The radiation exposure from portal images during the course of breast radiotherapy

Objectives: To evaluate the radiation dose to the contralateral breast (CB) from portal images taken during the course of conventional whole breast irradiation (WBI) and partial breast irradiation (PBI). Materials and Methods: The dose to CB from portal images was measured in a female anthropomorphic phantom with 12 thermoluminescent dosimeters placed in the contralateral (right) breast near the midline of the phantom. The average CB dose from portal imaging per monitor unit (MU) was calculated. Twenty WBI patients and 9 PBI patients were randomly selected to obtain the average total MU (ATMU), minimum and maximum MU used to obtain portal images during their treatment. The estimated radiation dose from portal imaging to CB was calculated as average total MU multiplied by average CB dose from portal imaging per MU. Results: In WBI, the mean dose and range from 20 patients to 2 points (ie, point 1 and 9 in Fig. 1) nearest to the midline of the phantom were 22.9 (15.1-43.4) cGy, and 8.4 (5.5-15.9) cGy, respectively. The doses to other points ranged from 0.9 cGy to 5.7 cGy. The PBI portal imaging techniques gave a significantly higher dose to most points in CB than the WBI. The doses to all the points ranged from 4.1 cGy to 27.4 cGy. The orthogonal portals contributed over 99% of the dose to CB for all the points. Conclusion: When acquiring portal images for breast radiotherapy, the number of images, corresponding exposure times, and portal image field size should all be considered.