Background and purpose: To evaluate geometric stability of applicator and rectum during pulsed dose rate (PDR) intracavitary brachytherapy.
Patients and methods: A total of 14 patients with cervical cancer (stages IIB-IVA) were analysed retrospectively. A dose of 10 Gy to point A was prescribed per brachytherapy session, and PDR was given with 1 Gy/pulse, 1 pulse/h, using a ring applicator (Varian). A rectal dosimeter consisting of five diodes spaced by 1.5 cm was routinely placed in the rectum. The diodes detected the progression of each pulse of radiation, as the stepping source was advanced through the applicator. A mathematical model has been developed for spatial analysis of the pattern of the dose readings. The model transforms dose measurement into a quantification of the geometric relationship between rectum diodes and applicator.
Results: The model could be used for all treatment sessions, and the relative positions of diodes and applicator were calculated for each pulse of radiation. The SD of displacements during the treatment was below 2.8mm in all directions for all patients. The mean SD in lateral, longitudinal and anterior-posterior directions were 1.2 +/- 0.7, 1.2 +/- 0.7 and 0.9 +/- 0.6 mm, respectively. The mean measurement uncertainty was below 0.8 +/- 0.5 mm in all directions.
Conclusions: A new mathematical method has been developed, enabling us to quantitate and monitor relative positions of applicator and rectal diodes during a PDR treatment. The spatial relation between rectal dosimeter and applicator was very stable during extended PDR treatments suggesting that the geometric stability of PDR treatment is at the same level as the stability reported for HDR brachytherapy.