ESTRO 2020 Abstract Book

S49 ESTRO 2020

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. 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).

of radiation-induced lung cancer. Reducing dose to the lungs during treatment planning for external beam APBI mitigates this risk, but also results in higher doses to the ipsilateral breast if target coverage and dose to the contralateral breast are kept constant. This could lead to a higher risk of breast fibrosis. Our purpose is to quantify the trade-off between secondary lung cancer and fibrosis risks. Material and Methods We conducted a treatment planning study on 20 female patients eligible for APBI, comparing coplanar and non- coplanar techniques, namely VMAT and CyberKnife robotic radiotherapy. We created 11 Pareto-optimal plans per patient per technique using automated treatment planning with the same constraints but gradually shifting priority from maximum sparing of the lungs to maximum sparing of the breast tissue. The excess absolute risk of developing lung cancer was based on lung dose and calculated with the model described by Schneider et al. (2011) accounting for fractionation, repair and repopulation. The risk of breast fibrosis was calculated using the model from Avanzo et al. (2012) for APBI with complete repair. Results The dose parameters, secondary lung cancer risks and fibrosis risks are summarized in Table 1. Prioritizing lung sparing resulted in a substantial reduction of the mean lung dose to as low as 0.3 Gy, and a five-fold median reduction of the secondary lung cancer risk compared to prioritizing sparing of breast tissue. The associated increase in breast dose resulted in a very small absolute increase in fibrosis risk of 0.4%-point. Figure 1 shows the Pareto-fronts of all patients of the trade-off between mean lung dose and mean ipsilateral breast dose. The thick lines depict the means per technique. The use of non- coplanar beams created more planning flexibility compared to coplanar techniques, as shown by the wider 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.

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