The Dosimetric Effects of Photon Energy on the Quality of Volumetric Modulated Arc Therapy for Lung Stereotactic Body Radiation Therapy

Abstract

Purpose: There is little published data on the optimal energy to use to minimize doses to Organs at Risk (OARs), while maintaining adequate Planning Target Volume (PTV) coverage in lung volumetric-modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT).

Methods: 35 lung lesions in 33 patients were treated at our institution by VMAT SBRT. Dosimetric plans using 6-Megavoltage (6-MV) and 10-Megavoltage (10-MV) energies were generated for each lesion. The median dose was 5000cGy delivered over 3-5 daily fractions. Various dosimetric parameters were recorded for both the 6-MV and 10-MV plans and the patients were stratified according to the tumor to chest wall distance (TCW), the tumor location (central versus peripheral), patient anterior-posterior (AP) diameter, and the diameter of an equivalent sphere encompassing the patient's body over the distance of the PTV (ESD).

Results: There was a statistically significant difference between 6-MV and 10-MV with respect to the sum lung dose, which favored 6-MV plans (p=0.04). For those stratified by TCW, there was a difference in conformity index (CI) for patients with peripheral tumors (p=0.04). For the group stratified by AP separation, there was a difference in mean sum lung dose favoring 6-MV (p=0.01). In the group stratified by ESD, there were statistically significant (SS) differences in the volume of lung receiving at least 13Gy (V13), mean sum lung dose, and CI, all favoring 6-MV plans (p=0.05, p<0.01, and p<0.01). For the cohort overall, and within each subgroup, there was a SS difference in the total number of monitor units (MUs), which consistently favored planning with 10-MV.

Conclusion: With the exception of thinner patients, for which 6-MV plans was superior with respect to OARs and conformity index, 10-MV should be considered for use in lung VMAT SBRT. 10-MV plans consistently resulted in fewer total MUs. Fewer MUs results in shorter treatment times, with the potential for improved target accuracy due to less intrafractional tumor motion.