Objective To study the dosimetric differences between intracavitary therapy and combined intracavitary/interstitial therapy in three-dimensional brachytherapy for cervical cancer at different high-risk clinical target volumes (HR-CTV) and to explore whether HR-CTV has a threshold value.

Methods The clinical data of 100 patients with advanced cervical cancer who received radical radiotherapy in the Fourth Affiliated Hospital of Guangxi Medical University from October 2019 to February 2021 were retrospectively analyzed. The age range was 35–63 years, with a median age of 51 years. According to different brachytherapy techniques, the patients were divided into two groups: intracavitary therapy group (45 cases) and intracavitary/interstitial therapy group (55 cases). According to the size of HR-CTV of each plan of a patient, the volume interval of 10 cm³ was used and divided into six volume ranges for analysis, namely, HR-CTV≤40 cm³, 40 cm³<HR-CTV≤50 cm³, 50 cm³<HR-CTV≤60 cm³, 60 cm³<HR-CTV≤70 cm³, 70 cm³<HR-CTV≤80 cm³, HR-CTV>80 cm³. Independent sample t-test was used in comparing the target dose (D₉₀ and D₁₀₀), target coverage (V₁₀₀), target high dose (D₅₀, V₁₅₀, and V₂₀₀), target conformity index (CI), and D_{2 cm}³ of organ at risks (OAR) (bladder, rectum, sigmoid colon, and small intestine) of the two brachytherapy techniques in various volume ranges.

Results When HR-CTV≤40 cm³, the target dose (D₉₀ and D₁₀₀), target coverage (V₁₀₀), and high dose of target (D₅₀, V₁₅₀, and V₂₀₀) in the intracavitary group were higher than those in the intracavitary/interstitial therapy group, and the differences were statistically significant(t=2.826–3.927, all P<0.05), but no significant difference in CI and D_{2 cm³} (mininum radiation dose received by the OAR volume of 2 cm³) of the bladder and rectum (t=0.186, 1.871, 0.258; all P>0.05). When 70 cm³<HR-CTV≤80 cm³ and HR-CTV>80 cm³, the target dose (D₉₀ and D₁₀₀), target coverage (V₁₀₀), and CI and D_{2 cm}³ of rectum and bladder in the intracavitary/interstitial group were significantly better, and the differences were statistically significant(t=−6.872–3.782, all P<0.05), while the high dose in target (D₅₀, V₁₅₀, and V₂₀₀) of the two techniques was similar, and the differences were statistically significant (t=0.613–1.918, all P>0.05). When 40 cm³<HR-CTV≤70 cm³, no significant differences in target dose (D₉₀, D₁₀₀), target coverage (V₁₀₀), and D_{2 cm}³ of rectum were found between the two groups (t=−1.759–0.710, all P>0.05), but the CI of intracavitary/interstitial group was significantly better than that of the intracavitary group, and the differences were statistically significant (t=−2.590, −4.577, −3.144; all P<0.05).

Conclusions For a small volume target (≤40 cm³), intracavitary therapy alone can better increase the dose of target and high dose volume in a target area without increasing the dose of OAR. When the target volume is large (>70 cm³), combined intracavitary/interstitial therapy can significantly improve the prescribed dose and conformity of the target while protecting the OAR.