Factors Affecting D90 High-risk Clinical Target Volumes (HR-CTV dose) of Intracavitary and Interstitial Brachytherapy in Locally Advanced Cervical Cancer.

Objective: Intracavitary brachytherapy alone covers a limited target volume; however, intracavitary and interstitial brachytherapy (IC/IS) can increase the dose coverage. We aim to assess the factors that impact D90 high-risk clinical target volume (HR-CTV) dose. We also assess clinical outcomes and toxicities for 3D image-based brachytherapy.

Methods: We included a total of 424 cervical cancer patients with FIGO stage IB1 to IVA who received chemoradiation and high-dose-rate brachytherapy between 2014 and 2023. Target delineation was per GEC-ESTRO guidelines with the aim to achieve total dose of ≥85 Gy (D90 HR-CTV) in equivalent dose (EQD2). Implantation, tumour size, lateral extension, and HR-CTV volume were analysed.

Results: The median follow-up time was 24 months (range 1-107). The overall 2-year local control, progression-free survival, and overall survival rate were 90.3%, 75%, and 95.5%, respectively. Of 424 patients, 86.8% received a total dose of at least 85 Gy of D90 HR-CTV in EQD2. In multivariate analysis, IC/IS brachytherapy and HR-CTV volume were significant factors associated with HR-CTV D90 ≥ 85Gy in EQD2 (P = 0.012 and P = 0.000, respectively). Subgroup analysis of patients with HR-CTV volume >35 ml found that IC/IS was a significant factor in achieving HR-CTV D90 ≥ 85Gy in EQD2 (P = 0.017). At the median follow-up, patients with D90 HR-CTV ≥85 Gy achieved local control rates of 72.08% in small volume (<20 cm3) group, 68.42% in intermediate volume (21-30 cm3) group, 71.68% in high intermediate volume (31-60 cm3) and 17.67% in larger volume (>60 cm3) group (P = 0.005). Grade 3 toxicities including proctitis, cystitis, and vaginal stenosis were 7.1%, 1.9% and 0.2%, respectively.

Conclusions: IC/IS brachytherapy may be used in patients with HR-CTV volumes greater than 35 ml to achieve total doses of D90 HR-CTV ≥85 Gy in EQD2. IC/IS brachytherapy also provide good local control with favorable toxicity profile.