Technical Aspects of a National Service for Paediatric Pelvic Brachytherapy

Brachytherapy is advantageous for localised rhabdomyosarcomas in children compared with external beam radiotherapy, sparing close organs at risk with highly conformal dosimetry. A methodology for planning and delivering fractionated high-dose-rate paediatric pelvic brachytherapy is detailed, and the dosimetric parameters are presented. This provides a practical template for radiotherapy departments with a similar patient cohort to implement this treatment technique.

Patients with localised pelvic rhabdomyosarcomas underwent transrectal ultrasound image-guided transperineal percutaneous implantation of brachytherapy catheters or insertion of intracavitary applicators. Clinical target volumes were delineated on computed tomography imaging, extended cranially to form an internal target volume (ITV) to allow for perineal oedema, and peer reviewed. Dosimetry was manually adjusted to achieve optimal coverage. The prescription dose was 27.5 Gy in five fractions. The dose to 90% (D90%) of the ITV, and the conformity number (CN) were calculated. Replanning was undertaken if measurements or imaging showed suboptimal dosimetry due to internal anatomical changes.

Thirty-five patients, aged 8 months to 17 years 6 months (median 2 years 10 months), were treated between 2009 and 2023. The median ITV was 18.5 cc. The median ITV D90% was 5.9 Gy per fraction. The majority of plans had a CN greater than 0.5. Replanning was required in two-thirds of patients.

Intraoperative transperineal interstitial implantation of flexible catheters, cervical intrauterine tubes, or vaginal applicators resulted in highly conformal plans, achieving excellent coverage with a D90% exceeding 90% of the prescribed dose in 34 out of 35 cases. Technical and practical measures are crucial for an optimal treatment for each child.