ESTRO 2021 Abstract Book

S778

ESTRO 2021

PD-0935 The evaluation of a spatially fractionated radiotherapy technique for large tissue sarcomas G. Borzov 1 , A. Nevelsky 1 , R. Bar-Deroma 1 , S. Billan 1 , M. Lutsyk 1 1 Rambam HealthCare Campus, Oncology, Haifa, Israel Purpose or Objective The promising clinical results of Spatially Fractionated GRID Radiation therapy (SFGRT) have caused a renewed interest in this field. The objective of this work is to evaluate the safety and feasibility of SFGRT for large soft tissue sarcoma in neoadjuvant radiotherapy. Materials and Methods Israel Ministry of Health ethical committee approval was obtained prior to study initiation. The treatment part of this study consisted of two RT courses: the conventional one with 25 fractions of 2 Gy delivered to PTV and the investigational SFGRT one with a single fraction of 20 Gy delivered to high dose nuclei (HDN) regions. HDN shaped as cylinders with a 1 cm diameter and 1 cm height were placed within the GTV. The number of HDNs and their position were determined by the physicist and the physician based on tumor size, shape and proximity to OARs. Also, the number of HDNs was limited by an acceptable dose fall-off criteria determined as a 5-6 Gy isodose region locked inside the CTV structure. Both courses of RT were planned using Monaco TPS with VMAT technique and delivered with an Elekta Versa HD linear accelerator. For each fraction patient positioning setup was performed using the HexaPOD 6D system based on CBCT data. In order to shorten the beam-on time the FFF modality was chosen for the delivery of the single fraction. QA measurements were performed using the Delta4+ phantom. The patients were immobilized using a VacLock cradle and an IGRT procedure was performed before each treatment. For SFGRT, IGRT procedure consisted of two CBCT prior to treatment and one post treatment. Results Two patients with large (202 and 181 cc) leg sarcomas were irradiated using the SFGRT technique. The number of HDNs was 8 and 7 respectively to the GTV volume. Nerve vessel bundles, contralateral leg and health tissues met dose constraints taking into account the doses from both courses of treatment. QA results showed a gamma-index higher than 95% for a 3%/2mm criterion. As a result of the geometric complexity and steep dose gradients, the investigational treatment plans consisted of a large amount of MU: about of 9 and 13 thousands. However, the overall treatment time including IGRT was less than 20 minutes in both cases. At the moment, one patient continues to receive the conventional part of the treatment while the second one is awaiting for a tumor resection procedure. Based on the regularly performed CBCT we can preliminary conclude that a significant and quick tumor volume reduction was observed in both cases. No complaints were received from patients.

Fig. 1. Dose distribution for a SFGRT plan.