Three-dimensional dosimetric considerations from different point A definitions in cervical cancer low-dose-rate brachytherapy

Purpose: To investigate the dosimetric difference due to the different point A definitions in cervical cancer low-dose-rate (LDR) intracavitary brachytherapy. Material and methods: Twenty CT-based LDR brachytherapy plans of 11 cervical patients were retrospectively reviewed. Two plans with point As following the modified Manchester system which defines point A being 2 cm superior to the cervical os along the tandem and 2 cm lateral (A(os)), and the American Brachytherapy Society (ABS) guideline definition in which the point A is 2 cm superior to the vaginal fornices instead of os (A(ovoid)) were generated. Using the same source strength, two plans prescribed the same dose to A(os) and A(ovoid). Dosimetric differences between plans including point A dose rate, treatment volume encompassed by the prescription isodose line (TV), and dose rate of 2 cc of the rectum and bladder to the prescription dose were measured. Results: On average A(ovoid) was 8.9 mm superior to A(os) along the tandem direction with a standard deviation of 5.4 mm. With the same source strength and arrangement, A(os) dose rate was 19% higher than A(ovoid) dose rate. The average TV(A(ovoid)) was 118.0 cc, which was 30% more than the average TV(A(os)) of 93.0 cc. D-2cc/D(A(prescribe)) increased from 51% to 60% for rectum, and increased from 89% and 106% for bladder, if the prescription point changed from A(os) to A(ovoid). Conclusions: Different point A definitions lead to significant dose differences. Careful consideration should be given when changing practice from one point A definition to another, to ensure dosimetric and clinical equivalency from the previous clinical experiences.