PubMed · 4882388
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1968. [Microcurie meter].. https://pubmed.ncbi.nlm.nih.gov/4882388/
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The dosimetric characteristics of three multileaf collimator (MLC) systems (Elekta, Siemens and Varian) having 10 mm leaf width are compared. A 6 MV photon beam was used from each unit for measurements. Film dosimetry was performed for the measurements and the analysis techniques were exactly duplicated in each system. Two of the collimators have rounded leaf ends (Elekta and Varian) and the third (Siemens) has a flat end that follows beam divergence. A scanning densitometer (Wellhöfer with 0.45 mm spot and 0.5 mm step size) was used for film analysis. The dosimetric characteristics studied include: penumbra width (80-20%) as a function of position of the leaf end in the field, inter- and intra-leaf radiation leakage, dose distribution of the tongue and groove, and isodose curves for stepped leaves forming 45 degrees angle beam edge. Results show that MLC designs with divergent and non-divergent leaves produce penumbra (80-20%) widths that are within 2.0 mm of each other. However, the distance of the collimator from the x-ray target plays an important role, and the smallest penumbra width was noted for the Varian MLC despite its rounded leaf-end design. Compared to the other systems, this collimator is positioned about 15 cm closer to the patient which affects the skin dose. The MLC with flat leaf end, although closer to the target, showed slightly poorer penumbra width. Inter-leaf leakage through the leaves is 1.3% for two of the collimators (Elekta and Varian) with the backup jaws and is nearly 1% for the third system (Siemens). The Siemens MLC produces reduced tongue-and-groove effect compared to the other two collimators (Elekta and Varian). The isodose undulation for a stepped edge is found to be significant for the collimator closest to the patient (Varian) and does not depend on the leaf-end shape. There is no perfect MLC system that can be recommended, rather each one has unique advantages and disadvantages that should be weighed with comfort, ease and cost effectiveness for clinical use.
The creation of intricate dose distributions produced by intensity-modulated radiotherapy (IMRT) depends on complex planning systems and specialized mechanical devices. The many possible sources of inaccuracy and the complexity of the dose maps themselves require that a substantial effort be made to ensure that calculated and delivered dose distributions agree. This review provides an overview of the current status of the validation of dose predictions of IMRT planning systems by comparisons with measurements. Emphasis is placed on multileaf collimator- (MLC) based IMRT. Discrepancies between calculations and measurements may be due to any of 3 causes: errors and uncertainties in the dose calculation algorithm, in measurements, or in beam delivery by the accelerator/MLC combination. Some of the factors affecting dosimetry include: the technique employed for modulating the fluence, the dose calculation algorithm and other aspects of the planning system, mechanical limitations of the MLC hardware, dosimetric characteristics of the MLC, such as MLC leakage and rounded leaf ends, the choice of dosimeter, and the measurement geometry and technique. The advantages and drawbacks of various dosimeters including film, ion chambers, thermoluminescent dosimetry, and electronic portal imaging devices are discussed. The steps involved in validating dosimetrically a planning system are outlined, including the various fields that need to be measured, the phantoms that may be used, and measurement techniques. The achievable accuracy of dosimetry for IMRT is discussed.