ESTRO 36 Abstract Book

S112 ESTRO 36 _______________________________________________________________________________________________

SP-0224 Brachytherapy for bladder/prostate rhabdomyosarcoma: clinical outcome and functional results C. Chargari 1 , H. Martelli 2 , F. Guérin 2 , R. Mazeron 1 , V. Minard-Colin 3 , E. Deutsch 1 , C. Haie-Meder 4 1 Institut Gustave Roussy, Brachytherapy Unit, Villejuif, France 2 Kremlin Bicêtre University Hospital, Pediatric Surgery, Kremlin Bicêtre, France 3 Institut Gustave Roussy, Pediatric oncology, Villejuif, France 4 Institut Gustave Roussy, Radiotherapy Department, Villejuif, France Historically, the standard treatment of children with bladder and/or prostate rhabdomyosarcoma (BP RMS) was based on total cystectomy or cysto-prostatectomy. The severe urinary and sexual sequelae of this radical surgical approach have prompted collaborative groups to look at alternative strategies, based on a multimodal conservative approach combining chemotherapeutic agents with radiation therapy. Although the probability of long-term survival with bladder preservation has improved with multimodal approaches, the risk of late gastrointestinal and genitourinary toxicities remains a major issue in children undergoing pelvic external beam radiotherapy (EBRT). Brachytherapy has been used in our center as part of the multimodal treatment of patients with BP RMS, in an effort to minimize sequelae. We report the results of an original conservative strategy based on surgery combined with brachytherapy. The outcome of children treated in our department between 1991 and 2015 for a BP RMS and undergoing a multimodal approach combining a conservative surgery (partial cystectomy and/or partial prostatectomy) and a perioperative interstitial low-dose rate or pulse-dose rate brachytherapy was prospectively documented. Prior to brachytherapy, children had received chemotherapy with modalities depending on their risk group of treatment. A total of 100 patients were treated, median age of 28 months (5.6 months-14 years). According to the Intergroup Rhabdomyosarcoma Study (IRS) Group group, 84 were IRS- III and 12 were IRS-IV tumors. Four patients were treated at relapse. Median number of chemotherapy cycles before local therapy was 6 (4–13). After surgery, 63 patients had a macroscopical tumor residuum. Five patients underwent a brachytherapy boost before pelvic external beam radiotherapy (EBRT) because of nodal involvement and 95 had exclusive brachytherapy. Median follow-up was 64 months (6 months-24.5 years). Five year disease-free and overall survival rates were 84% (95%CI: 80–88%) and 91% (95%CI: 87–95%), respectively. At last follow-up, most survivors presented with only mild to moderate genito- urinary sequelae and a normal diurnal urinary continence. Five patients required a secondary total cystectomy: 3 for a nonfunctional bladder and 2 for relapse. A specific analysis of the urinary outcome of patients treated according to this strategy showed that 75% of long-term male survivors considered they had a normal quality of life after the combined conservative local treatment of their BP RMS. Therefore, brachytherapy is effective as part of a conservative strategy on BP RMS, with a relatively low delayed toxicity as compared with previously published studies using EBRT. Longer follow-up is required to ensure that the functional results are maintained over time. SP-0225 Intraoperative HDR brachytherapy for pediatric cancers S. Wolden 1 1 Memorial Sloan Kettering Cancer Center, New York- NY, USA Brachytherapy remains the most conformal technique for delivering therapeutic radiation, making it an ideal option for pediatric patients. The normal tissue sparing is even

superior to proton therapy. I will discuss high dose rate (HDR) intraoperative brachytherapy (IORT) as a specific technique for the treatment of pediatric tumors. IORT may be used as an adjunct or even in place of external beam radiotherapy to maximize local control while minimizing normal tissue complications. We will review various IORT techniques as well as specific indications. Long term outcomes of relatively large series of children with neuroblastoma and pediatric sarcomas will be reviewed. A brief overview of other unique forms of brachytherapy for rare childhood tumors will also be presented. OC-0226 Towards consistency of TPS dose calculations: converting dose to medium to dose to water N. Reynaert 1 , F. Crop 1 , E. Sterpin 2 , H. Palmans 3 1 Centre Oscar Lambret, PHYSIQUE MEDICALE, Lille, France 2 Katholieke Universiteit Leuven, Department of Oncology- Laboratory of Experimental Radiotherapy, Leuven, Belgium 3 National Physical Laboratory, Radiation Dosimetry, Teddington, United Kingdom Purpose or Objective The aim of the current work is to demonstrate that conversion factors between dose to medium and dose to water calculated by different treatment planning systems for photon beams should be based on mass energy attenuation coefficients and that stopping power ratios should not be considered. Material and Methods A theoretical explanation is introduced establishing the inadequacy of stopping power values when converting dose to medium to dose to water (when considering TPS dose calculations). Monte Carlo calculations (EGSnrc) are performed in a simple bone phantom, validating the theoretical model. A bone slab is modeled and calculations are performed in bone and in water having the same electron density as bone. Special attention was paid on the importance and the range of the interface effects. Calculations were performed for 6 and 20 MV photons beams and 6-18 MeV electron beams are also considered. Results The Monte Carlo simulations clearly confirm the theoretical model. In the framework of TPS reporting/prescription, the dose to medium to dose to water conversion problem cannot be considered as a cavity problem as the composition of all voxels is modified simultaneously, leading to large electron fluence differences. For photon beams, the secondary electron fluence is modified by two effects. On one hand, fewer electrons are generated in bone because of the lower attenuation coefficients compared to water with the same electron density, which tends to increase the secondary electron fluence in water compared to bone. On the other hand, the range of these secondary electrons is larger in bone than in water with bone density which leads to an inverse effect. The first effect is defined by the ratio of mass attenuation coefficients; while the second by the ratio of stopping powers which is compensating the stopping power ratio present in the formal equation of the ratio of dose to water to dose to medium. Only at Proffered Papers: Dose measurement and dose calculations