Breast Deformations Measured With Cone Beam Computed Tomography During Adjuvant Radiotherapy for Breast Cancer.

PURPOSE/OBJECTIVE(S) This study aims to assess breast deformations during adjuvant radiotherapy for breast neoplasms with cone beam computed tomography (CBCT) and to correlate the deformations assessed by multiple observers with characteristics of the tumor and the treatment plan. MATERIALS/METHODS Patients were eligible if they received adjuvant radiotherapy for breast cancer. Tumor characteristics and details of the treatment plan were collected retrospectively. The deformations of the external contour of the mammary gland at the level of the tumor cavity and at the isocenter were evaluated by two radiation oncologists after image registration between the planning computed tomography (pCT) scan and CBCTs obtained during treatment. For analysis purposes, deformations of less than 5 millimeters (mm) have been grouped together in the same category of deformation, as have those of 5 mm to 10 mm and those of 10 mm and more. RESULTS 56 patients treated in our institution between March 2013 and October 2015 are included in this analysis. Radiation oncologists observed similar deformations for 79% of measurements and deformations of less than one category of deviation for 89% of measurements. At the start of treatment, there were breast contractions at level of the tumor cavity in 38.9% of the participants and breast dilatation in 46.3% compared to the anatomy of the pCT. 78% of the participants had a breast deformation of less than 5 mm and 9% had a deformation between 5 mm and 10 mm at the first CBCT. Deformations remained stable during treatment in 40.4% of the patients. Dilatations or contractions between 5 mm and 10 mm during treatment occurred in 8.5% of participants. The mean tumor cavity was 13.2 cubic centimeters (cc) when the deformations were less than 5 mm at the first CBCT and 38.3 cc when the deformations were greater than 10 mm (P = 0.01). Tumors located in the lower outer quadrant of the breast were associated with more deformations during treatment than tumors located in the other quadrants (P = 0.04). Smaller deformations at the end of radiotherapy were correlated with longer delays between surgery and the end of the radiotherapy (P = 0.03), with a mean of 92 days when the deformations were less than 5 mm and 67 days when de deformations were greater than 5 mm. CONCLUSION The increase in the volume of the tumor cavity, the localization of the tumor in the lower outer quadrant of breast and a smaller delay between surgery and the end of the radiotherapy correlate with breast deformations during radiotherapy. These results, combined with the planned dose, could be used to target patients who might benefit from a closer monitoring of their positioning during treatment. Further investigations are needed to determine whether these findings correlate with received doses and exit doses, measured with electronic portal imaging device.