Intensity modulated radiotherapy and 3D conformal radiotherapy for whole breast irradiation: a comparative dosimetric study and introduction of a novel qualitative index for plan evaluation, the normal tissue index

We report on a retrospective dosimetric study, comparing 3D conformal radiotherapy ( 3DCRT ) and hybrid intensity modulated radiotherapy ( hIMRT ). We evaluated plans based on their planning target volume coverage, dose homogeneity, dose to organs at risk ( OAR s) and exposure of normal tissue to radiation. The Homogeneity Index ( HI ) was used to assess the dose homogeneity in the target region, and we describe a new index, the normal tissue index ( NTI ), to assess the dose in the normal tissue inside the tangent treatment portal. Methods Plans were generated for 25 early‐stage breast cancer patients, using a hIMRT technique. These were compared with the 3 DCRT plans of the treatment previously received by the patients. Plan quality was evaluated using the HI , NTI and dose to OAR s. Results The hIMRT technique was significantly more homogenous than the 3 DCRT technique, while maintaining target coverage. The hIMRT technique was also superior at minimising the amount of tissue receiving D 105% and above ( P  < 0.0001). The ipsilateral lung and contralateral breast maximum were significantly lower in the hIMRT plans ( P  < 0.05 and P  < 0.005), but the 3 DCRT technique achieved a lower mean heart dose in left‐sided breast cancer patients ( P  < 0.05). Conclusion Hybrid intensity modulated radiotherapy plans achieved improved dose homogeneity compared to the 3 DCRT plans and superior outcome with regard to dose to normal tissues. We propose that the addition of both HI and NTI in evaluating the quality of intensity modulated radiotherapy ( IMRT ) breast plans provides clinically relevant comparators which more accurately reflect the new paradigm of treatment goals and outcomes in the era of breast IMRT .