Use of an intravaginal spacer in young girls treated with brachytherapy for bladder neck rhabdomyosarcoma: Dosimetric impact for organs at risk sparing and acute tolerance

Purpose: Interstitial brachytherapy is indicated as part of a conservative strategy for children with bladder and/or prostate rhabdomyosarcoma (RMS), providing high local control probability with acceptable functional results. Vaginal and/or rectal complications were however reported, due to the close proximity to the implanted volume. We investigated the dosimetric impact of a vaginal spacer in terms of rectal and vaginal doses. Methods and patients: Medical records of 12 consecutive female patients with bladder neck RMS, median age 32 months (range: 1.3–6 years), were reviewed. Five patients were treated prior to 2017 without a vaginal spacer and seven patients treated after 2017 had their brachytherapy delivered with a vaginal spacer placed at time of implant. Results: Minimal doses delivered to the most exposed 2 cm³, 1 cm³, and 0.5 cm³ of the rectum were all statistically significantly lower among patients treated with a vaginal spacer, as compared to those treated without a spacer. Median rectal D2cm³ was 22 Gy_EQD2 versus 38 Gy_EQD2 (P = 0.02), D1cm³ was 29 Gy_EQD2 versus 51 Gy_EQD2 (P = 0.013), and D0.5cm³ was 32 Gy_EQD2 versus 61 Gy_EQD2 (P = 0.017), with and without the vaginal spacer, respectively. The posterior vaginal wall D0.5cm³ dose was also significantly decreased, with median D0.5cm³ of 92 Gy_EQD2 versus 54 Gy_EQD2 (P < 0.0001), with and without the spacer, respectively. Acute tolerance was excellent in all patients, with no need for replanning and no acute complication. Conclusions: The use of vaginal spacers in brachytherapy of female pediatric patients with bladder neck RMS resulted in significantly decreased doses to the rectum and the posterior vaginal wall. Though the clinical impact of such dose reduction remains undemonstrated, routine utilization of a vaginal spacer could be a method to decrease long-term morbidity in these patients.