Exploring Appropriate Offset Values for Pencil Beam and Monte Carlo Dose Optimization in Lung Stereotactic Body Radiotherapy Encompassing the Effects of Respiration and Tumor Location

Purpose: To explore offset values in dose optimization with pencil beam (PB) algorithm
to minimize dosimetric differences with plans calculated with Monte Carlo (MC) for lung cancer
treatment with Stereotactic Body Radiotherapy (SBRT). Methods: 20 cases of Non-Small Cell
Lung Cancer, treated with gated full motion range SBRT were selected. According to the
proximity of the Gross Tumor Volume (GTV) to the chest wall, two groups are defined.
Treatment plans were created on 4D average intensity projection (AIP) CT set with Brainlab
iPlanDose® 4.1.2 planning system. The D97 of PTV was normalized to 50Gy using the fast PB
and compared with MC. The optimized plan was then recomputed over each 4D respiratory
phase, and compared with MC. Results: The mean difference in the PB and MC D97 of the ITV
was 10.5% (±0.8%) of the prescription dose (50Gy). PB algorithm showed 2.3-2.4% less
overestimation to the D97 of the ITV, when comparing to MC, in the maximum exhalation phase
than in the maximal inhalation phase. Significantly smaller dose difference between PB and MC
is also shown in plans for peripheral lesions (7.7 ± 0.7%) versus for central lesions (12.7±0.8%)
(p< 0.01). Conclusions: The dosimetric differences between PB and MC can be reasonably
predicted depending on the location of lesion in the lung, and may be used as offset value in dose
optimization with PB. Caution is suggested when using the maximum inhalation phase for
treatment planning.