Indrawati Hadi1, Rieke von Bestenbostel1, Stephan Schönecker1, Katrin Straub1, Lukas Nierer1, Raphael Bodensohn1, Michael Reiner1, Guillaume Landry1, Claus Belka1,2, Maximilian Niyazi1,2, Stefanie Corradini1
1LMU Munich University Hospital, Radiation Oncology, Munich, Germany; 2German Cancer Consortium (DKTK), Radiation Oncology, Munich, Germany
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.
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.
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/m2 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 EQD210 71.3Gy (69.3 – 83.9Gy10). An optimized PTV (PTVopt) was obtained from subtracting organs at risk (OAR) with a 3mm isotropic expansion from the PTV. Median PTVopt was 43.5ccm (24.2 – 131.35ccm). Using OAR constraints of BT, a cumulative median EQD23 for the rectum D2ccm was 63.7 Gy3 (51.5 – 72.6Gy3); bladder D2ccm 72.2Gy3 (67.1 – 83.6Gy3); sigmoid D2ccm 45.7Gy3 (43.2 – 58.7Gy3); bowel D2ccm 59Gy3 (47.7 – 70.0Gy3). 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.
These early results report the feasibility of an MR-guided SBRT boost approach in patients with LA(R)GC who were not candidates for BT. When classical BT-OAR constraints are followed, acute toxicity was favorable. However, long-term follow-up is needed to validate the results.