PHANTOM AND CLINICAL STUDIES ON LOW-DOSE CT PROTOCOL FOR APPLICATOR RECONSTRUCTION IN THREE-DIMENSIONAL BRACHYTHERAPY USING SIZE-SPECIFIC DOSE ESTIMATES BASED ON WATER EQUIVALENT DIAMETER.

Abstract

To verify the feasibility of low-dose computed tomography (CT) protocol for applicator reconstruction in three-dimensional brachytherapy among patients of different sizes, using size-specific dose estimate based on water equivalent diameter (SSDEdw) in phantom and clinical studies. Pre-scans of a female pelvic phantom were followed by reconstruction of each image set with iDose4 levels 3-5. Imaging data from 64 cervical cancer patients were divided into low, standard and high weight groups. Among two to five CT scans required for applicator reconstruction, the first scan was adopted by routine-dose CT protocol (tube voltage = 120 kV, tube current-exposure time product = 320 mA.s) and the remaining by low-dose CT protocol (tube voltage = 120 kV, tube current-exposure time product = 80 mA.s). The SSDEdw and image quality parameters were compared among the groups, and correlations between SSDEdw and body mass index, area of reference plane (AreaROI3) and mean CT value of reference plane (CTROI3) were analyzed. According to the phantom test results, we determined tube voltage to 120 kV and tube current-exposure time product to 80 mA.s as the low-dose protocol. Clinical study revealed no statistically significant differences in signal-to-noise ratio (SNR) and contrast-noise-ratio (CNR) between low-dose and routine-dose CT in Groups A and B; in Group C, these were significantly lower in the former. The SSDEdw was significantly lower under low-dose than routine-dose protocol in all groups, with strong negative correlation with BMI and AreaROI3 in Groups A and B and moderate-to-strong negative correlation in Group C. Because of the characteristics of three-dimensional brachytherapy, in patients with BMI < 24.0 kg per m2, low-dose CT protocol can minimize radiation exposure and achieve precise, individualized treatment.