3D Printing in Combination With Kelowna GYN Template-Based High-Dose Rate Interstitial Brachytherapy: Design and Dosimetric Results

Abstract

The Kelowna GYN template is a perineal template for interstitial GYN brachytherapy designed for compatibility with a single channel vaginal cylinder. We present the first reported series of patients treated with this system and an additional 3D printed modification for utilization of this template with an intrauterine tandem. Indications generally included large tumors not amenable to intracavitary +/- free hand needles approach, and/or vaginal involvement (tumor thickness greater than 0.5 cm), and/or bladder invasion, and primary vaginal carcinomas and recurrent endometrial carcinomas with depth of invasion >0.5cm per GEC-ESTRO and ABS guidelines. The Kelowna template allows placement of interstitial needles with straight angled holes to provide coverage of the craniocaudal and lateral extension up to 3 cm from the vaginal cylinder. A modified applicator system using a custom 3D printed adaptor piece allows secure attachment of the tandem to the template for patients with an intact uterus. Transabdominal ultrasound was utilized to guide needle placement. CT treatment planning was performed with stepwise dwell weight adaptation and needle loading to achieve optimal dose coverage. Dosimetric data was evaluated including the D90 of high risk-clinical target volume (HR-CTV) with total EQD2 including whole pelvis radiotherapy. D2cc of OARs were recorded. Between October 2018 and January 2020, 17 patients (6 cervical; 7 recurrent endometrial; 2 vaginal; 2 recurrent cervical) with GYN malignancies were treated using the Kelowna template. All of the patients got external beam initially while all brachytherapy was done at a single NCI designated comprehensive cancer center. Patients with cervical cancer had bulky tumors at the time of implant (median HR-CTV volumes of 48 cm3). Indications for treatment were tumor >4cm (n = 5), vaginal involvement (n = 8), >0.5cm depth of invasion (n = 10). Median number of needles utilized was 12 (range 5 to 17). Treatments consisted of 3 or 4 fractions on consecutive days (BID). Dosing was 21 Gy in 3 fractions (n = 10), 24 Gy in 4 fx (n = 1), 28 Gy in 4 fx (n = 5), and 35 Gy in 5 fx (n = 1). For cervical and recurrent tumors, the median D90 was 81.6 Gy (range 86.3 - 92.6). For vaginal and recurrent endometrial, the median D90 was 86.9Gy (range 74.8 - 103.2). The median dose to OARs was within standard limits for intracavitary brachytherapy alone (EQD2cc for bladder of 72.4 Gy (IQR 67.2 - 77.5 Gy) and rectum of 61.2 Gy (IQR 58.7 - 62.7)). No patients needed adjustment in needle position between treatments. No patients had grade 3 acute toxicities. No patients had recurrence on follow-up. The Kelowna template allows for customized safe, robust, and reproducible implants with interstitial only or combined interstitial-intracavitary applicator for selected cases of bulky, asymmetric cervical, vaginal, and recurrent endometrial cancer and works well with customized 3D printed components.