To analyze the effect of intra fractional and inter fractional motion during breast intensity modulated radiation therapy (IMRT) by calculating dose distribution based on four-dimensional computed tomography (4D-CT).
Twenty patients diagnosed with left breast cancer were enrolled. Three-dimensional (3D)-CT along with ten phases of 4D-CT were collected for each patient, with target volumes independently delineated on both 3D-CT and all phases of 4D-CT. IMRT plans were generated based on 3D-CT (43.2 Gy in 16 fractions). The plan parameters for each segment were split into phases based on time duration estimates for each respiratory phase, with phase-specific dose distributions calculated and summated (4D-calculated dose). The procedure is repeated for 16 fractionations by randomly allocating starting phase using random-number generation to simulate inter fractional discrepancy caused by different starting phase. Comparisons of plan quality between the original and 4D-calculated doses were analyzed.
There was a significant distortion in 4D-calculated dose induced by respiratory motion in terms of conformity and homogeneity index compared to those of the original 3D plan. Mean doses of the heart and the ipsilateral lung were significantly higher in the 4D-calculated doses compared to those of the original 3D plan (0.34 Gy, = 0.010 and 0.59 Gy, < 0.001), respectively). The mean IMN dose was significantly greater in the 4D-calculated plan, compared to the original 3D plan (1.42 Gy, < 0.001).
IMN doses should be optimized during the dose-calculation for the free-breathing left breast IMRT.
The interplay effect between respiratory motion and MLC modulation caused discrepancies in dose distribution, particularly in IMN.