Abstract
For stereotactic ablative body radiotherapy (SABR) in lung cancer patients, Radiation Therapy Oncology Group (RTOG) protocols currently require radiation dose to be calculated using tissue heterogeneity corrections. Dosimetric criteria of RTOG 0813 were established based on the results obtained from non‐Monte Carlo (MC) algorithms, such as superposition/convolutions. Clinically, MC‐based algorithms are now routinely used for lung SABR dose calculations. It is essential to confirm that MC calculations in lung SABR meet RTOG guidelines. This report evaluates iPlan MC plans for SABR in lung cancer patients using dose‐volume histogram normalization per current RTOG 0813 compliance criteria. Eighteen Stage I‐II non‐small cell lung cancer (NSCLC) patients with centrally located tumors, who underwent MC‐based lung SABR with heterogeneity correction using X‐ray Voxel Monte Carlo (XVMC) algorithm (BrainLAB iPlan version 4.1.2), were analyzed. Total dose of 60 Gy in 5 fractions was delivered to planning target volume (PTV) with at least V100%=95%. Internal target volumes (ITVs) were delineated on maximum intensity projection (MIP) images of 4D CT scans. PTV (ITV+5 mm margin) volumes ranged from 10.0 to 99.9 cc (mean=36.8±20.7 cc). Organs at risk (OARs) were delineated on average images of 4D CT scans. Optimal clinical MC SABR plans were generated using a combination of non‐coplanar conformal arcs and beams for the Novalis‐TX consisting of high definition multileaf collimators (MLCs) and 6 MV‐SRS (1000MU/min) mode. All plans were evaluated using the RTOG 0813 high and intermediate dose spillage criteria: conformity index (R100%), ratio of 50% isodose volume to the PTV (R50%), maximum dose 2 cm away from PTV in any direction (D2cm), and percent of normal lung receiving 20 Gy (V20) or more. Other organs‐at‐risk (OARs) doses were tabulated, including the volume of normal lung receiving 5 Gy (V5), maximum cord dose, dose to <15 cc of heart, and dose to <5 cc of esophagus. Only six out of 18 patients met all RTOG 0813 compliance criteria. Eight of 18 patients had minor deviations in R100%, four in R50%, and nine in D2cm. However, only one patient had minor deviation in V20. All other OARs doses, such as maximum cord dose, dose to <15 cc of heart, and dose to <5 cc of esophagus, were satisfactory for RTOG criteria, except for one patient, for whom the dose to <15 cc of heart was higher than RTOG guidelines. The preliminary results for our limited iPlan XVMC dose calculations indicate that the majority (i.e., 2/3) of our patients had minor deviations in the dosimetric guidelines set by RTOG 0813 protocol in one way or another. When using an exclusive highly sophisticated XVMC algorithm, the RTOG 0813 dosimetric compliance criteria such as R100% and D2cm may need to be revisited. Based on our limited number of patient datasets, in general, about 6% for R100% and 9% for D2cm corrections could be applied to pass the RTOG 0813 compliance criteria in most of those patients. More patient plans need to be evaluated to make recommendation for R50%. No adjustment is necessary for OAR dose tolerances, including normal lung V20. In order to establish new MC specific dose parameters, further investigation with a large cohort of patients including central, as well as peripheral lung tumors, is anticipated and strongly recommended.PACS number: 8087
Highlights
SABR with hypofractionated dose schemata has emerged a viable alternative treatment for medically inoperable early-stage lung cancer patients.[1]
Dosimetric criteria of Radiation Therapy Oncology Group (RTOG) 0813 were established based on the results obtained from non-Monte Carlo (MC) algorithms, such as superposition/convolutions.[3,4] Recently, several commercial treatment planning systems (TPS) have employed MC-based dose calculation algorithms, and many researchers have investigated whether MC-based dose calculation algorithms can meet the dosimetric criteria of RTOG 0813.(5,6) For instance, Li et al[5] evaluated the MC algorithm employed in Monaco TPS
In the study by Rana et al,(6) it was found that Acuros XB resulted in lower magnitudes of R100%, R50%, and D2cm by 5%, 1.2%, and 1.6%, respectively, on average, than the analytical algorithm (AAA), except for the normal lung tissue (V20) where it was higher by 1.1%
Summary
SABR with hypofractionated dose schemata has emerged a viable alternative treatment for medically inoperable early-stage lung cancer patients.[1]. Dosimetric criteria of RTOG 0813 were established based on the results obtained from non-Monte Carlo (MC) algorithms, such as superposition/convolutions.[3,4] Recently, several commercial TPS have employed MC-based dose calculation algorithms, and many researchers have investigated whether MC-based dose calculation algorithms can meet the dosimetric criteria of RTOG 0813.(5,6) For instance, Li et al[5] evaluated the MC algorithm employed in Monaco TPS (Computerized Medical System, St. Louis, MO) for SBRT lung plans and compared the results against the superposition algorithm in XiO TPS (Computerized Medical System, St. Louis, MO). In one of the most recent studies, Rana et al[6] evaluated the MC-based Acuros XB employed in Eclipse TPS (Varian Medical Systems, Palo Alto, CA) for the SBRT lung cases, and compared the results against the anisotropic analytical algorithm (AAA) in Eclipse TPS. Li et al[5] used intensity modulated radiation therapy (IMRT) and Rana et al[6] used volumetric modulated arc therapy (VMAT), which is referred as RapidArc in Eclipse TPS
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