Abstract
A patient specific quality assurance program has been developed to facilitate the clinical implementation of intensity modulated radiotherapy (IMRT) delivered using a micro‐multileaf collimator. The methodology includes several dosimetric tasks that are performed prior to the treatment of each patient. Film dosimetry is performed for each individual field and for the multifield composite plan. Individual field measurements are performed at a depth of 5 cm in a water equivalent slab phantom; export of dose calculations from the treatment planning system is similarly specified. For the composite distribution, parameters from the patient plan are applied to an IMRT phantom, and film is exposed in an axial orientation. Distributions are compared with the aid of software developed for the specific tasks. The measured and calculated dose distributions can be superimposed and positioned graphically using move, rotate, and mirror tools, as well as by specifying isocenter coordinates and using fiducial marks. Horizontal and vertical profiles are available for analysis. Dose difference, distance‐to‐agreement, and γ index, the minimum scaled multidimensional distance between a measurement and a calculation point determined in combined dose and physical distance space, are calculated along a specified isodose line and displayed. γ provides an excellent measure of disagreement between measurement and calculation for complex intensity distributions. We specify 3% dose difference and 3 mm distance as our scaling acceptability criteria. Absolute dosimetry for each composite plan is performed using an ionization chamber. To date, excellent agreement between measurements and calculations has been observed. © 2003 American College of Medical Physics.PACS number(s): 87.53.–j, 87.66.–a
Highlights
The concept of conforming the dose in three-dimensions is not new; the use of compensators, wedges, and dynamic asymmetric jaws is targeted toward that process
A number of studies have demonstrated the superiority of the physical dose distribution of IMRT compared to other modalities, with applications in brain tumors, head and neck cancers, and prostate cancer treatments.[1,2,3,4,5]
We have developed a patient specific QA protocol for IMRT patient pretreatment verification
Summary
The concept of conforming the dose in three-dimensions is not new; the use of compensators, wedges, and dynamic asymmetric jaws is targeted toward that process. A number of studies have demonstrated the superiority of the physical dose distribution of IMRT compared to other modalities, with applications in brain tumors, head and neck cancers, and prostate cancer treatments.[1,2,3,4,5] The reason and rationale for the effort put in this area is to reduce the risk or severity of complications where radiotherapy is successful, and to escalate the dose while reducing or keeping a comparable level of complications, where the failure of radiation therapy is the lack of local control.[6,7,8,9,10] Conventional devices, such as wedges and compensators, modulate the intensity of a beam, and subsequently the energy fluence, such that all points in a field are continuously irradiated, .
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