Abstract
PurposeOur purposes are to compare the accuracy of RaySearch's analytical pencil beam (APB) and Monte Carlo (MC) algorithms for clinical proton therapy and to present clinical validation data using a novel animal tissue lung phantom.MethodsWe constructed a realistic lung phantom composed of a rack of lamb resting on a stack of rectangular natural cork slabs simulating lung tissue. The tumor was simulated using 70% lean ground lamb meat inserted in a spherical hole with diameter 40 ± 5 mm carved into the cork slabs. A single‐field plan using an anterior beam and a two‐field plan using two anterior‐oblique beams were delivered to the phantom. Ion chamber array measurements were taken medial and distal to the tumor. Measured doses were compared with calculated RayStation APB and MC calculated doses.ResultsOur lung phantom enabled measurements with the MatriXX PT at multiple depths in the phantom. Using the MC calculations, the 3%/3 mm gamma index pass rates, comparing measured with calculated doses, for the distal planes were 74.5% and 85.3% for the APB and 99.1% and 92% for the MC algorithms. The measured data revealed up to 46% and 30% underdosing within the distal regions of the target volume for the single and the two field plans when APB calculations are used. These discrepancies reduced to less than 18% and 7% respectively using the MC calculations.ConclusionsRaySearch Laboratories' Monte Carlo dose calculation algorithm is superior to the pencil‐beam algorithm for lung targets. Clinicians relying on the analytical pencil‐beam algorithm should be aware of its pitfalls for this site and verify dose prior to delivery. We conclude that the RayStation MC algorithm is reliable and more accurate than the APB algorithm for lung targets and therefore should be used to plan proton therapy for patients with lung cancer.
Highlights
Lung and bronchus is the most common cancer site in 16 states and second overall in the United States, with only 12% fewer estimated cases than female breast.[1]
We hypothesize that the error introduced by the infinite‐slab‐approximation in the analytical pencil beam (APB) algorithm was the cause of the increasing discrepancy between APB and Monte Carlo (MC) as the beam traversed the lung tumor
This phantom enabled us to determine the errors resulting from using an analytical pencil beam algorithm for lung targets
Summary
Lung and bronchus is the most common cancer site in 16 states and second overall in the United States, with only 12% fewer estimated cases than female breast.[1]. The APB algorithm sometimes suffers loss of accuracy in areas of inhomogeneity, such as in lung, where even small algorithmic deficiencies can result in significant shifts of dose distributions.[10]. It has been shown by Taylor et al that the APB algorithm is “doing a poor job” of predicting dose in lung tumors, with over‐predictions up to 46% in the PTV.[11]. MC is viewed as the gold standard for dose calculation for most radiation transport calculations, but traditional MC algorithms such as MCNPX15,16 and Geant[416] are too slow to keep up with clinical workflow To combat this problem, Fast Monte Carlo (FMC) dose engines have been developed for clinical use.[10,17]. They found that FMC corresponded well with data measured in an inhomogeneous phantom made of water surrounding a long insert of bone tissue substitute, whereas more than half of the APB dose distributions failed gamma‐index analysis.[16]
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