ESTRO 2024 - Abstract Book

S225

Brachytherapy - Gynaecology

ESTRO 2024

463

Digital Poster

Dosimetric evaluation of dose optimisation in HDR brachytherapy using Venezia and Geneva applicators

Natalie Kar Wei Yip 1 , Tsz Ching Fok 1 , Tin Lok Chiu 1 , Siu Ki Yu 1 , Amy Tien Yee Chang 2

1 Hong Kong Sanatorium & Hospital, Medical Physics Department, Hong Kong, Hong Kong. 2 Hong Kong Sanatorium & Hospital, Comprehensive Oncology Centre, Hong Kong, Hong Kong

Purpose/Objective:

With increased popularity of CT/MR imaging and CT/MRI compatible high dose rate brachytherapy applicators, 3D treatment planning is made possible and accessible in typical treatment planning systems. Graphical dose optimization (GrO) is an interactive forward planning technique, in which the user manually reshapes isodose lines on the CT/MR images; dwell positions, weights and times of radioactive sources along the catheters are altered to achieve optimal target coverage and maximal sparing of OAR. With the aid of MR images, improved accuracy in target and OAR delineation became possible due to better soft tissue contrast. Theoretically, this should improve ease in treatment planning, thus achieving superior dosimetric qualities for both targets and OAR. Our goal is to compare treatment plans of GrO and conventional 2D brachytherapy technique, and to quantify the extent of dosimetric variance between the two. The most recent MRI-compatible intracavitary/interstitial brachytherapy applicators include Venezia applicator (Elekta), which is a lunar shaped applicators producing a ring dose distribution, and the newer tandem and ovoids Geneva applicator (Elekta), both allow image-guided adaptive brachytherapy using MRI on each treatment day. It can potentially provide advanced tailoring of conformal dose distribution when parallel and/or oblique needles are inserted along with the applicator. Both applicators are widely used for cervical cancer brachytherapy in recent years, understanding their dosimetric characteristics will prove value. Sixteen cervical cancer patients who completed external beam radiotherapy (25 or 28 fr, 1.8-2.2 Gy/fr, total dose to tumour: 45-50.4 Gy with or without simultaneous integrated boost to pathological lymph nodes) with concurrent chemotherapy, followed by High Dose Rate brachytherapy (Elekta Flexitron afterloader with Ir-192 source, 3 or 4 fr, prescription dose at Point A: 4-8 Gy) using Venezia applicator (n = 5, intrauterine tube: 5 or 7 cm, IU tube angle: 30 degrees, ovoid size: 22 mm) or Geneva applicator (n = 11, intrauterine tube: 3, 5 or 7 cm, IU tube angle: 30 degrees, ovoid size: 13, 15 or 22 mm), were retrospectively recruited. Parallel interstitial needles were used on 8 patients to boost dose to extensive parametrial tumour invasion. Vaginal cap was present for 3 fractions for one Venezia patient to ensure the upper two thirds of the vagina was treated appropriately. T2-weighted sequence MR images with 2 mm slice thickness were obtained after the first insertion of Venezia or Geneva applicator, followed by MR or CT in subsequent fractions. Following the GEC-ESTRO recommendations, target (GTV, HR-CTV, IR-CTV) and OAR (bladder, rectum, sigmoid, bowel) were contoured by the case oncologist and radiotherapist on these images. All treatment planning was done on the same treatment planning system (TPS) console – Oncentra TPS (version 4.6.3). Graphically optimised plans (GrOP) were approved by the oncologist and delivered to the patients; conventional plans (ConP) with normalisation to Point A were generated from delivered plans retrospectively. D90, D98 of GTV, HR-CTV, IR-CTV, D2cc of OAR (bladder, rectum, sigmoid and bowel) were extracted from Oncentra TPS. Significance of differences between parameters were evaluated using paired t-tests. p-value of < 0.05 was considered statistically significant. Material/Methods: