Evaluation of positional error during radiotherapy for breast cancer after modified radical mastectomy

Accurate positioning of Radiation Therapy (RT) is critical to its safety and efficacy for postmastectomy patients. We measured the Planning Target Volume (PTV) margins using the daily Cone-Beam Computed Tomography (CBCT) for patients who received Modified Radical Mastectomies (MRMs). We analysed setup error data using CBCT for 58 postmastectomy patients, including 28 with left-sided breast cancers (15 patients who received single chest wall (CW) RT and 13 who received CW plus supraclavicular region (CW/SR) RT) and 30 with right-sided cancers (15 who received single CW RT and 15 who received CW/SR RT). The positional error was corrected and compared in x, y, and z translational planes for different breast cancer locations (left or right) and different target volumes (single CW or CW/SR). We analysed a total of 925 digital images using the On Board Imaging system (OBI). The Z translational errors in treating CW/SR were significantly greater than in treating CW alone for both left-sided (P=0.035) and right-sided (P=0.048) cancers. The Y translational errors were significantly greater for the left-sided cancers than for the right-sided cancers (P=0.001) for both CW alone (P=0.026) and CW/SR (P=0.001). The Z translational errors of CW/SR were significantly greater than of single CW, and the Y translational errors were significantly greater on the left side than on the right side. In treating postmastectomy patients with daily image-guided radiation therapy, positioning errors should be checked and corrected.