Abstract
Despite the improvements in the dose calculation models of the commercial treatment planning systems (TPS), their ability to accurately predict patient dose is still limited. One of the limitations is caused by the simplified model of the multileaf collimator (MLC). The aim of this study was to develop a Monte Carlo (MC) method‐based independent patient dose validation system with an elaborate MLC model for more accurate dose evaluation. Varian Clinac 2300 IX was simulated using Geant4 toolkits, after which MC commissioning with measurements was performed to validate the simulation model. A DICOM‐RT interface was developed to obtain the beam delivery conditions including the hundreds of MLC motions. Finally, the TPS dose distributions were compared with the MC dose distributions for water phantom cases and a patient case. Our results show that the TPS overestimated the absolute abutting leakage dose in the closed MLC field, with about 20% more of the maximum dose than that of the MC calculation. For water phantom cases, the dose distributions inside the target region were almost identical with the dose difference of less than 2%, while the dose near the edge of the target shows difference about 10% between Geant4 and TPS due to geometrical differences in MLC model. For the patient analysis, the Geant4 and TPS doses of all organs were matched well within 1.4% of the prescribed dose. However, for organs located in areas with high ratio of leaf pairs with distances less than 10 mm leaf pair (LP (<10mm)), the maximum dose of TPS was overestimated by about 3% of the prescribed dose. These dose comparison results demonstrate that our system for calculating the patient dose is quite accurate. Furthermore, if the MLC sequences in treatment plan have a large ratio of LP (short), more than 3% dose difference in normal tissue could be seen.
Highlights
Volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) are techniques for treating cancer that utilize highly conformal dose distributions generated by multileaf collimator (MLC) motion
The full width at half maximum (FWHM) for the Geant[4], treatment planning systems (TPS), and experimental measurement were 0.65, 0.30, and 0.59 cm, respectively. These results show that the TPS peak is sharper than the experimental measurement and Geant[4] peaks, which implies that the TPS does not properly reflect the abutting leakage dose with the closed field, while that of the measurement and Monte Carlo (MC) were almost identical
The developed system was validated by three processes: MC commissioning of the modeled linac, experimental validation of the modeled MLC, and dose comparison in water between the commercial TPS and Geant[4]
Summary
Volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) are techniques for treating cancer that utilize highly conformal dose distributions generated by multileaf collimator (MLC) motion. The conformity and uncertainty in dose delivery of VMAT/IMRT are sensitive to the structural details of the MLC; accurate MLC modeling is very important for successful patient treatment and for reducing side effects.[1] detailed modeling of the complex MLC geometry to reflect its precise dosimetric properties is challenging in commercial treatment planning systems (TPS).[2] Molineu et al.[3] reported the results of an analysis of a multi‐ institutional IMRT clinical trial using the anthropomorphic head and neck IMRT phantom at the Radiological Physics Center. In the case of dynamic MLC, the dosimetric effect of the radiation transmitted and scattered from the rounded leaf ends can exceed 10% of the total dose.[10,11,12] Even a 1% improvement in dose delivery precision has been reported to increase the cure rate for early stage tumors by 2%.13 a 5% change in dose can result in 10–
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