Optimal Number of Needle Applicators Inserted in Combined Intracavitary and Interstitial Brachytherapy for Cervical Cancer

Background/Aim: Combined intracavitary and interstitial brachytherapy (IC/IS-BT) is an effective treatment for extensive and bulky cervical cancer. However, the optimum number of interstitial needle applicators ("needles") inserted in IC/IS-BT can be difficult to determine. To examine the number of needles required for adequate dose coverage of cervical tumors, we retrospectively analyzed IC/IS-BT plans. Patients and Methods: IC/IS-BT plans for cervical cancer patients treated from January 2014 to January 2021 were analyzed. All tumors were controlled locally at the time of analysis (August 2022). The relationship between the number of needles and several volumetric parameters of high-risk clinical target volume (CTV_HR) were analyzed, including maximum diameter, maximum cross-sectional area, and the volume of CTV_HR. Spearman's rank correlation coefficients (r) were used to evaluate correlations. Results: Eighty-two plans in 32 patients were analyzed. The median maximum cross-sectional area and volume of CTV_HR were 18.9 (12.3-42.5) cm² and 53.8 (30.1-152.2) cm³, respectively. The mean D_90% and D_98% of CTV_HR at each BT session were 7.0±0.8 Gy and 5.9±0.8 Gy, respectively. There was a positive correlation between the number of needles and the maximum cross-sectional area of CTV_HR (r=0.53). The average numbers of needles were 1.3, 1.9, 2.2, 3.1, and 4.0 when the maximum cross-sectional area of CTV_HR were ≤15 cm², 15-20 cm², 20-25 cm², 25-30 cm², and >30 cm², respectively. Conclusion: The optimal number of needles can be determined from the maximum cross-sectional area of CTV_HR.