Abstract
Intensity Modulated Radiation Therapy (IMRT) is widely accepted as an appropriate method to treat tumors at many different anatomic locations including lung. Dose calculation algorithms that have different degrees of accuracy are used to produce clinical IMRT treatment plans. In this study, Monte Carlo (MC) dose calculation was used to evaluate the reliability of plan evaluation parameters compared to a pencil beam (PB) dose calculation for IMRT of the lung.Twenty fi ve lung IMRT cases were randomly selected for analysis. Plan evaluation parameters were calculated using PB and MC methods for the targets and organs at risk (OARs). Comparisons were made using dose-volume histograms, mean dose, and equivalent uniform dose. The following doses-volume histogram points were compared: D98, D95 of the GTV and PTV, V20 and V30 for the lungs, D33 for the heart and esophagus and Dmax for the spinal cord. Mean dose differences were 3.6 ± 2.3% and 4.3 ± 2.8% for the GTV and PTV, respectively. The average EUD differences were 4.1 ± 2.4% for the GTV and 5.7 ± 4.9% for the PTV. Less than 2% differences were observed between the MC and PB algorithms for all OAR plan evaluation parameters. However, minimum and maximum differences for some plan evaluation parameters ranged from about ±20%.There are appreciable differences in plan evaluation parameters between the PB and MC calculations for the targets. The mean dose and EUD have a weak but statistically signifi cant inverse dependence on the number of fi elds, total MU, GTV volume and PTV volume for the targets. There can be large case-to-case differences between PB and MC for both the targets and OARs. Accurate MC calculations can remove those remaining systematic errors from treatment plans compared to PB calculations.
Highlights
It is well known that intensity modulated radiation therapy (IMRT) provides improved target coverage while sparing surrounding healthy tissue compared to three dimensional conformal radiotherapy
Accurate Monte Carlo (MC) calculations can remove those remaining systematic errors from treatment plans compared to pencil beam (PB) calculations
For all the organs at risk (OARs), the mean dose difference between PB and MC vary within an average of 1%, but the mean dose difference in terms of individual patient’s comparison shows a minimum dose difference as low as -2.6% in the case of esophagus and up to 2.8% in the case of the ipsilateral lung
Summary
It is well known that intensity modulated radiation therapy (IMRT) provides improved target coverage while sparing surrounding healthy tissue compared to three dimensional conformal radiotherapy. Monte Carlo and pencil beam (PB) IMRT dose calculations have been previously compared for prostate and head and neck patients (Ma et al, 1999; Wang et al, 2002; Boudreau et al, 2005; Yang et al, 2005). It has been shown that pencil beam calculations overestimate the dose by 5-10% compared to MC calculations and measurements in slab lung phantoms (Boyer and Mok, 1985; Metcalfe et al, 1993; Butson et al., 2000; Charland et al, 2003; Carrasco et al, 2004; Krieger and Sauer, 2005; Paelinck et al, 2005; Blazy et al, 2006). Similar results were found in anthropomorphic thorax phantom studies (Laub et al, 2001; Ma et al, 2003; McDermott et al, 2003; Davidson et al, 2007)
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