Abstract
ObjectiveThe objective of this study was to evaluate the interplay effects in proton-based stereotactic body radiotherapy (SBRT) using 4D robust optimization combined with iso-energy layer repainting techniques for non-small cell lung cancer (NSCLC).Materials and MethodsTwelve patients with early-stage NSCLC who underwent 4DCT were retrospectively selected. A robust CTV-based 4D plan was generated for each based on commercial Treatment planning system (TPS), considering patient setup errors, range uncertainties, and organ motion. The 4D static dose (4DSD) and 4D dynamic dose (4DDD) were calculated using a hybrid deformable algorithm and simulated proton delivery system. An index was developed to quantitatively evaluate the interplay effects. The interplay effects of the 4D robust plan and multiple iso-energy layers (3, 4, 5, 6, and 7) of the robust repainting 4D plan were calculated based on to select the optimal times for layer repainting.ResultsDue to the interplay effects, the mean target values D2 and D5 increased to 1.28 and 1.01%, and the target values D98 and D95 decreased to 2.01 and 1.77%, respectively, for the 4D Robust SBRT plan. After multiple iso-energy repainting times, the interplay effects of the target values D98 and D95 tended to migrate, from 2.01 to 0.92% in target value D98 and from 1.77 to 0.89% in target value D95, respectively. Moreover, a positive linear correlation was observed between the optimal interplay effect mitigation and target range of motion.ConclusionIn proton-based 4D Robust SBRT, the interplay effects degraded the target dose distribution but were mitigated using iso-energy layer repainting techniques. However, there was no significant correlation between the number of repainting layers and improvements in the dose distributions. The optimal layer repainting times based on the interplay effect index were ascertained according to the patient characteristics.
Highlights
Proton spot scanning-based stereotactic body radiotherapy (SBRT) has been shown to outperform photon-based SBRT in patients with early-stage non-small cell lung cancer (NSCLC)
In early-stage NSCLC, approximately 50% of lung tumors move more than 5 mm and some even move more than 2 cm in the superior–inferior direction [5], where interplay effects caused by the interference between the beam spot and intrafractional respiratory motion is dominant and should be taken into account
The interplay effects can be estimated by calculating the single-fraction 4D static dose (4DSD) and single-fraction 4D dynamic dose (4DDD) based on 4DCT images [10, 11]
Summary
Proton spot scanning-based stereotactic body radiotherapy (SBRT) has been shown to outperform photon-based SBRT in patients with early-stage non-small cell lung cancer (NSCLC). The motion of lung tumors has been evaluated based on 4DCT imaging. In early-stage NSCLC, approximately 50% of lung tumors move more than 5 mm and some even move more than 2 cm in the superior–inferior direction [5], where interplay effects caused by the interference between the beam spot and intrafractional respiratory motion is dominant and should be taken into account. There are few clinical reports on the effectiveness of combining 4DRP and repainting with proton-based SBRT for IMPT in patients with early-stage NSCLC. The 4DSD is calculated based on the assumption that the tumor moves in 4DCT images without considering the delivery system’s time dependence. The 4DDD is calculated by taking into account the delivery system’s time dependence This method is not very intuitive and cannot provide changes in the target volume or organs at risk (OARs), such as target conformity, homogeneity, and OAR volume dosimetry
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