Influence of variations in photon beam quality index on tissue inhomogeneity correction factors in radiation therapy treatment planning

This study was carried out to investigate how variations in the beam quality index (QI)-so-called tissue phantom ratio (TPR20,10) affect tissue inhomogeneity correction factors (ICFs) in external beam radiotherapy treatment plans. A total of 90 three-dimensional conformal radiotherapy (3DCRT) treatment plans for lung, gynecological, and prostate cancers and 15 stereotactic body radiation therapy (SBRT) plans for lung cancer were analyzed. For the 3DCRT plans, ICFs were evaluated across a range of beam QI values. For 6 MV photon beams, the TPR20,10 values were set at 0.670 ± 3%, while for 15 MV photon beams, the range was 0.760 ± 3%. SBRT plans were generated using two 6 MV photon beam configurations from a Varian TrueBeam accelerator – one employing a flattening filter (6 MV, TPR20,10 = 0.688) and the other operating in flattening filter-free (FFF) mode (6 MV FFF, TPR20,10 = 0.632). All dose calculations were performed using the Eclipse treatment planning system with the anisotropic analytical algorithm. For the calculations for 3DCRT plans, a 6.0% variation in the beam QI resulted in maximum differences in ICFs of 16.3% for lung cases with 6 MV beams and 12.5% for 15 MV beams. In gynecological and prostate cases, the ICF differences remained below 2.0% and 1.0%, respectively. In addition, for lung SBRT plans, a 5.6% discrepancy in TPR20,10 between 6 MV and 6 MV FFF beams led to ICF variations of <3.0%. These findings suggest that while variations in the beam QI significantly influence ICFs in lung cancer 3DCRT plans, their impact is less pronounced in gynecological and prostate treatments. This underscores the necessity of carefully accounting for beam quality variations during radiotherapy treatment planning to ensure accurate dose delivery and optimal patient outcomes.