Poster - Thurs Eve-27: Method of estimating imaging dose to patients from on-line cone-beam computed tomography using...

The use of on-line kilovoltage cone-beam computed tomography (CBCT) is increasing as part of the current evolution of image-guided radiotherapy. At our institution, we use Varian's On Board Imager® (OBI) mainly for imaging prostate cancer patients. Since daily CBCT can add significant dose, we have performed a comprehensive set of dose measurements using acrylic cylindrical phantoms using ionization chambers as well as skin dose measurements using Thermoluminescence Dosimeters (TLD). The TLD were calibrated under chosen reference conditions (10×10 cm² field size at 100 cm SSD) using the OBI beam with bowtie filters (125 kVp, 6.0 mm Al). For the patients, TLD were placed on the anterior, left and right lateral locations to give the peripheral dose. The CBCT dose values (in units of mGy/100mAs) were then used to model the central (phantom only) and peripheral dose (phantom and patients) as a function of equivalent diameter, d_eq , using exponential functions. Dose values measured in Rando phantom as well as published dose values agreed with the model quite well for d_eq ⩾ 24 cm (body), but there was higher variation in CBCT dose for d_eq ⩽ 18 cm (head). Hence, it is recommended that skin dose be measured for head scans to validate the estimate. This method provides a quick estimate of CBCT dose so that a decision can be made whether to incorporate it into the treatment prescription. Also, knowledge of CBCT dose as a function of patient size may enable reduction of the total mAs for smaller body scans.