Abstract
BackgroundIntensity modulated arc therapy (IMAT) has been widely adopted for Stereotactic Body Radiotherapy (SBRT) for lung cancer. While treatment dose is optimized and calculated on a static Computed Tomography (CT) image, the effect of the interplay between the target and linac multi-leaf collimator (MLC) motion is not well described and may result in deviations between delivered and planned dose. In this study, we investigated the dosimetric consequences of the inter-play effect on target and organs at risk (OAR) by simulating dynamic dose delivery using dynamic CT datasets.MethodsFifteen stage I non-small cell lung cancer (NSCLC) patients with greater than 10 mm tumor motion treated with SBRT in 4 fractions to a dose of 50 Gy were retrospectively analyzed for this study. Each IMAT plan was initially optimized using two arcs. Simulated dynamic delivery was performed by associating the MLC leaf position, gantry angle and delivered beam monitor units (MUs) for each control point with different respiratory phases of the 4D-CT using machine delivery log files containing time stamps of the control points. Dose maps associated with each phase of the 4D-CT dose were calculated in the treatment planning system and accumulated using deformable image registration onto the exhale phase of the 4D-CT. The original IMAT plans were recalculated on the exhale phase of the CT for comparison with the dynamic simulation.ResultsThe dose coverage of the PTV showed negligible variation between the static and dynamic simulation. There was less than 1.5% difference in PTV V95% and V90%. The average inter-fraction and cumulative dosimetric effects among all the patients were less than 0.5% for PTV V95% and V90% coverage and 0.8 Gy for the OARs. However, in patients where target is close to the organs, large variations were observed on great vessels and bronchus for as much as 4.9 Gy and 7.8 Gy.ConclusionsLimited variation in target dose coverage and OAR constraints were seen for each SBRT fraction as well as over all four fractions. Large dose variations were observed on critical organs in patients where these organs were closer to the target.
Highlights
Stereotactic body radiation therapy (SBRT) has been increasingly employed in the treatment of medically inoperable early stage non-small cell lung cancer (NSCLC)
One way to account for tumor motion [10,11] is to create margins using an internal target volume (ITV) that encompasses the motion envelope of the tumor derived from a respiratory-gated computed tomography (4DCT) [12]
Note that even for a single fraction, the dose deviation was small. This may be attributed to the fact that the dose per fraction was large in lung Stereotactic Body Radiotherapy (SBRT) which in this case was more than six times the conventional dose per fraction (12.5 Gy vs. 1.8 to 2.0 Gy)
Summary
Stereotactic body radiation therapy (SBRT) has been increasingly employed in the treatment of medically inoperable early stage non-small cell lung cancer (NSCLC). SBRT involves hypofractionation to deliver a large dose per fraction in a small number (usually 1–5) of treatments to the target volume while minimizing normal is widely adopted for lung SBRT [4] due to the reduced treatment time compared with IMRT and 3Dconformal treatment [5,6,7,8]. Studies [13,14,15,16] have shown that under multiple fields and after a large number of fractions, the interplay effect in IMRT delivery results in a smeared dose distribution where the standard deviation of the dose is generally within 1% of the expected value [13]. While treatment dose is optimized and calculated on a static Computed Tomography (CT) image, the effect of the interplay between the target and linac multi-leaf collimator (MLC) motion is not well described and may result in deviations between delivered and planned dose. We investigated the dosimetric consequences of the inter-play effect on target and organs at risk (OAR) by simulating dynamic dose delivery using dynamic CT datasets
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