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Whole-pelvic radiotherapy in combination with intracavitary brachytherapy (ICBT) is the standard treatment for locally advanced cervical cancer. However, some patients are ineligible. Stereotactic body radiotherapy (SBRT) boost is a promising treatment for patients with cervical cancer who are ineligible for ICBT. The aim of this multicenter, single-arm, phase I/II study was to prospectively evaluate the efficacy and toxicity of SBRT boost.

ICBT-ineligible patients with untreated cervical cancer (International Federation of Gynecology and Obstetrics [2008] stage IB1–IIIB) were enrolled between April 2016 and October 2020. Patients underwent whole-pelvic radiotherapy (45 Gy in 25 fractions) with SBRT boost to the primary lesion. In the phase I dose-escalation cohort (3 + 3 design), patients were treated with SBRT boost of 21 or 22.5 Gy in 3 fractions in 3 days. Although dose-limiting toxicity was not confirmed, a dose of 21 Gy was selected for the phase II cohort, because of safety concerns. The primary endpoint was 2-year progression-free survival.

Twenty-one patients (phase I, n = 3; phase II, n = 18) were enrolled between April 2016 and October 2020; 17 (81%) had clinical stage III–IV disease with para-aortic lymph node metastases. The median (range) follow-up was 29 (10–72) months. The initial response was complete response in 20 patients and partial response in one patient. The 2-year locoregional control, progression-free survival, and overall survival rates were 84%, 67%, and 81%, respectively. Grade ≥ 3 toxicity was confirmed in one patient each in the acute (diarrhea) and late (urinary tract obstruction) phase.

SBRT boost was effective and safe, even in patients with advanced disease in whom ICBT was not possible. These findings suggest that SBRT boost is an important treatment option for patients with cervical cancer who are ineligible for ICBT.