Application of Silicone-Filled Vaginal Spacers for Pediatric Proton Therapy

Abstract

Tissue spacers, including vaginal wall spacers, are used in a number of cancer diagnoses including soft tissue sarcomas, prostate cancer, cervical and uterine cancers. In proton therapy, the benefit of tissue spacers can be even greater as the device can serve as a beam stopper where protons reach their end of range in the material of the spacer instead of adjacent tissues.Commercially available hollow vaginal spacers which come in sizes ranging from 1.46 cm to 3.65 cm in diameter were filled with air-free silicone in a custom-made vacuum chamber. Two adolescent patients with pelvic soft tissue sarcomas receiving proton therapy were CT-simulated with and without a vaginal spacer. The relative stopping power of silicone was measured in a water tank to estimate the water equivalent thickness from 1.56 cm to 3.91 cm used in the treatment planning system for dosimetric comparisons. The dosimetric advantages of the silicone-filled vaginal spacer were investigated in three different dosimetric scenarios: (1) no vaginal spacer, (2) hollow vaginal spacer, (3) silicone-filled vaginal spacer in terms of plan quality and sparing of contralateral vaginal wall during primary and boost phases for two patients.While the target coverages remained comparable for the phase I (CTV, Rx = 36 Gy) and the phase II (GTV, Rx = 59.4 Gy), the dose distributions of ipsilateral and contralateral vaginal walls varied across these scenarios. After the phase II, the dosimetric sparing of contralateral vaginal wall was chiefly achieved in the patient A as the mean dose was 8.4 Gy with the silicone-filled spacer, and 17.1 Gy with hollow spacer, and 30.8 Gy with no spacer. However, due to the size and location of the tumor relative to the vagina, the sparing of contralateral vaginal was not observed in the patient B. While the patient A had a unilateral tumor with smaller GTV volume (23.3 cc) irradiated by straight lateral and posterior beams, the patient B had bilaterally located larger GTV volume (342.2 cc) covered by two posterior oblique beams.The study suggested that when the vagina is adjacent but located distal to the tumor, a significant dose reduction can be achieved with the insertion of a silicon-filled vaginal spacer.