A comparison of 18-MV and 6-MV treatment plans using 3D dose calculation with and without heterogeneity correction

Homogeneity of the dose distribution in irradiation of the intact breast for stage I and II cancers is an important factor, particularly for larger breasts. In the present work, we have studied dose homogeneity for 6- and 18-MV treatment plans in 10 patients, typically with larger breasts. For each patient, 6 3-dimensional (3D) dose distributions were calculated using patient computed tomography data and the ADAC Pinnacle3 treatment planning system. First, a dose distribution was calculated, assuming the patient was water, with the 6-MV beam parameters used to treat the patient. Second, the calculation was repeated using the actual patient anatomy. Comparison of these 2 distributions showed how patient heterogeneity affected dose. Third, individual beam weights were optimized, and the dose calculation was repeated. Each of these 3 dose calculations was repeated at 18 MV. Results showed that: (1) at 6 MV, the ratio of mean dose in the target volume calculated with heterogeneity considerations to that without was 1.014 +/- 0.006, and the ratio of the standard deviation of dose in the target volume was 0.919 +/- 0.042; (2) at 18 MV, the ratio of mean dose to the target volume calculated with heterogeneity considerations to that without was 1.001 +/- 0.005, and the ratio of the standard deviation of dose in the target volume was 1.15 +/- 0.09; and (3) the dose homogeneity, measured by the standard deviation of the dose distribution in the target volume, was 25% less for the 18-MV treatment plan for patients with breast volumes greater than 1600 cm3. We conclude that: (1) 3D, heterogeneity-corrected dose calculation is necessary to fairly evaluate any advantage of 18 MV over 6 MV; (2) excluding the dose buildup region, 18 MV produces a significantly more homogeneous dose distribution for breast volumes greater than 1600 cm3; and (3) when prescribing dose using heterogeneity-corrected dose distributions, dose prescriptions should be increased by 1.5% at 6 MV, but no increase is needed for 18 MV.