The Analysis of Dose in a Rectum by Multipurpose Brachytherapy Phantom / 대한방사선종양학회지

ABSTRACT

PURPOSE: In this work we designed and made MPBP (Multi Purpose Brachytherapy Phantom). The MPBP enables one to reproduce the same patient set-up in MPBP as the treatment of the patient and we tried to get an exact analysis of rectal doses in the phantom without need of in-vivo dosimetry. MATERIALS AND METHODS: Dose measurements were tried at a point of rectum 1, the reference point of rectum, with a diode detector for 4 patients treated with tandem and ovoid for a brachytherapy of a cervix cancer. Total 20 times of rectal dose measurements were made with 5 times a patient. The set-up variation of the diode detector was analyzed. The same patient set-ups were reproduced in self-made MPBP and then rectal doses were measured with TLD. RESULTS: The measurement results of the diode detector showed that the set-up variation of the diode detector was the maximum 11.25+/-0.95 mm in the y-direction for Patient 1 and the maximum 9.90+/-2.40 mm, 20.85+/-4.50 mm, and 19.15+/-3.33 mm in the z-direction for Patient 2, 3, and 4, respectively. In analyzing the degree of variation in 3 directions the more variation was showed in the z-direction than x- and y-direction except Patient 1. The results of TLD measurements in MPBP showed the relative maximum error of 8.6% and 7.7% at a point of rectum 1 for Patient 1 and 4, respectively and 1.7% and 1.2% for Patient 2 and 3, respectively. The doses measured at R1 and R2 were higher than those calculated except R point of Patient 2. This can be thought to related to the algorithm of dose calculation, whcih corrects for air and water but is guessed not to consider the correction for the scattered rays, but by considering the self-error (+/-5%) TLD has the relative error of values measured and calculated was analyzed to be in a good agreement within 15%. CONCLUSION: The reproducibility of dose measurements under the same condition as the treatment could be achieved owing to the self-made MPMP and the dose at the point of interest could be analyzed accurately. If a treatment is performed after achieving dose optimization using the data obtained in the phantom, dose will be able to be minimized to important organs.