International Journal of Radiation Oncology*Biology*Physics
Physics contributionMonte Carlo–based dosimetry of head-and-neck patients treated with SIB-IMRT
Introduction
Intensity-modulated radiation therapy (IMRT) has taken on a significant role in radiotherapy because of its ability to deliver radiation conformally around structures in a highly heterogeneous medium. Head-and-neck (HN) cancer patients are well suited to benefit from IMRT’s conformality, because the tumor volumes are often irregularly shaped and are directly adjacent to many critical structures. Studies have shown that compared with 3D conformal planning, IMRT is not only more conformal, but it results in a better dose homogeneity and a sharper dose gradient at the boundaries (1, 2, 3, 4, 5, 6). Furthermore, IMRT planning techniques such as simultaneous integrated boost (SIB) can increase the conformality of treatment, shorten the overall treatment course, and limit the growth of aggressive diseases such as HN squamous cell carcinoma (6, 7, 8, 9, 10, 11).
Several studies document the dosimetric benefits of IMRT combined with SIB techniques (12, 13). Previously, our institution reported on the dosimetric analysis of a group of HN patients treated with SIB-IMRT (11, 13). The dosimetric calculations in that report were based upon superposition/convolution (SC) algorithms with the intensity modulation incorporated into the dose calculation via a fluence transmission matrix. This method of including intensity modulation is similar to that implemented in commercial treatment planning systems (TPS) (14, 15, 16, 17), although, instead of SC dose computation algorithms, pencil beam–based dose computation algorithms are more commonly used (18, 19, 20). Although beam-by-beam film-based quality assurance in a homogeneous phantom agreed with predictions using commonly accepted criteria in our previous study, discrepancies between the planned and delivered dose distributions were observed. However, the clinical impacts of these dose differences were not evaluated.
Intensity-modulated radiation therapy treatment fields, and SIB-IMRT treatment fields in particular, often have large intensity fluctuations that result in complex multileaf collimator (MLC) patterns and present challenges to dose calculation algorithms, because of the effects of radiation transmitted through and scattered from the MLC (21). A possible way to improve dose calculation accuracy is to use an algorithm such as Monte Carlo (MC), which can explicitly account for MLC scatter and leakage radiation during the dose calculation simulation. When compared to measurements, MC dose algorithms provide more accurate estimate of a dose delivered to a patient and, consequently, have been used to evaluate dose calculation accuracy (22, 23, 24, 25, 26, 27, 28, 29).
Wang et al. (29) investigated the use of MC to evaluate dosimetric effects of inhomogeneities for 5 clinical HN IMRT plans and 5 lung IMRT plans. A measurement-based pencil beam algorithm with an equivalent path length inhomogeneity correction was compared with MC, with the fluence modulation incorporated into the MC using the treatment planning system’s effective MLC transmission matrix. Although the majority of the dose indices in this study agreed well, 2 of the 10 plans showed dose indices with deviations >5%.
Leal et al. (24) studied the use of MC for IMRT verification. Step-and-shoot IMRT plans were calculated using the Plato treatment planning system (Veenendall, Netherlands) and compared with EGS4-based MC calculations and film dosimetry for 3 cases. Major differences were found in heterogeneous situations such as head and neck.
Francescon et al. (22) compared step-and-shoot IMRT dose distributions calculated using the Pinnacle3 TPS (Philips Medical Systems, Milpitas, CA) collapsed cone convolution algorithm with EGS4-based MC calculations for two plans. In the MC algorithms, particles were simulated through the MLC using the BEAM (30) MLC component module, which simplifies the MLC leaf geometry by ignoring MLC interleaf transmission. Isocenter point dose differences of 2.1% for a prostate plan and 2.9% for a head-and-neck plan were observed, and dose–volume histogram (DVH) data comparisons showed deviations of up to 6% for doses below 85% of the prescribed dose and higher deviations for doses above 85% of the prescription dose.
A study by Ma et al. (26) compared IMRT plans from Corvus TPS (Corvus, Nomos Corp., Sewickley, PA) with MC for two patient plans. In the MC simulation, an independently developed effective MLC fluence transmission matrix was used to incorporate intensity modulation into the MC dose calculation. Leaf leakage radiation was included as an empiric correction term to the model. Corvus’s finite-size pencil beam algorithm–calculated doses were found to have discrepancies of >5% for target structures and over 20% for critical structures, compared with the MC results.
The goal of this study was to evaluate the accuracy of previously reported SIB-IMRT dose distributions (11, 13) that were originally computed using SC by comparing results with detailed MC dose computations. To ensure accurate characterization of the radiation transmitted through the MLC during IMRT delivery, the intensity modulation was incorporated into the MC dose calculation using an algorithm that tracked particles through the moving MLC leaves (27). As a result, the MC fully simulated the effect of the MLC on treatment delivery and on radiation transport through the patient. The results of this paper will allow quantitative assessment of the dosimetric accuracy of previously reported SIB-IMRT treatments.
Section snippets
Methods and materials
Thirty-one SIB-IMRT plans from 24 different patients who participated in our institutional review board–approved locally advanced phase I/II head-and-neck squamous cell carcinoma protocol were used in this study (11). Details of the plans used for treatment are covered in detail elsewhere (11, 13), but are summarized here for completeness. All plans were created with 9 6-MV equally spaced coplanar beams, with the exception of Plan 28 (Patient 21), in which the location of the target structure
Results
Film measurements from 7 patient plans were analyzed and are summarized in Fig. 1. The numbers given in the figure are the percentage of points with gamma ≤1 with a 2% max dose and 2 mm distance-to-agreement distance criteria. The arithmetic average of all 7 plans with percentage of points passing gamma test is 76.6% with a range of 65.9–95.8% for SC and 95.4% with a range of 91.9–97.2% for the MC. This result demonstrates that our MC implementation better predicts in-phantom doses and can be
Discussion
This study shows that MC predicts higher doses than the SC for the dose calculation methods used for this patient cohort. Film dosimetry results confirm that the MC results can be considered to be a benchmark standard. The correlation between phantom dosimetry results and patient results is demonstrated in Fig. 7, which shows the average dose difference with respect to film dosimetry (MC − SC) observed for points with doses >10% of Dmax dose for the phantom calculations and the local percent
Conclusions
Monte Carlo was used to recompute dose distributions for 31 SIB-IMRT plans based on 24 different patients. On phantom, measurements demonstrated that MC dose calculations agree better with film than the SC algorithm as implemented using our in-house IMRT system, justifying its use as reference standard for patient-based dose computations. In the patient geometry, the SC algorithm results from the VCU-IMRT system agreed, on average, with the MC-based algorithm. Over all plans, the arithmetic
Acknowledgment
The authors would like to thank Devon Murphy-Stein for her meticulous editing of this manuscript.
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Measurement-based Monte Carlo simulation of high definition dose evaluation for nasopharyngeal cancer patients treated by using intensity modulated radiation therapy
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2014, Radiation Physics and ChemistryCitation Excerpt :Table 1 shows that absorbed dose to the middle ears obtained from MBMC is, on average, higher than that from TPS. Sakthi et al. (2006) studied 31 SIB IMRT plans and showed that MC calculations predicted higher doses than the TPS computations. Also, measured doses agreed better with MC results than TPS data.
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2007, Seminars in Radiation OncologyCitation Excerpt :Because RAIR reduces setup error and margins around structures decreases, the importance of dose-calculation accuracy increases because dose errors, which often occur in the periphery of an irradiated volume, will now reside more closely to the clinical target volume as it moves through the course of therapy. One reason for using MC for RAIR is that MC dose calculation is the benchmark standard by which other dose computation algorithms are measured.94-102 For intensity-modulated radiation therapy (IMRT) and RAIR, MC can accurately account for both details of the fluence delivery (via direct transport of particles through moving MLC segments) and patient tissue heterogeneities.103,104
Dosimetric and Monte Carlo verification of jaws-only IMRT plans calculated by the Collapsed Cone Convolution algorithm for head and neck cancers
2019, Reports of Practical Oncology and RadiotherapyCitation Excerpt :MC predicts about a 4.2% higher mean dose to the parotid glands as compared to TPS. This also accords with Sakthi's (2006) observations.12 As shown in Fig. 6, a good agreement between the experimental measurements, the TPS and Monte Carlo simulation of PDD curves along the central beam axis was achieved.
This work was supported by Grant CA98524 from the National Cancer Institute.
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Rupert Schmidt-Ullrich, M.D., passed away in December 2004.