Dosimetric evaluation of different cylinder diameters in three-dimensional vaginal brachytherapy for early-stage...

To evaluate the dosimetric, radiobiological, and toxicity differences between different cylinder diameters (d) in high-dose-rate three-dimensional computed-tomography-guided vaginal brachytherapy (VBT) for early-stage endometrial cancer (EC).

From January 2019 to January 2024, postoperative EC patients treated with exclusive VBT using cylinders were classified by the cylinder diameter (d ≤ 2.6 cm: small-size; d ≥ 3.0 cm: large-size) and matched according to 1:2 propensity score matching. Vaginal clinical target volume (CTV) was a 3-mm expansion around the cylinder surface. Dosimetric parameters in equivalent dose in 2 Gy (EQD2) (α/β = 3 Gy) and equivalent uniform dose (EUD) of vaginal_CTV and organs at risk (OARs) were evaluated. Urinary, gastrointestinal, and vaginal toxicities were assessed using CTCAE v5.0.

After matching, 132 patients (small-size: 44; large-size: 88) were analyzed. For vaginal_CTV, the small-size group had higher doses to 2%, 5%, 0.1 cc, 1 cc, and 2 cc of the volume (D₂, D₅, D_0.1 cc, D_1cc, and D_2cc) than the large-size group while lower doses to the 95%, 98%, and 100% volume (D₉₅, D₉₈, and D₁₀₀). The D_2cc and D_5cc of bladder and all dosimetric parameters of rectum were smaller in the small-size group. The EUD of vaginal_CTV, bladder, and rectum showed no significant differences. No significant differences in toxicities were found within the median follow-up of 26.8 months.

Cylinders with smaller diameters produced more nonuniform dose distributions in the target and delivered lower doses to bladder and rectum than large-size cylinders. However, the dosimetric differences did not translate into significant differences of radiobiological parameters or outcomes.