SU‐E‐J‐55: Dosimetric Evaluation of Centrally Located Lung Tumors: A Monte Carlo (MC) Study of Lung SBRT Planning

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.

Local control rates in stereotactic body radiotherapy (SBRT) of lung metastases associated with the biologically effective dose

“…Other authors [43][44][45][46][47] have presented similar results to our work for different fractionation schemes, where the greatest differences between dose algorithms were found for the smaller lesions, embedded in lung tissue, thus the doses received by the tumour were highly dependent on the size, tissue density and location. In all cases, the dose calculated by PB was higher than that obtained by algorithms with adequate heterogeneity correction as MC or Acuros XB.…”

Technical Note: Dosimetric evaluation of Monte Carlo algorithm in iPlan for stereotactic ablative body radiotherapy (SABR) for lung cancer patients using RTOG 0813 parameters

“…That is due to the lack of electronic equilibrium in the regions near low‐density tissues heterogeneities interface. In our most recent clinical study (18) with 10 lung cancer patients, the mean PTV dose was as high as 13%, on average, when using heterogeneities‐corrected PB algorithm compared to XVMC using same beam configurations, MLCs, and the same number of MUs. However, the volume covered by 5 Gy, 10 Gy, and 20 Gy isodose lines of the normal lung were comparable (within 3.0%), on average, when calculated by heterogeneities‐corrected PB compared to XVMC.…”

Monte Carlo evaluation of tissue heterogeneities corrections in the treatment of head and neck cancer patients using stereotactic radiotherapy

“…XVMC algorithm accurately predicted both the dose delivered to the isocenter and the dose at the boundaries of tumors whereas PB-hete overestimated the dose at the lung-tumor interfaces due to the lack of electronic equilibrium in the regions near low-density tissues and heterogeneous interfaces. In our most recent clinical study (16,17) using a large cohort of lung SBRT patients, the mean PTV dose was overestimated by 15%, on average, when using PB-hete algorithm compared to XVMC using identical beam configurations, MLCs margins, and total number of MUs. Also, the volume of lung receiving 5 Gy, 10 Gy, and 20 Gy was overestimated by about 3.0%, on average, when calculated by PB-hete compared to XVMC.…”

“…Our own initial clinical experience (15,16,17) on validating and implementing iPlan XVMC algorithm using QUASAR (Modus Medical Devices Inc., London, Canada) phantom study has demonstrated an excellent agreement (within ± 2%) between doses calculated using XVMC versus ion chamber measurements for 6 MV-SRS beams in heterogeneous lung equivalent material. In our phantom study, (15) the dose difference between PB-hete and measured value was as large as 9%.…”

“…(11,12,13,14) The peer-reviewed literature has documented the robust experimental validation and clinical implementation of the XVMC algorithm to accurately predict dose delivered to heterogeneous tissue interfaces. (11)(12)(13)(14)(15)(16)(17) Inspired by our initial clinical experience in Monte Carlo-based lung SBRT planning, (15,16,17) we extended the use of XVMC algorithm to tumors in the head and neck regions. The goal of this article is to present comparative dosimetric analysis of our MC-based dose calculation for the head and neck SRT patients as a part of clinical validation of a complex, yet, more accurate algorithm in real patients who underwent head and neck reirradiation.…”