Comparison of dosimetric and clinical outcomes between short- and long-channel cylinder applicators for vaginal brachytherapy in intermediate- and high-risk endometrial cancer

Purpose: Vaginal brachytherapy (VBT) using a cylinder applicator is a standard treatment of intermediate- and high-risk endometrial cancer. We conducted a retrospective study of the dosimetric and clinical outcomes at our institution with 2 single-channel applicators in patients receiving VBT.

Methods and materials: One hundred thirty-six patients with endometrial cancer treated from 2006 to 2016 receiving VBT after definitive surgery were evaluated. Two cylinders were used with the distal dwell position 7.1-12.8 mm from the apex varying by diameter (short channel), and 3.2 mm from the apex (long channel). We prescribed 18-26 Gy in 3-4 fractions at 0.5 cm depth. Measurements of the distance from the apex to the prescription isodose line were taken from CT imaging. Student's t test and the Wilcoxon rank-sum test were used with corrections for multiple comparisons.

Results: Patients had International Federation of Gynecology and Obstetrics 2009 Stage I-II disease (70 Stage IA, 58 Stage IB, 9 Stage II). Mean cylinder apex dose was 95.2% and 154.7% of prescription (p < 0.001), and mean distance from apex to the prescription isodose line was 0.54 mm and 3.5 mm (p < 0.001) for the short- and long-channel cylinders, respectively. There were no significant differences in any toxicity between cylinders. Four patients (2.9%) had vaginal recurrence, all of whom were treated with the short-channel cylinder. Cylinder type was not associated with vaginal recurrence (p = 0.27).

Conclusions: A cylinder applicator with a distal dwell position closer to the apex results in higher doses to the vaginal cuff and increased D_2cc to the bladder. All four recurrences were in the short-channel cylinder. Additional investigation into applicator design and impact on patient outcomes in larger cohorts with sufficient followup is warranted.