Abstract
This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%, 24.9% and 27.7% for the MGS, FFTC and Clarkson algorithms, respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%, 4.9% and 6.9% for the MGS, FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%, 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than ±4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within ±7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Metcalfe, P. E., Wong. T. P. Y. and Hoban, P. W.,Radiotherapy X-ray beam inhomogeneity correction: The problem of lateral electronic disequilibrium in lung, Aust. Phys. Eng. Sci. Med. 16(4): 155–167, 1993.
Hoban, P. W., Murray, D. C., Metcalfe, P. E. and Round, W. H.Superposition dose calculation in lung for 10MV photons, Aust. Phys. Eng. Sci. Med. 13(2): 81–92, 1990.
Mackie, T. R., Scrimger, J. W. and Battisra, J. J.,A convolution method of calculating dose for 15 MV x-rays, Med. Phys. 12: 188–196, 1985.
Sharpe, M. B. and Battista, J. J.,Dose calculations using convolution and superposition principles: The orientation of dose spread kernels in divergent x-ray beams, Med. Phys. 20(6): 1685–1694, 1993.
Miften, M., Wiesmeyer, M., Kapur, A. and Ma, C. M. C.,Comparison of RTP dose distributions in heterogeneous phantoms with the BEAM Monte Carlo Simulation system, Jour. of App. Cli. Med. Phys. 2(1): 21–31, 2001.
Dunscombe, P., McGhee, P. and Lederer, E.,Anthropomorphic phantom measurements for the validation of a treatment planning system, Phys. Med. Biol. 41: 399–411, 1996.
Mifte, M., Wiesmeyer, M., Monthofer, S. and Krippner, K.,Implementation of FFT convolution and multigrid superposition models in the FOCUS RTP system, Phys. Med. Biol. 45: 817–833, 2000.
Aspradakis, M. M. and Redpath, A. T.,A technique for the fast calculation of three-dimensional photon dose distributions using the superposition model, Phys. Med. Biol. 42: 1475–1489, 1997.
Metcalfe, M. M., Hoban, P. W., Murry, D. C. and Round, W. H.,Beam hardening of 10 MV radiotherapy x-ray: analysis using a convolution/superposition method, Phys. Med. Biol. 35: 1533–1549, 1990.
Hoban, P. W., Murray, D. C. and Round, W. H.,Photon beam convolution using polyenergetic energy deposition kernels, Med. Biol. 39: 669–685, 1994.
Papanikolaou, N., Mackie, T. R., Meger-Wells, C., Gehring, M. and Reckwerdt P.,Investigation of the convolution method for polyenergetic spectra, Med. Phys. 20: 1327–1336, 1993.
O’Connor, J. E.,The variation of scattered x-rays with density on an irradiated body, Phys. Med. Biol. 1: 352–369, 1957.
Clarkson, J. R.,A note on depth doses in fields of irregular shape. Brit. J. Radiol. 14: 140–144, 1941.
Lewis, R. D., Ryde, S. J. S., Seaby, A. W., Hancock, D. A. and Evans, C. J., Use of Monte Carlo computation in benchmarking radiotherapy treatment planning system algorithms, Phys. Med. Biol. 45: 1755–1764, 2000.
Butson, J. M., Elferink, R., Cheung, T., Yu, P., Stokes, M., Quach, K. Y. and Metcalfe, P.,Verification of lung dose in an anthropomorphic phantom calculated by the collapsed cone convolution method, Phys. Med. Biol. 45: N143-N149, 2000.
Metcalfe, P., Kron, T. and Hoban, P.,Radiotherapy X-Rays Linear Accelerators, Medical Physics Publishing, Madison, Madison, Wisconsin, USA. P: 315, 1997.
Wong, T. P. Y., Metcalfe, P., Kron, T. and Emeleus, T.,Radiotherapy x-ray dose distribution beyond air cavities, Aust. Phys. Eng. Sci. Med. 13 (3): 138–146, 1992.
Klein, E. and Purdy, J.,Entrance and exit dose regions for a Clinac 2100C, Int. J. Radiat. Oncol. Biol. Phys. 27: 429–435, 1993.
Instruction Manual for Markus-chamber type 23343. NE TECHNOLOGY LIMITED, Berkshire RG 75PR England.
Gagnon, W. F. and Horton, J. L.,Physical factors affecting absorbed dose to the skin from cobalt-60 gamma rays and 25-MV x rays, Med. Phys. 6: 285, 1979.
Purdy, J.Build up/surface dose and exit dose measurements for a 6MV linear accelerator, Med. Phys. 13: 259–262, 1986.
Dose Calculation-FFT Convolution and Multigrid Superposition, P37, The document for FOCUS Release 3.0.0., 2000.
Woo, M. K.,Analysis of photon beam exit dose using photon kernels, Phys. Med. Biol. 39: 687–702, 1994.
Tannous, N. B. J., Gagnon, W. F., and Almond, P. R.,Build-up region and skin-dose measurements for the Therac 6 Linear Accelerator for radiation therapy, Med. Phys., 8(3): 378–381, 1981.
Shahine, B. H., Al-Ghazi, M. S. A. L. and El-Khatib, E.Experimental evaluation of interface doses in the presence of air cavities compared with treatment planning algorithms, Med.Phys. 26(3): 350–355, 1999.
Wong, T. P. Y., Metcalfe, P. E. and Chan, C. L.,The effects of low-density media on X-ray dose distribution. Med. Phys. 39(3): 134–143, 1994.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Ding, W., Johnston, P.N., Wong, T.P.Y. et al. Investigation of photon beam models in heterogeneous media of modern radiotherapy. Australas. Phys. Eng. Sci. Med. 27, 39 (2004). https://doi.org/10.1007/BF03178375
Received:
Accepted:
DOI: https://doi.org/10.1007/BF03178375