PubMed · 5272718
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C Takei. 1970. [Transmitting ionization dosimeter].. https://pubmed.ncbi.nlm.nih.gov/5272718/
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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.
BACKGROUND AND PURPOSE: A relevant part of radiotherapy treatment planning system QA concerns dose calculation verification. Report 55 by AAPM TG-23 is an instrument for performing dosimetric evaluation of treatment planning systems in case of external photon beams. It was employed by different groups in three radiotherapy departments for controlling performances of RTPS CadPlan Varian-Dosetek, versions 2.7.9, 3.0.6 and 3.1.1. MATERIALS AND METHODS: Once the basic data of the AAPM 4 MV and 18 MV X-ray units had been converted into the CadPlan format and the AAPM units configured, the whole set of TG23 tests were carried out on three different systems. According to Report 55, comparisons between values measured by TG-23 and calculated by RTPS were made in terms of dose at selected points and radiological field width at different depths. RESULTS: As far as dose is concerned, 266 data were compared for 4 MV and 297 for 18 MV. Ninety-five-point-nine percent of dose deviations for 4 MV and 92.6% for 18 MV are less than 2%. Most of the relevant discrepancies for both energies occur in a test case where dose has to be calculated under a long narrow block centred on the beam axis. Deviations as much as 6.1% for 4 MV and -7.5% for 18 MV were observed in points at 1 cm depth under the block. Poor results were also observed in the rectangular field 25 x 5, in points outside the field edges under collimators. As regards radiological field width, 58 out of 64 comparisons for 4 MV occurred in the range +/- 2 mm. For 18 MV the biggest deviation was -2.2 mm. CONCLUSIONS: The TG-23 tests demonstrated that the accuracy of the RTPS in dose calculation is good in most of the typical radiotherapy applications. Our results are better than those recently published for other RTPS. The TG-23 package turned out to be an effective instrument for QA and calculation verification, as well as being a powerful method for training purpose in configuring and using a RTPS.