Combined application of block and modulation factors to reduce the volume of the low dose area in helical tomotherapy plans for lung cancer.

BACKGROUND

The helical tomotherapy (HT) system can expose more normal lung tissue to low radiation doses, resulting in extensive low-dose distribution in both lungs which may induce radiation pneumonitis (RP).

PURPOSE: This study aims to optimize dosimetric parameters and identify a clinically feasible treatment plan by analyzing the impact of different block settings and modulation factor (MF) combinations on HT plans for lung cancer to reduce the low-dose exposure volumes in normal lung tissue.

METHODS

We retrospectively reviewed 14 lung cancer patients who received radiotherapy. These cases were optimized using different modulation factors (MFs: 3, 4, 5) and block techniques (Unblocked, Directional, Complete). The impact of MF and block combinations on reducing low-dose bath (e.g., V5, V10) in lung tissue was evaluated by analyzing dose distribution maps, dose-volume histograms (DVH), homogeneity index (HI), conformity index (CI), and treatment time for each optimized HT plan combination.

RESULTS

Block settings and higher MF exerted minimal influence on the average dose to the target volume. With stricter block constraints and higher MF, HI increased (range: 17.39%-27.54%) and CI decreased (range: 2.76%-17.43%); although both indices showed slight deterioration, they remained within acceptable clinical limits. Treatment time increased substantially (range: 15.77%-131.58%). The block technique significantly reduced V5 in the bilateral lungs and the contralateral (healthy) lung, with less impact on the ipsilateral (affected) lung. The combination of block and high MF effectively reduced V5.

CONCLUSIONS

To balance target volume coverage (high dose), dose distribution uniformity, and treatment duration while reducing low-dose irradiation to normal lung tissue, we recommend implementing Directional block with an MF range of 3-4 to optimize the HT plan for patients with unilateral lung cancer.