Comparison Study of the Measured Rectal Point Dose by Detector In Vivo and the Recommended Point Dose of Rectum in Brachytherapy of Cervical Cancer

Abstract

To explore the difference and the relevance of the measured rectal point dose by detector in vivo and the recommended point dose of rectum to estimate radiation doses to rectum in patients with cervical cancer treated with 3-dimensional (3D) brachytherapy. Between January 2012 and December 2014, 150 cervix cancer patients (320 person-time) in our hospital who were treated with radical radiation therapy (RT) combined with the high-dose-rate (HDR) brachytherapy after external beam RT (EBRT) for total dose of 45 to 50 Gy in 25 fractions were analyzed. The computed tomography (CT)-based and ultrasound-guided 3D brachytherapy was performed using an applicator with 112 cases for interstitial combined with intracavity brachytherapy and 38 cases for intracavity only. The prescribed dose of brachytherapy was 7 Gy in 3 to 4 fractions. The reference point of rectal dose recommended by the International ICRU Report 38 (DICRU), the rectal D2cc values recommended by the GEC-ESTRO, the measured rectal point dose in vivo by an IDF1-3G detector (P), which were inserted into the rectum at the level of ICRU reference point and the calculated dose of the same point on the detector in planning system (P1) were recorded for all individual fractions respectively as indicators for the rectal dose evaluation. The features and the relevance of these indicators were determined by the paired t test and Pearson test. The P value was 3.48±0.68 Gy, it was higher than P1 (3.25±0.69 Gy) in the same point (P<.05). Both the P and P1 were significantly lower than the DICRU (3.71±0.62 Gy) and the D2cc (3.87±0.68 Gy). Moreover, the P value has a high relationship with the DICRU (P=.000; correlation coefficient=0.722), but not with D2cc (P=.002; correlation coefficient=0.284). The rectal point dose determined with the use of rectal detector in vivo has a big deviation with the same point in planning system, and it underestimated the rectal dose comparing with the DICRU and D2cc. Therefore, we did not recommend it as a perfect indicator for rectal dose evaluation for treatment planning. However, we argued that it is a good method in monitoring the real-time rectal dose for quality control and quality assurance during brachytherapy.