Clinical evaluation of monitor unit software and the application of ...

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a b Frequency Frequency 120 100 80 60 40 20 70 60 50 40 30 20 10 0 0 -11% -10% -11% -10% -9% -8% -7% -6% -5% -4% -3% -2% tient safety regulations because all treatment plans would be subjected to automated independent dose calculations. Ideally, such an automated procedure is followed by a retrospective analysis that continuously scans through the database looking for systematic sources of dose calculation error/uncertainties. This may consequently be an additional QA strategy where small but systematic error/uncertainties, either in the planning calculation or in the independent dose calculation, can be found and possibly also explained and fixed. However, for such an automated QA-procedure it is still necessary to check the treatment plan by an experi- D. Georg et al. / Radiotherapy and Oncology 85 (2007) 306–315 313 All data Deviation (MUV-TPS)/MUV [%] Only radiological depth data -9% -8% -7% -6% -5% -4% -3% -2% -1% 0% 1% 2% 3% Deviation (MUV-TPS)/MUV [%] thorax HN pelvic -1% 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% RD thorax RD HN RD pelvis enced professional in order to avoid hot spots, dose prescription and plan normalisation errors. Independent dose calculations based on export files from TPS do not check correct data file transfer from the TPS to the treatment unit and the actual performance of the treatment unit, which are other sources of error in the radiotherapy chain. However, these aspects could be included by measuring leaf settings and MU measured with an EPID [23]. Moreover, such information could be used as well as input information for an independent dose calculation. Finally, there is also a tangible risk for errors to be introduced by the user(s) during 10% 4% 5% 6% 7% 8% 9% 10% Fig. 3. Treatment site dependent frequency distributions of deviations between dose calculations performed with MUV and the local TPS. (a) Using input data based on either geometric or radiological depth, (b) using input data based on radiological depth only.
314 Clinical evaluation of fluence based MU software the planning and preparation procedure [25], often due to simple human mistakes in combination with inappropriate clinical routines. As long as patient anatomy is not included in verification calculations the accuracy of independent dose checks is influenced by treatment site specific factors. This is the most striking limitation of independent dose calculation procedures that are employed in clinics today. For some treatment areas, such as thorax and head-and-neck, accurate results cannot be achieved in simple calculation conditions, i.e. a semi-infinite homogeneous phantom. With radiological depth corrections, for head-and-neck treatments almost as good results as for pelvic treatment could be achieved (see Fig. 1a and b). Interestingly, the impact of radiological depth correction was limited for pelvic treatments. This might be biased by the class solutions, i.e. beam geometry, applied in the centres participating in the study. Although for thorax fields the agreement between TPS and independent MU calculations was largely improved when using radiological depth information, the overall agreement was inferior compared to the other treatment areas. This can be explained by the overestimated scatter when performing calculations in a water equivalent homogeneous medium and comparing them with calculations accounting for differences in scatter conditions from inhomogeneous media. When defining action levels for any QA process the uncertainties of procedures or calculations, which are used to derive a certain value, need to be considered. A proper selection of action level is a compromise between probabilities of dose deviations outside the clinical tolerance level and workload generated by false alarms. To ensure narrow tolerance levels with an acceptable workload it is important to use a verification tool capable of high accuracy. Further, when the accuracy of the verification tool is dependent on irradiation parameters inherent uncertainty estimation will help optimising the QA-procedure. Unfortunately, uncertainties are not accounted for in an open and sincere manner, even though they are inevitable when aiming to model complex processes such as dose deposition in a patient. A well-known fact concerning computer calculations is that in practice some sort of ‘‘priority-compromise’’ needs to be found between speed and accuracy. In that aspect the importance of presenting also flaws and weaknesses of a dose calculations model by the companies that market TPSs must be emphasized. Such issues are often discussed in a scientific context, but probably not as frequently in the clinical routine work. Because the accuracy of independent dose verification depends on input data (e.g. geometric versus radiological depth), treatment area (e.g. missing tissue), and treatment specific beam data (e.g. beam weight to overall plan, weight of wedge) clinical AL should be defined treatment site and treatment technique specific. Conclusion The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators, such as regular and irregular open beams, physical and dynamic wedges, step-and-shoot and dynamic IMRT. The highly accurate dose calculation model implemented in MUV allows investigating systematic TPS deviations by performing calculations in homogeneous conditions. Information on the uncertainty from any dose calculation algorithm would both enhance and simplify the process of setting clinical action levels that identify cases where the discrepancies between planning and independent dose calculations need follow up. However, further research is needed in that aspect. Based on the current findings treatment site and treatment technique dependent action levels between 3% and 5% seem to be clinically realistic if a radiological depth correction is performed. Acknowledgements This work was partly supported by ESTRO and performed within the framework of the ESQUIRE project and partly supported by the Cancer Research foundation in Northern Sweden. * Corresponding author. Dietmar Georg, Division of Medical Radiation Physics, Department of Radiotherapy, Medical University Vienna/AKH Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria. E-mail address: Dietmar.Georg@akhwien.at Received 13 September 2006; received in revised form 9 March 2007; accepted 24 April 2007; Available online 27 September 2007 References [1] Ahnesjö A, Saxner M, Trepp A. A pencil beam model for photon dose calculation. Med Phys 1992;19:263–73. [2] Brahme A. Dosimetric precision requirements in radiation therapy. Acta Radiol Oncol 1984;23:379–91. [3] Dutreix A, Svensson H, Bjärngard BE, Bridier A, Mijnheer A, Shaw J. Monitor unit calculation for high energy photon beams, Physics for clinical radiotherapy. Booklet No. 3, ESTRO; 1997. [4] Ferreira IH, Dutreix A, Bridier A, Chavaudra J, Svensson H. The ESTRO-QUALity assurance network (EQUAL). Radiother Oncol 2000;55:273–84. [5] Fiorino C, Corletto D, Mangili P, Broggi S, Bonini A, Cattaneo GM, et al. Quality assurance by systematic in vivo dosimetry: results on a large cohort of patients. Radiother Oncol 2000;56:85–95. [6] Georg D, Olofsson J, Künzler T, Karlsson M. On empirical methods to determine scatter factors for irregular MLC shaped beams. Med Phys 2004;31:2222–9. [7] Gillis S, De Wagter C, Bohsung J, Perrin B, Williams P, Mijnheer BJ. An inter-centre quality assurance network for IMRT verification: results of the ESTRO QUASIMODO project. Radiother Oncol 2005;76:340–53. [8] Hansson H, Bjork P, Knöös T, Nilsson P. Verification of a pencil beam based treatment planning system: output factors for open photon beams shaped with MLC or blocks. Phys Med Biol 1999;44:N201–7. [9] Heeren G. The bright but ephemeral life of a rainbow: a chronicle of seventeen years of intensive ESTRO-EU cooperation. Radiother Oncol 2005;75:253–7. [10] IAEA Technical Reports Series No. 430: Commissioning and quality assurance of computerized planning systems for radiation treatment of cancer. International atomic energy agency, IAEA; 2005.
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