ESTRO 2023 - Abstract Book

S1931

Digital Posters

ESTRO 2023

Stockholm, Sweden) and the imaging advantages of MRI-based while preserving the stability, geometry, and robustness of the implant and including an intrauterine component to provide a central BT dose. Pre-planning techniques are not frequently employed in BT. Potential reasons of this are the possible anatomic changes that are produced after the applicator insertion (uterus straighten). The purpose of this work is to present the noticeable benefice of the implementation of a preplanning technique using a multi-interstitial perineal and endocavitary MRI gynecological brachytherapy in-house developed applicator. Materials and Methods A pre-BT MRI T2 acquisition is performed with the template (without intrauterine tandem and interstitial needles but with the vaginal cylinder) in place 3-5 days before the BT implant. The required needles and their depths are selected to encompass the CTV (as conformal as possible). To facilitate this task, a Java based application linked to the TPS (Oncentra Prostate v 4.3 Elekta) has been developed. An applicator library has been specifically developed for this applicator, using free available software. The anchor points are three inserted A-vitamin pellets. The main inconvenient of preplan is the lack of an IC/IS component doing that the position of the uterus varies in the majority of the patients being straight after the insertion of the intrauterine tube. To evaluate the benefit of the preplan, implant coverage for a group of patients in which it was used was compared with another group in which it was not. Results Forty three implants with preplan have been compared with 132 without it, in patients treated of LACC and primary/vaginal recurrences (Table 1). The D90 in CTV obtained were 80.4 Gy and 75.93 Gy respectively.

Conclusion A virtual MRI preplanning has been presented. Although a statistical study has not been carried out, the data show an improvement in the D90 obtained in the patients in whom the preplan has been carried out, due to the better distribution and depth of the needles used.

PO-2147 Theoretical maximum lateral target thickness for Intensity-modulated vaginal brachytherapy

F. Svahn 1,2 , M. Lecavalier-Barsoum 3,4 , M. Morcos 5,6 , S. Abbasinejad Enger 7,8,9,2

1 McGill University, Medical Physics Unit, Montreal, Canada; 2 Jewish General Hospital, Lady Davis Institute for Medical Research, Montreal, Canada; 3 McGill University, Department of Oncology, Montreal, Canada; 4 Jewish General Hospital, Department of Radiation Oncology, Montreal, Canada; 5 Miami Cancer Institute, Radiation Oncology, Miami, USA; 6 Florida International University, Herbert Wertheim College of Medicine, Miami, USA; 7 McGill University, Medical Physics Unit, Montreal, Canada; 8 McGill Unversity, Department of Oncology, Montreal, Canada; 9 McGill University Health Centre, Research Institute of the McGill University Health Centre, Montreal, Canada Purpose or Objective Current recommendations by the American Brachytherapy Society for the treatment of vaginal cancers is the use of interstitial brachytherapy for patients with bulky (> 5 mm thick) disease. Rotating-shield intensity modulated brachytherapy (IMBT) has been shown to be able to deliver highly conformal and asymmetric dose distributions without needles in various sites. We aim to find a theoretical maximum target thickness which can be safely treated with centered single channel applicators commonly used in IMBT. We also present proof-of-concept shield designs for vaginal IMBT and retrospectively investigate the IMBT dosimetry in patient geometry using a single IS/IC-BT fraction. Materials and Methods Simulations were performed with the Monte Carlo-based dose calculation software RapidBrachyMCTPS. The four cylindrical shields are 120 mm long and either 18 or 14 mm in diameter and consist of tungsten with collimation windows of 120 or 150 degrees, named X_Y with X corresponding to the shield diameter in mm and Y corresponding to degrees of the emission window. Shields were placed inside the pre-contoured MIAMI applicator with an outer diameter of 30 mm. A 3 mm thick vaginal mucosa was contoured around the applicator in the patient geometry. The maximum Hausdorff distance between the CTV and the applicator was 12.8 mm. For ease of comparison, all simulations were normalized to the clinical prescription dose. Artificial lateral lesions in the water phantom were all contoured as 70 mm tall lateral lesions with an initial 4.9 mm thick 9.2697cc lesion circumferentially grown by 2.9mm into a final 28.1 mm thick lesion of 156.898cc. Plans for artificial lesions were generated with a centered single channel 30 mm applicator. Maximum type A uncertainty for water phantom simulations was 1.28% for the lesion with 28.1 mm thickness, and <0.47% for all patient geometry simulations. Total EQD2 dose was extrapolated from the single fraction dose with α / β = 3 for OAR and a 1.8x25 fraction EBRT protocol. Results Bowel D2cc was on average increased by 0.85 +- 1.69%. Bladder D2cc was on average decreased by 21.26 +- 3.20%. Rectum D2cc was on average decreased by 1.38 +- 1.80%. Urethra D2cc was on average decreased by 4.68 +- 1.12%. Vaginal mucosa D2cc was on average decreased by 10.75 +- 2.42%. Vaginal mucosa D1cc was on average decreased by 10.24 +- 2.14%. Total