Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators

Abstract

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

Intensity modulated radiotherapy dosimetry with ion chambers, TLD, MOSFET and EDR2 film

“…The dosimetric accuracy of IMRT treatment planning systems for single fields has been reported to be better than 3% or 3 mm. 19 A study 20 using the MIMiC multileaf collimator, has shown that direct dose measurements at the isocenter agreed with the calculations within +3%. Recent studies have attempted to address the accuracy of calculated dose for the total plan transferred to a phantom compared with measurements for a series of patients.…”

A Review of intensity modulated radiation therapy: Incorporating a report on the seventh education workshop of the ACPSEM — ACT/NSW branch

“…The commissioning and quality assurance of IMRT and its associated treatment verification methods, have been the subject of numerous publications [55][56][57][58][59][60][61][62] . The main focus of the workshop was conventional MLC based IMRT, and the various delivery methods (segmental MLC and dynamic MLC).…”

Characterization and evaluation of an integrated quality monitoring system for online quality assurance of external beam radiation therapy

“…Intensity modulated radiation therapy (IMRT),1 tomotherapy,2 image‐guided radiation therapy (IGRT),3, 4 volumetric‐modulated arc therapy (VMAT),5, 6 and small field treatments utilized in SRT and SBRT are examples of radiation delivery practice with more intricate work flow than “conventional” radiation therapy. Due to this increased complexity, new quality assessment (QA) strategies have been developed, including patient‐specific dose verification 7, 8, 9, 10. This patient‐specific QA measurement is only performed once for a treatment course that can include as many as 44 sessions.…”