Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning J. E. Cygler, a) G. M. Daskalov, and G. H. Chan b) Medical Physics Department, Ottawa Regional Cancer Centre, 503 Smyth Road, Ottawa, Ontario K1H 1C4, Canada G. X. Ding Clinical Physics, Fraser Valley Cancer Centre, British Columbia Cancer Agency, 13750-96th Avenue, Surrey, British Columbia V3V 1Z2, Canada Received 7 August 2003; revised 17 October 2003; accepted for publication 21 October 2003; published 22 December 2003 The purpose of this study is to perform a clinical evaluation of the first commercial MDS Nordion, now Nucletron treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow’s VMC ++ voxel-based Monte Carlo cal- culation algorithm. The accuracy of the dose distribution calculations is evaluated by direct com- parisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances SSDs and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calcula- tions for homogeneous water phantom at two different SSDs. All electron beams in the range 6 –20 MeV are from a Siemens KD-2 linear accelerator. We used 10 000 or 50 000 histories/cm 2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10 000 histories/cm 2 ) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 1010 cm 2 field size, incident on the phantom 151510 cm 3 consisting of 31 CT slices and voxels size of 3 3 3 mm 3 total of 486 720 voxels. We find good agreement discrepancies smaller than 5% for most of the tested dose distributions. We also find excellent agreement discrepancies of 2.5% or less for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors ( 5%) occur when large voxel sizes interfere with the extensions of heteroge- neities or dose gradients because of differences between the experimental and calculated geom- etries. Therefore, user control over voxelization is important for high accuracy electron dose calculations. © 2004 American Association of Physicists in Medicine. DOI: 10.1118/1.1633105 Key words: electron beam, Monte Carlo treatment planning, inhomogeneous phantoms, monitor unit calculations I. INTRODUCTION Recently, a Monte Carlo dose calculation for electron beams has been released commercially to cancer clinics and is be- coming a routine tool used for radiotherapy patients. This is a significant advance in radiotherapy, since it is widely ac- cepted that Monte Carlo simulation of the radiation transport is one of the most accurate methods for calculating absorbed dose distributions in radiation therapy. 1–4 No other existing algorithm can handle accurately the dose from electron beams in a variety of frequently met clinical situations such as scatter perturbations by air cavities or other heterogene- ities, as well as the backscatter from high-density materials such as bone. 1–7 Some of the current computational algo- rithms used in commercial planning systems can give errors in calculated dose as high as 20%. 5–7 Calculations of monitor units for nonstandard situations such as at extended source- to-surface distances SSDs require the user to create a sepa- rate virtual machine for each SSD to get reasonable results. Application of custom-made cutouts or other beam field- shaping devices often require extensive and time-consuming measurements of the output factors in order to confirm the estimates of the treatment planning system. In the present study we have followed the guidelines of Van Dyk et al. 8 and have evaluated the accuracy of the dose calculations by the new commercial Monte Carlo based treatment planning sys- tem for electron beams, recently released by MDS Nordion, now Nucletron. We have also verified the monitor unit cal- culations of the new system at two SSDs and benchmarked the speed of dose calculations under various conditions. II. MATERIALS AND METHODS II.A. Electron beam calculation algorithm The new Monte Carlo algorithm for electron beams is implemented in the Dose Calculation Module DCM, which 142 142 Med. Phys. 31 „1…, January 2004 0094-2405Õ2004Õ31„1…Õ142Õ12Õ$22.00 © 2004 Am. Assoc. Phys. Med.