ESTRO 2021 Abstract Book

S868

ESTRO 2021

Conclusion From our group of patients we discovered that more than two brain metastatic lesions are associated with a worse prognosis in terms of local recurrence (BPFS). This would suggest SRS as a proper option, related to an appropriate treatment of the lesions, and an adequate control of further dissemination in the brain, delaying the use of WBI and its toxicity. PO-1041 Skull base chordomas and chondrosarcomas: Trento Protontherapy Centre early experience M. Cianchetti 1 , D. Amelio 2 , B. Rombi 2 , S. Vennarini 2 , D. Scartoni 2 , I. Giacomelli 2 , A. Turkaj 2 , F. Dionisi 2 , M. Amichetti 2 1 Azienda Provinciale per i Servizi Sanitari, Unità Operativa di Protonterapia, Trento , Italy; 2 Azienda Provinciale per i Servizi Sanitari, Unità Operativa di Protonterapia, Trento, Italy Purpose or Objective Skull base chordomas (C) and chondrosarcomas (CS) are locally aggressive tumors located adjacent to critical structures. Radiotherapy is used both as an adjuvant treatment to surgery and a radical one when the first is not feasible. The aim of our retrospective study is to report our experience with skull base chordomas and chondrosarcomas treated at Trento Proton Therapy in terms of toxicity profile and clinical outcome Materials and Methods Between June 2015 and November 2020, 51 patients (pts) with skull base C and CS were irradiated with proton therapy (PT) at our institution. Mean age was 48.7 years (range: 3.7 -83.1). There were 44 (96.4%) C, and 7 (3.6%) CS. Tumors were located in clivus n= 46, petroclival region n=2 and sellar region n=3. Two lesions were only biopsied and treated with exclusive PT. Forty nine lesions had been treated with≥1 surgery; 41 cases had gross residual disease at the beginning of PT, 8 cases were irradiated without gross disease. Five cases were re-irradiation, 3 of them after photons (total dose range 16-54 Gy) and two after hadrons (70,4-74 GyRBE). Registered side effects were graded according to CTCAE5.0. Results PT was well tolerated, all but two pts completed their treatment (one didn't due to distant disease progression and one to general conditions decline). Mean overall treatment time was 50.2 days (range: 36-58) Technique was conventional in 36 cases, Simultaneous Integrated Boost in 2 and mixed in 13. All pts were treated with active beam scanning PT. Single field optimizations was used for 35 pts, multifield optimization for 15 and mixed for 1. Mean high-risk (HR) PTV volume was 63.4 cc(range 2.2-335.8); mean low-risk (LR) PTV volume was 101.3 cc(range19,4 - 266,9). Mean prescribed total dose was 70.0 GyRBE (range 54-74 GyRBE) for HR PTV and 54.2 GyRBE (range 50-60 GyRBE) for LR PTV. No acute or late ≥ Grade (G)3 side effects were reported. Acute G3 toxicity occurred 6 times, late G3 toxicity 15. Median follow-up was 25.7 (range 1.3- 63.6) months. All the 8 pts treated without gross disease are alive without any local or distant relapse. Of the 43 pts with gross disease: 31 are alive with local controlled disease, 3 died of other causes; 3 died of local progression; 3 are alive with local disease in progression; 1 is alive with local and distant disease controlled; 1 died with local and distant disease controlled. Local relapses were 8 in total occurring between 4.53 and 53.8 months (mean: 26.5 months), in 3 cases associated with distant ones. Conclusion PT allows to deliver high dose of radiation therapy in a very critical setting. Early results are encouraging but a longer follow-up is obviously needed to gain more robust data for late toxicity and long term disease control. PO-1042 Dose-response-toxicity relationship to treatment in patients treated with Cerebral SBRT. L. Sonera-Marcos 1 , R.P. PEDRO 2 , M. Elena 3 , M. David M 4 1 Hospital Universitario Virgen Del Rocio, HUVR Radiation Oncology Service, Seville, Spain; 2 hospital Universitario Virgen Del Rocio, Huvr Radiation Oncology Service, Seville, Spain; 3 hospital Universitario Virgen Del Rocío, Huvr Radiation Oncology Service, Seville, Spain; 4 hospital Universitario Virgen Del Rocio, Huvr Radiation Oncology Service, Seville, Spain

Purpose or Objective