Dosimetric comparison of prone versus supine hypofractionated partial breast irradiation via external beam radiotherapy in early breast cancer

Objectives: To analyze the dosimetry of PTV and OARs - heart and lungs in prone versus supine hypofractionated partial breast irradiation. Methods and Materials: Between October 2014 and December 2016, 23 histologically proven early breast cancer patients were enrolled on the study. All patients underwent breast conservation surgery followed by Adjuvant radiotherapy. A CTV of 1.5cm was generated from the cavity followed by 0.5 mm PTV margins. 3DCRT or IMRT plans were generated in both the positions. Patient was treated in supine position to a dose of 40Gy delivered in 15 fractions at 2.66Gy per fraction, 1 fraction per day over 5 fractions per week. Dose constraints were applied to the OARs of ipsilateral lung V30 <10%, V20<20%, V30<10% and the heart V20<5%, V10<10% and Dmean <5Gy. In our study the PTV Dosimetric parameters were assessed using PTV D90, PTV V95 and PTV Dmax. Lung Dosimetric parameters were mean ipsilateral lung V30, V20 and V10 and Heart Dosimetric parameters were V20, V10 and mean heart dose. Results: The PTV D90 was 37.91Gy in supine position and 38.03Gy in prone position. The PTV Dmax was 106.22Gy in supine position and 105.87Gy in prone position. The PTV V95 was 91.73% in supine and 93.77% in prone position. The mean ipsilateral lung V30 was 6.69% in supine and 2.47% in prone position and was statistically significant (p =0.002). The mean ipsilateral lung V20 was 11.05% in supine and 4.87% in prone position and was statistically significant (p=<0.001). The mean ipsilateral lung V10 was 17.82% in supine and 7.97% in prone position and was statistically significant (p=<0.001). The mean ipsilateral lung dose was 6.37Gy in supine position and 3.10Gy in prone position which was also statistically significant (p=<0.001). The Heart V20 was noted to be 1.28% in supine position and 1.53% in prone position. The Heart V10 was 3.99% in supine position and 3.19% in prone position. The mean heart dose was 1.86Gy in supine position and 1.48Gy in prone position. None of these parameters were statistically significant. Conclusion: Prone position in breast irradiation had better dosimetry with ipsilateral lung. Although the cardiac dose was reduced by prone position compared to supine position, DVH parameters were not statistically significant.