On the validity of density overrides for VMAT lung SBRT planning

Modeling dose to a moving target in lung is a very difficult task. Current approaches to planning lung stereotactic body radiotherapy (SBRT) generally calculate dose on either free breathing or average computed tomography (CT) scans, which do not always accurately predict dose to parts of the target volume not occupied by tumor on the planning scan. In this work, the authors look at using density overrides of the target volumes to more accurately predict dose for lung SBRT using the analytic anisotropic algorithm (AAA).

Volumetric modulated arc therapy plans were created on free breathing scans (FBP), time average scans (AVGP), free breathing scans with the internal target volume overridden to tumor density (ITVP), free breathing scans with the planning target volume overridden to tumor density (PTVP), and free breathing scan using a hybrid scheme with the internal target volume set to tumor density and the planning target volume minus the internal target volume set to a density intermediate between lung and tumor (HP) for the case of a 4D motion phantom and five patient cases. Radiochromic film measurements were made for the phantom plans, with gamma analysis used to compare the planned to delivered dose. The patient plans were recalculated on each of the phases of a 4DCT to evaluate tumor coverage and conformity index (CI). A modified modulation complexity score (MCSv) and average open area per control point (AA) metrics were used to evaluate multileaf collimator (MLC) modulation for each of the plans.

The HP plans showed significantly higher gamma passing rates (p < 0.05) than the FBP, AVGP, and ITVP for criteria of 2 mm/2% and 1 mm/1%. No significant correlation was observed between gamma values and AA or MCSv. The tumor volume was covered by the prescription dose on all phases of the 4DCT for all patient plans. The PTVP and HP yielded lower mean CI than the other plans for all five patients, with three of the cases showing statistically significant differences (p < 0.05). No meaningful correlation was observed between the mean CI and AA or MCSv.

These measurements suggest that the HP planning method may provide more accurate dose modeling and decreased normal lung irradiation for lung SBRT compared to the commonly used FBP and AVG planning methods when used with the AAA. The HP method does not appear to have a strong relationship with MLC modulation.