Abstract
Purpose:To compare dose distributions calculated using the iPlan XVMC algorithm and heterogeneities corrected/uncorrected Pencil Beam (PB‐hete/PB‐homo) algorithms for SBRT treatments of lung tumors.Methods:Ten patients with centrally located solitary lung tumors were treated using MC‐based SBRT to 60Gy in 5 fractions for PTVV100%=95%. ITV was delineated on MIP‐images based on 4D‐CT scans. PTVs(ITV+5mm margins) ranged from 10.1–106.5cc(mean=48.6cc). MC‐SBRT plans were generated with a combination of non‐coplanar conformal arcs/beams using iPlan‐XVMC‐algorithm (BrainLABiPlan ver.4.1.2) for Novalis‐TX consisting of HD‐MLCs and 6MV‐SRS(1000MU/min) mode, following RTOG 0813 dosimetric criteria. For comparison, PB‐hete/PB‐homo algorithms were used to re‐calculate dose distributions using same beam configurations, MLCs/monitor units. Plans were evaluated with isocenter/maximal/mean doses to PTV. Normal lung doses were evaluated with V5/V10/V20 and mean‐lung‐dose(MLD), excluding PTV. Other OAR doses such as maximal spinal cord/2cc‐esophagus/max bronchial tree (BT/maximal heart doses were tabulated.Results:Maximal/mean/isocenter doses to PTV calculated by PB‐hete were uniformly larger than MC plans by a factors of 1.09/1.13/1.07, on average, whereas they were consistently lower by PB‐homo by a factors of 0.9/0.84/0.9, respectively. The volume covered by 5Gy/10Gy/20Gy isodose‐lines of the lung were comparable (average within±3%) when calculated by PB‐hete compared to XVMC, but, consistently lower by PB‐homo by a factors of 0.90/0.88/0.85, respectively. MLD was higher with PB‐hete by 1.05, but, lower by PB‐homo by 0.9, on average, compared to XVMC. XVMC max‐cord/max‐BT/max‐heart and 2cc of esophagus doses were comparable to PB‐hete; however, PB‐homo underestimates by a factors of 0.82/0.89/0.88/0.86, on average, respectively.Conclusion:PB‐hete significantly overestimates dose to PTV relative to XVMC ‐hence underdosing the target. MC is more complex and accurate with tissue‐heterogeneities. The magnitude of variation significantly varies with ‘small‐island‐tumor’ surrounded by low‐density lung tissues ‐PB algorithms lacks later electron scattering. Dose calculation with XVMC for lung SBRT is routinely performed in our clinic, its performance for head' neck/sinus cases will also be investigated.
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