Dosimetric validation for multileaf collimator-based intensity-modulated radiotherapy: a review.

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.