The impact of tumor size and histology on local control when utilizing high-dose-rate interstitial brachytherapy for gynecologic malignancies

Objective: To report long-term results of an outpatient template-based high-dose-rate interstitial brachytherapy (HDR ISBT) program for the treatment of gynecologic malignancies. Methods: Patients treated between 2006 and 2020 at an academic hospital with outpatient template based HDR ISBT without spinal or general anesthesia were reviewed. Patients who had previously received HDR ISBT were excluded. Baseline patient, tumor, and treatment characteristics, such as tumor size, histology, and/or total EQD2 including prior external beam radiation therapy (EBRT) were recorded. Local control and overall survival were estimated using the Kaplan-Meier method, and factors associated with local control and overall survival were evaluated using Cox regression analyses. Results: 150 patients received HDR ISBT for a gynecologic tumor and the median follow-up time was 2.98 years (0.89–4.82). Of those, 74/150 (49%) were treated definitively, 69/150 (46%) were treated for tumor recurrence/persistence, and 7/150 (5%) were treated for durable palliation. Median tumor size was 3.00 cm (1.50–4.00). 124/150 (83%) patients received EBRT prior to HDR ISBT. Median HDR ISBT dose was 18 Gy delivered in eight fractions. Local control was 71% (64%–79%), 58% (50%–68%), and 57% (48%–67%) at one, three, and five years, respectively. On multivariate analysis, non-endometrial adenocarcinoma histology (HR = 2.423, 95% CI = 1.011–5.808, p = 0.047) and tumor size ≥ 3 cm (HR = 2.903, 95% CI 1.053–3.441, p = 0.033) were associated with lower local control. Conclusions: The majority of patients who received outpatient-based twice daily HDR ISBT had long-term local control. Larger tumor size and non-endometrial adenocarcinoma histology were detrimental to local control.