Rectal doses in intracavitary brachytherapy of gynecological malignancies: Comparison of two dosimetric methods

Purpose: To compare calculated rectal doses obtained by two dosimetric methods in intracavitary brachytherapy of gynecological malignancies Materials and methods: This analysis included 124 intracavitary applications performed in 102 patients with cervical or endometrial cancer. The pelvic dose distribution based on orthogonal intracavitary placement films was calculated with the computer planning system. In each application the rectal dose was defined in the specific rectal point determined by both the use of a wire marker inserted into the rectum (R1) and by packing the vagina with radio-opaque gauze - the method recommended by the ICRU Report 38 (R2). The comparison included R1 and R2 doses as well as the respective radiobiological equivalent doses determined by the linear-quadratic model (r1 and r2). Results: In 83% of applications the absolute value of R1 was lower than R2. The mean difference between R1 and R2 was 3.7 Gy (95% CI 3.03-4.41 Gy) and between r1 and r2 7.2 Gy (95% CI 5.77-8.56 Gy). These differences were significant (P < 0.001 for both comparisons). The difference between the doses was not influenced by the type of applicator and remained significant even when a systemic ± 10% error of method was assumed. Conclusion: The rectal point dose determined with the use of rectal wire marker may be underestimated, therefore this method should be discouraged in gynecological brachytherapy.