelvic nodes irradiation in a SIB approach. Material and Methods Our clinical protocol consists in delivering 74.2Gy to prostate and proximal seminal vesicles (PTV high ), 65.6Gy to cranial portion of seminal vesicles (PTV int ) and 51.8Gy to pelvic lymph nodes (PTV LN ) in 28 fractions. One-hundred- two HTT clinical plans were selected to train a KB-model using the RapidPlan (RP) tool implemented in the Eclipse TPS (v 13.6, Varian Inc.). RP is configured to model plans delivered with VMAT - RapidArc (RA) plans. Hence, all plans were exported from the HTT-TPS to Eclipse and linked to virtual RA-plans. The resulting KB-model was interactively fine-tuned in terms of statistical DVH- estimation and optimized template for the optimization, aiming at maximizing its robustness. Then, an internal (20 patients inside the model) and an external validation (30 new patients) were performed to assess the performances of the model. All automatic HTT-plans (KB-TP) were compared against the original plans (TP) in terms of OARs/PTVs dose-volume parameters. Wilcoxon-tests were performed to assess statistically significant differences (p < 0.05). To automatize the entire HTT-planning workflow, the individually optimized KB-based templates are converted in HTT-like template and sent automatically to the HTT-TPS through scripting. The individual template is then associated to the patient in the HTT-TPS and the full dose calculation is set after 300 iterations, without any additional planner intervention (Figure 1).

Results KB-TP plans were generally better than or equivalent to TP plans, in both validation cohorts (Figure 2). PTVs coverage were comparable for the internal sets, meanwhile PTV high and PTV int coverages were slightly improved for the external ones. Moreover, a significant improvement in PTVs and OVERLAP (between rectum and PTV high ) homogeneity were observed for both set. OARs sparing for KB-TP was slightly improved, more evidently in the external validation group. Of note, V 20Gy , V 40Gy and D max for the bladder were significantly better in KB-TP plans, V 20Gy and D mean for the bowel, as well as for V 68Gy and D max of the rectum. The automatic KB-based technique was successfully implemented in the clinical routine with a consequent large reduction of the planning time and better plan homogeneity, hopefully avoiding any risk of sub-optimal plans.

Conclusion The risk of secondary lung cancer of external beam APBI can be dramatically reduced by prioritizing lung sparing during treatment planning. The associated increase in breast dose does not lead to a relevant increase in fibrosis risk. Prioritizing lung sparing also reduced the mean heart dose. The use of non-coplanar beams systematically resulted in the lowest risks of secondary lung cancer and fibrosis. Based on our results, we recommend prioritizing lung sparing to reduce the mortality risk from secondary lung cancer and cardiovascular disease for patients treated with APBI. A non-coplanar beam set-up can reduce these risks even further compared to a coplanar beam set-up.