ESTRO 2021 Abstract Book

S1076

ESTRO 2021

of field 34.6%) and 14/40 had either synchronous or metachronous metastasis (Lung-2, Liver-3, Bone-4, Others-5). In patients with metastasis, a total of 19 sites were treated with either intensity modulated or stereotactic RT. Overall only 15/26 patients with nodal relapse (60%) and 6/14 with visceral metastasis (40%) received systemic therapy (Carboplatin+Paclitaxel). The most common reason for omission of systemic chemotherapy was poor performance status, patient reluctance or nodal only relapse. None of the patients received anti VEGF or Immunotherapy. Median re-RT dose to nodal regions at relapse in EQD2 was 42.5 Gy (23.3Gy to 44.3 Gy). Median RT dose to oligometastasis was 31.3 Gy/5 fractions (23.3Gy to 42.3Gy). For those received in field reirradiation, the cumulative dose was 92.1 Gy ( 80.5 Gy to 100 Gy). The median PFS and OS after relapse was 8.5 months (4-16 months) and 18.5 months (13-26 mths) respectively. At the time of the present analysis only 42.5% patients had died suggesting expectation of further improvement in PFS and OS. Grade>II toxicity was observed in 12.5% of patients. Conclusion Use of potentially radical doses of radiation is associated with encouraging PFS and OS in patients with cervix cancer. Prospective clinical studies are needed to better understand and define the role of radiation in oligorecurrent or oligometastatic disease in combination with systemic therapy including immunotherapy. PO-1311 MR-guided SBRT boost for patients with recurrent gynecological cancers ineligible for brachytherapy I. Hadi 1 , R. von Bestenbostel 1 , S. Schönecker 1 , K. Straub 1 , L. Nierer 1 , R. Bodensohn 1 , M. Reiner 1 , G. Landry 1 , C. Belka 1,2 , M. Niyazi 1,2 , S. Corradini 1 1 LMU Munich University Hospital, Radiation Oncology, Munich, Germany; 2 German Cancer Consortium (DKTK), Radiation Oncology, Munich, Germany Purpose or Objective External beam radiotherapy (EBRT) with concurrent chemotherapy followed by a brachytherapy (BT) boost is the standard of care for most patients with locally advanced or recurrent gynecological cancer (LA(R)GC). However, not every patient is suitable for a BT boost due to the extent or localization of the tumor. Therefore, we investigated the feasibility of an MR-guided SBRT boost (MR-SBRT-boost) following EBRT. Materials and Methods Patients with LA(R)GC treated at our institution with a MR-SBRT-boost using a 0.35T hybrid MR-Linac (Viewray Inc., Mountain View, CA), were retrospectively analyzed. The patients were not suitable for BT due extensive infiltration of the pelvic wall (37.5%) and other adjacent organs (62.5%). Online-adaptive treatment planning was performed to control for interfractional anatomical changes. Treatment parameters and toxicity were evaluated to assess the feasibility of MR-SBRT-boost. Results Eight patients with recurrent cervical cancer (n=5; 62.5%), locally advanced cervical cancer and rectal infiltration (cT4) (n=1; 12.5%), as well as recurrent vaginal cancer (n=2; 25.0%) were treated between 03/2020 - 01/2021 (Fig. a-c) . EBRT of the primary tumor and the pelvic (62.5%)/paraaortic lymphatics (37.5%) was applied using a VMAT with a median dose of 1.8Gy/fraction (fx) (range: 1.8 – 2.2Gy) to a median total dose (TD) of 45.0Gy (45.0 – 55.0Gy). All patients received a concurrent Cisplatin 40mg/m 2 q1w. A simultaneous integrated boost (SIB) to positive lymph nodes was given in 3 patients with a TD of 55-57.5Gy in 25 fx. The MR- SBRT-boost was delivered every other day (q.a.d) with a median dose of 5.0Gy/fx (4.0 – 7.0Gy) to a median TD of 20.0Gy (8.0 – 28.0Gy). The median HR-CTV was 28.9ccm (12.9 – 111.75ccm) and the median cumulative TD of the combined EBRT+MR-boost of the HR-CTV was EQD2 10 71.3Gy (69.3 – 83.9Gy 10 ). An optimized PTV (PTV opt ) was obtained from subtracting organs at risk (OAR) with a 3mm isotropic expansion from the PTV. Median PTV opt was 43.5ccm (24.2 – 131.35ccm). Using OAR constraints of BT, a cumulative median EQD2 3 for the rectum D2ccm was 63.7 Gy 3 (51.5 – 72.6Gy 3 ); bladder D2ccm 72.2Gy 3 (67.1 – 83.6Gy 3 ); sigmoid D2ccm 45.7Gy 3 (43.2 – 58.7Gy 3 ); bowel D2ccm 59Gy 3 (47.7 – 70.0Gy 3 ). The median net beam on time/fx was 5.9 minutes (1.53 – 11.67 min), and the median overall treatment time (OTT)/fx was 79 min (44.0 – 89.5 min), including the adaptive workflow in 100% of fractions. The median OTT from the beginning of EBRT to the last SBRT fraction was 50 days (39 – 56 days). The most common side effects were diarrhea CTC°I-II (n= 4, 50.0%) and dysuria CTC°I-II (n= 5; 62.5%). No CTCAE ≥ °III and no worsening of acute side effects after the MR-SBRT- boost were observed.