A Phase II Trial of Stereotactic Ablative Radiation Therapy as a Boost for Locally Advanced Cervical Cancer

Purpose: Our purpose was to assess the feasibility, safety, and efficacy of stereotactic ablative radiation therapy (SAbR) as an alternative for intracavitary/interstitial brachytherapy boost for locally advanced cervical cancer (LACC) after initial chemoradiation. Methods and Materials: A single arm institutional phase II study of SAbR as a boost for LACC was conducted. Eligible patients had LACC FIGO 2009 stage IB2-IVB, performance status 0 to 3, and one of the following: medically unfit or refused intracavitary or tumor extent required interstitial brachytherapy for coverage. The cervix planning target volume boost (PTV_boost) received 28 Gy in 4 fractions. Results: The study was closed with 15 of 21 patients completed owing to concern for toxicity. Median follow-up for this cohort was 19 months. Patients had predominantly advanced stage (III-IV, 53%) with median Charlson comorbidity score of 4. Most tumors were large with a median SAbR boost PTV size of 139 cc (range, 51-268 cc). Tumor size and patient comorbidities probably contributed to the lower-than-expected 2-year local control, progression free, and overall survival of 70.1%, 46.7%, and 53.3%, respectively. The SAbR boost 2 year cumulative grade ≥ 3 toxicity of 26.7% was predominantly rectal (ulcer/fistula).The median SAbR PTV volume was 225 cc versus 95 cc for patients with and without grade ≥ 3 toxicity. On dosimetric analysis, only the percentage of rectal circumference receiving 15 Gy (PRC₁₅) for the SAbR boost was associated with development of grade 3 ulcer or rectovaginal fistula (P = .04), with PRC₁₅ > 62.7% being the strongest predictor of toxicity (AUC, 0.93; sensitivity, 100%; specificity, 90%). Conclusions: In this SAbR boost series suboptimal outcomes were probably related to patient selection and very large tumor volume. This approach may still be considered in patients with smaller tumors unable to undergo standard brachytherapy for cervix cancer.