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An MLC‐based linac QA procedure for the characterization of radiation isocenter and room lasers’ position
Author(s) -
Rosca Florin,
Lorenz Friedlieb,
Hacker Fred L.,
Chin Lee M.,
Ramakrishren,
Zygmanski Piotr
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2198171
Subject(s) - isocenter , collimator , optics , linear particle accelerator , laser , radiation , medical imaging , physics , dosimetry , radiosurgery , nuclear medicine , materials science , computer science , beam (structure) , radiation therapy , medicine , artificial intelligence , imaging phantom
We have designed and implemented a new stereotactic linac QA test with stereotactic precision. The test is used to characterize gantry sag, couch wobble, cone placement, MLC offsets, and room lasers’ positions relative to the radiation isocenter. Two MLC star patterns, a cone pattern, and the laser line patterns are recorded on the same imaging medium. Phosphor plates are used as imaging medium due to their sensitivity to red light. The red light of room lasers erases some of the irradiation information stored on the phosphor plates enabling accurate and direct measurements for the position of room lasers and radiation isocenter. Using film instead of the phosphor plate as imaging medium is possible, however, it is less practical. The QA method consists of irradiating four phosphor plates that record the gantry sag between the 0° and 180° gantry angles, the position and stability of couch rotational axis, the sag between the 90° and 270° gantry angles, the accuracy of cone placement on the collimator, the MLC offsets from the collimator rotational axis, and the position of laser lines relative to the radiation isocenter. The estimated accuracy of the method is ± 0.2 mm . The observed reproducibility of the method is about ± 0.1 mm . The total irradiation/illumination time is about 10 min per image. Data analysis, including the phosphor plate scanning, takes less than 5 min for each image. The method characterizes the radiation isocenter geometry with the high accuracy required for the stereotactic radiosurgery. In this respect, it is similar to the standard ball test for stereotactic machines. However, due to the usage of the MLC instead of the cross‐hair/ball, it does not depend on the cross‐hair/ball placement errors with respect to the lasers and it provides more information on the mechanical integrity of the linac/couch/laser system. Alternatively, it can be used as a highly accurate QA procedure for the nonstereotactic machines. Noteworthy is its ability to characterize the MLC position accuracy, which is an important factor in IMRT delivery.