Dosimetric impact of accurately delineating of the left anterior descending artery in photon and proton radiotherapy

The dosimetric impact of accurately delineating the left anterior descending artery (LAD) was investigated for routine treatment planning deep inspiration breath-hold (DIBH) CT where cardiac motion was not accounted for. The LAD was contoured in the routine DIBH CT images by an expert radiologist in a population of 10 patients with cancer of the left breast. The motion blurring of the LAD in the DIBH CT images was extracted from the contoured LAD to create a corrected LAD volume. This was used to compare the maximum and mean LAD dose over 3D conformal radiotherapy (3DCRT), uniform scattering (US) proton, and pencil beam scanning (PBS) proton plans. Using a corrected LAD volume reduced the maximum dose LAD DVH indicator by 2% (3DCRT), 4% (US), and 25% (PBS). A corrected LAD volume increased the mean dose LAD DVH indicator by 25% (3DCRT), 61% (US), and 35% (PBS). In terms of absolute dose the impact of contouring on LAD is higher for photon therapy due to the higher doses delivered using this modality. Overall, the results demonstrate that the LAD volume could potentially be the source of inconsistencies in correlation between dose and radiation-induced cardiac toxicity when uncompensated motion is present in the treatment planning images.