Session Item

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 (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 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.