ESTRO 2023 - Abstract Book

S1921

Digital Posters

ESTRO 2023

overcome the problem, the aim of this study is to propose the brachytherapy planning using three-ovoid applicators to improve insufficient dose distribution. Materials and Methods Clinical dosimetry was calculated using HDRPlus software and the patient was treated using the BEBIG MultiSource® High Dose Rate brachytherapy treatment unit with a cobalt-60 source. In this study, three types of applicators (cylinder, two- ovoid, and three-ovoid applicators) were used, and three brachytherapy plans were compared for optimal dose delivery. The three-ovoid applicators are two-ovoids and tandem with multiple holding discs. In accordance with American Brachytherapy Society guidelines, the total treatment dose is 5Gy in 5 separate fractions. For each individualized treatment planning, dose optimization was performed in a standardized plan according to the applicator types. To compare the three brachytherapy plans, prescription point, vaginal apex point, bladder, and rectal dose were evaluated. The dose at the vaginal apex point, which was defined as 5 mm above the surface of the vaginal apex corresponding to the midline of the applicator tips and perpendicular to the anteroposterior axis of the patient, was calculated. Results In the planning using cylinder, two-ovoid, and three-ovoid applicators, the calculated values for the vaginal apex point are 4.40 Gy, 2.18 Gy, and 4.68 Gy, respectively. These values correspond to 88%, 43.7%, and 93.7% of the prescribed dose, respectively. The bladder doses are 3.69 Gy, 1.35 Gy, and 3.63 Gy, respectively. The rectal doses ranged from 1.0-4.8 Gy, 1.1-2.3 Gy, and 1.8-5.3 Gy, respectively. Conclusion We confirmed the feasibility of a vaginal-cuff brachytherapy planning using three-ovoid applicators to deliver optimal dose distribution leading to better clinical outcomes. Purpose or Objective External beam radiotherapy (EBRT) and high-dose-rate brachytherapy (BT) with or without concomitant chemotherapy are the standard of care in cervical cancer. Image-guided adaptive brachytherapy (IGABT) has been established by GEC-ESTRO working group and EMBRACE studies in the last decades. The purpose of this study is to evaluate the benefit of IGABT as compared to conventional point-A based brachytherapy. Materials and Methods A total of 138 patients received definitive radiotherapy between October 2015 and September 2020 at our hospital. Fourteen patients who had distant metastasis at treatment initiation, could not finish planned definitive radiotherapy, or declined chemotherapy were excluded. Thus, 124 patients were included in this study. Definitive radiotherapy consisted of 50-50.4 Gy of whole pelvis irradiation including the central shield and two to four fractions of intracavitary with or without interstitial brachytherapy. While, 97 patients received point-A based brachytherapy (P group), 27 received volume- based adaptive brachytherapy (V group). Every BT procedure was based on a computed tomography image. High-risk clinical target volume (HR-CTV) for the P group was delineated retrospectively. The equivalent doses in 2 Gy fraction (EQD2) of the total dose of EBRT and BT were calculated with the Linear-quadratic model with α / β = 10 Gy for HR-CTV and α / β = 3 Gy for organs at risk. Dose-volume parameters and oncological outcomes were retrospectively analyzed. Results Squamous cell carcinoma (SCC) histology accounted for 86.4% in the P group and 77.8% in the V group (p=0.364). The mean volume of HR-CTV at the first fraction of brachytherapy was 29.6 cm ³ in the P group and 38.6 cm ³ in the V group (p = 0.020). The mean EQD2 of the D90% to the HR-CTV was 73.28 Gy in the P group and 71.52 Gy in the V group (p=0.278). Mean EQD2 of the D2 cm3 to rectum were 68.78 Gy in P group and 64.26 Gy in V group (p=.374), that to sigmoid were 56.12 Gy in P group and 57.74 Gy in V group (p=0.355), that to bladder were 72.59 Gy in P group and 77.86 Gy in V group (p=0.022). The 2-year local control rate (LCR) was 84.4% in P group and 88.9% in V group (p=0.574). In the univariate analysis using Fisher’s exact test, small HR-CTV volume (<20 cm ³ vs >20 cm ³ , p=0.043), histology of SCC (SCC vs non-SCC, p=0.011), higher cumulative dose of EBRT+BT (EQD2 of the D90% to the HR-CTV >73 Gy vs <73 Gy, p=0.002), and higher cumulative dose of BT only (EQD2 of the D90% to the HR-CTV >35 Gy vs <35 Gy, p<0.001) were significantly correlated to LCR. In the multivariate analysis using Cox proportional hazard analysis, higher cumulative doses of EBRT+BT (p=0.017) and SCC histology (p=0.017) were significantly correlated to LCR. Conclusion Volume-based adaptive brachytherapy achieved an equivalent dose to HR-CTV and local control, albeit for patients with larger target volume compared with point-based brachytherapy. PO-2135 Impact of image guided adaptive brachytherapy in cervical cancer M. Sawada 1 , Y. Shiraishi 1 , H. Toyama 1 , T. Tanaka 1 , R. Kota 1 , K. Yoshida 1 , N. Shigematsu 1 1 Keio University, Radiation oncology, Shinjuku, Japan

PO-2136 retrospective study of cervical cancer with radiation dose reduction in concurrent chemoradiotherapy

N. Nagai 1 , M. Kawamura 1 , S. Ishihara 1 , Y. Oie 1 , Y. Kozai 1 , Y. Takase 1 , M. Okumura 1 , Y. Shindo 1 , R. Yasui 1 , Y. Yanagi 1 , S. Naganawa 1

1 Nagoya University Hospital, Radiology, Nagoya, Japan

Purpose or Objective