Technical Aspects of the Integration of Three-Dimensional Treatment Planning Dose Parameters (GEC-ESTRO Working Group) into Pre-implant Planning for LDR Gynecological Interstitial Brachytherapy

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This study investigates the technical feasibility of pre-implant image-based treatment planning for LDR GYN interstitial brachytherapy(IB) based on the GEC-ESTRO guidelines. Initially, a virtual plan is generated based on the prescription dose and GEC-ESTRO defined OAR dose constraints with a pre-implant CT. After the actual implant, a regular diagnostic CT was obtained and fused with our pre-implant scan/initial treatment plan in our planning software. The Flexi-needle position changes, and treatment plan modifications were made if needed. Dose values were normalized to equivalent doses in 2 Gy fractions (LQED 2 Gy) derived from the linear-quadratic model with α/β of 3 for late responding tissues and α/β of 10 for early responding tissues. D90 to the CTV, which was gross tumor (GTV) at the time of brachytherapy with a margin to count for microscopic disease, was 84.7 ± 4.9% of the prescribed dose. The OAR doses were evaluated by D2cc (EBRT+IB). Mean D2cc values (LQED2Gy) for the rectum, bladder, sigmoid, and small bowel were the following: 63.7 ± 8.4 Gy, 61.2 ± 6.9 Gy, 48.0 ± 3.5 Gy, and 49.9 ± 4.2 Gy. This study confirms the feasibility of applying the GEC-ESTRO recommended dose parameters in pre-implant CT-based treatment planning in GYN IB. In the process, this pre-implant technique also demonstrates a good approximation of the target volume dose coverage, and doses to the OARs.

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