2388 - Dosimetric Evaluation of Adaptive Stereotactic Radiotherapy Boost for Locally Advanced Cervical Cancer

Purpose/Objective(s):

Definitive chemoradiation with pelvic external beam radiotherapy (EBRT) followed by a brachytherapy (BT) boost remains the standard of care for locally advanced cervical cancer (LACC). Stereotactic body radiotherapy (SBRT) has been explored as a substitute for BT boost in patients who are not candidates for BT, however due to interfraction motion, SBRT often results in higher dose to organs at risk (OAR).  We hypothesize that daily CT-guided online adaptive radiotherapy (OART) using the Ethos adaptive platform allows for more effective delivery of SBRT that better emulates the dose distribution of BT.   

Materials/

Methods:

This is a single-institution retrospective dosimetric analysis of patients with pathologically confirmed LACC, stages IB-IVA, treated with EBRT followed by BT boost from September 2016 through October 2023. We identified 12 patients who received a BT boost dose of 28Gy in 4 fractions following 45 Gy EBRT, and also had cone-beam CT (CBCT) imaging from their EBRT course capturing their entire cervix. We used each of the patients’ last 4 CBCT images to simulate the creation of 4 unique OART plans. OART plans were generated by contouring CTV-HR, bowel, sigmoid, rectum and bladder on each of the CBCTs.  A 3 mm PTV margin was added to CTV-HR.  These volumes were imported into the Ethos platform where 4 unique daily OART plans were generated. BT dosimetry data was extracted from our BT planning software. For both BT and OART plans, each OAR was considered to have received 45 Gy in 25 fractions during EBRT. The total EQD2 (a/b=3) to D 2.0 cc was calculated for each treatment course. The dosimetric goal for bowel and sigmoid were EQD2 <70 Gy, for rectum EQD2 <65 Gy and bladder EQD2 < 80 Gy. Statistical analysis was performed using a paired two-sided T test and α of 0.05.

Results:

Twelve patients with LACC who received pelvic EBRT followed by BT boost to 28 Gy in 4 fractions were analyzed for a total of 48 fractions.  OART plans achieved 100% prescription dose coverage to at least 90% of the CTV-HR . The mean EQD2 for each OAR and their p values can be found in Table 1. OART plans had a statistically significant lower dose to sigmoid and higher dose to rectum likely due to the absence of vaginal packing that is routinely placed at the time of BT.  Despite the higher dose to rectum, the mean EQD2 of 61.3 Gy was still below our goal of 65 Gy and only 2 patients exceeded this constraint (17%).  There were no significant differences in cumulative bowel or bladder dose.

Conclusion:

Daily online adaptive SBRT can achieve comparable dosimetric outcomes compared to BT by minimizing the impact of interfraction variability on both target coverage and OAR dose compared to non-adapted SBRT.  The use of daily OART to deliver SBRT in patients who are not BT candidates appears promising and further investigation is warranted.

Table 1