Sparing lung tissue with virtual block method in VMAT planning for locally advanced non-small cell lung cancer

This study aimed to exploit a new virtual block method to spare normal lung tissue in VMAT planning for patients with locally advanced non-small cell lung cancer (LA-NSCLC). The previous method was used to manually restrict the angle of the beam passing through, which ignored the location and shape of large targets that varied between different slices and did not block the beamlets precisely. Unlike the previous method, this new virtual block method was used to block the beamlets when necessary by closing the multi-leaf collimator (MLC) at prerequisite angles. The algorithm for closing the MLC depended on the thickness of the beamlets passing through the lungs and avoided only the entrance radiation beamlet. Moreover, this block can be automatically contoured. A retrospective study was performed to compare the VMAT plans with and without the virtual block method for 17 LA-NSCLC patients, named the block plan (B-plan)/non-block plan (N-plan). All cases were selected in this study because of the large tumor size and unmet dose constraints of the lungs. In addition to the maximum dose constraint for the virtual block, B-plans adopted identical optimization parameters to N-plans for each patient. These two types of plans were compared in terms of dosimetric indices and plan scores. The results were statistically analyzed using the Wilcoxon nonparametric signed-rank test. B-plans have advantages in the following dosimetric metrics that have statistical significance (p < 0.05): (1) lower V-5/V-10/D-mean/normal tissue complication probability (NTCP) of total lungs; (2) reductions in V-5/V-10 for the contralateral lung; (3) decrease in D-mean/V-40 of the heart; (4) decrease in esophagus V-40; (5) reductions in D-mean, V-5/V-10 of normal tissue. B-plans (82.51 +/- 7.07) achieved higher-quality scores than N-plans (80.74 +/- 7.22). The new virtual block spared the lungs as well as other normal structures in VMAT planning for LA-NSCLC. Thus, the block method may decrease the risk of radiation-related toxicity in patients.