Dosimetric evaluation of a commercial 3-D treatment planning system using Report 55 by AAPM Task Group 23

Background and purpose: A relevant part of radiotherapy treatment planning system QA concerns dose calculation verification. Report 55 by AAPM TG-23 i s an instrument for performing dosimetric evaluation of treatment planning systems in case of external photon beams. It was employed by different grou ps in three radiotherapy departments for controlling performances of RTPS C adPlan 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 config ured, the whole set of TG23 tests were carried out on three different syste ms. According to Report 55, comparisons between values measured by TG-23 an d calculated by RTPS were made in terms of dose at selected points and radi ological field width at different depths. Results: As far as dose is concerned, 266 data were compared for 4 MV and 2 97 for 18 MV. Ninety-five-point-nine percent of dose deviations for 4 MV an d 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 bl ock. Poor results were also observed in the rectangular field 25 x 5, in po ints outside the held edges under collimators. As regards radiological fiel d width, 58 out of 64 comparisons for 4 MV occurred in the range +/-2 mm. F or 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 c onfiguring and using a RTPS. (C) 1999 Elsevier Science Ireland Ltd, All rig hts reserved.