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S. Plaude et al. / Medical Physics in the Baltic States 7 (2009) 52 - 55 5. References 1. Escude L., Linero D., Molla M. et al. Quality assurance for radiotherapy in prostate cancer: point dose measurements in intensity modulated fields with large dose gradients. Int J Radiat Oncol Biol Phys, 66, 2006. p. 136 – 140. 2. Moran J. M., Radawski J., Fraass B. A. A dose -gradient analysis tool for IMRT QA. J of Applied Clin Med Phys, Vol. 6, No. 2, 2005. p. 62 – 73 3. González – Castaño D., Pena J., Sánchez – Doblado F. et al. The change of response of ionization chambers in the penumbra and transmission regions: impact for IMRT verification. Med Biol comput. 46, 2008. p. 373 – 380. Fig. 5. Comparison of Farmer and Semiflex chamber responses. 55 4. Sánchez Doblado F., Hartmann G. H, Pena J. et al. Uncertainty estimation in intensity – modulated radiotherapy absolute dosimetry verification. Int. J. Radiation Oncology Biol. Phys., Vol. 68, No 1, 2007. p. 301 – 310. 5. Sánchez – Doblado F., Capote R., Rosello J. V. et al. Micro ionisation chamber dosimetry in IMRT verification: Clinical implications of dosimetric errors in the PTV. Radiat Oncol, 75, 2005. p. 324 – 348. 6. Kawamorita R., Iwai K., Takeushi Y. et al. Examination of Dose Verification in Intensity Modulated Radiation Therapy (IMRT) Treatment Planning Using Averaging Dose in Chamber Volume. Jpn J Radiol Technol 58 (6), 2002. p. 783-792. 7. Technical Data, PTW – Ionization chambers, PTW Freiburg.
MEDICAL PHYSICS IN THE BALTIC STATES 7 (2009) Proceedings of International Conference “Medical Physics 2009” 8 - 10 October 2009, Kaunas, Lithuania MODELING OF FIELD PARAMETERS FOR DOSE VERIFICATION IN EXTERNAL BEAM RADIOTHERAPY Arturs MEIJERS*, Sergey POPOV* * Latvian Oncology Centre, Riga Eastern Clinical University Hospital, Riga, Latvia, LV1079 Abstract: In radiation therapy it is necessary to perform manual dose verification for each treatment plan. Several dosimetric parameters are dependent on treatment field size, however usually they are measured only for square fields. Therefore method allowing approximation of irregularly shaped field with equivalent square is necessary. Methods: Spreadsheet based equivalent square calculation tool based on Clarkson integration method was developed. Results: Basic verification tests suggested that by using developed method equivalent square can be determined with 1.32 % error; such error would result in calculation of treatment field monitor units with 0.042 % error. Conclusions: Developed tool is easy to implement and use for manual treatment plan verifications. Keywords: equivalent square; manual dose verification 1. Introduction According to international approach and recommendations, it is necessary to perform manual treatment plan verification for each patient. Manual monitor units (MU) verifications most commonly are done by using point dose calculation methods, such as isocentric method, SSD method, Clarkson’s method etc. For example, SSD method allows calculating point dose by following equation [1]: where: K – calibration index of accelerator rc – collimator field size Sc – collimator scatter factor Sp – phantom scatter factor TD – tumor dose (1) Parameters of linear accelerator (linac) are mostly dependant on beam energy, field size and depth. Parameters dependant on field size are usually acquired for square fields, however such fields are rarely used for treatment delivery. More significant are irregularly shaped fields. However, since beam parameters are acquired for square fields, it is necessary to approximate irregularly shaped fields with square field. Therefore concept of equivalent square (EQSQ) was introduced. Development of simple and easy-to-use EQSQ calculation method is significant for manual treatment plan verification. Since previously widely used dose calculation algorithms, such as pencil beam convolution 56 algorithm or PBC, had implemented EQSQ concept, it was possible to acquire this parameter directly from treatment planning system (TPS) reports, however analytical algorithms, such as anisotropic analytical algorithm or AAA, do not use EQSQ for dose calculation. Therefore, to perform manual plan verifications alternative EQSQ calculation method is necessary. 2. Methodology EQSQ can be defined by geometric or dosimetric approach. Dosimetric approach takes into account physical characteristics of the beam, therefore it is considered more precise. Let us consider square field equivalent to irregularly shaped field if percentage depth dose (PDD) of square field at reference depth is equivalent to the PDD of irregular field. EQSQ calculation method is based on Clarkson’s integration method. However Clarkson’s method is based on use of parameters such as scatter air ratio (SAR) and tissue air ratio (TAR). Since acquisition of these parameters require additional measurements, scatter maximum ratio (SMR) and tissue maximum ratio (TMR), which are special cases of scatter phantom ratio (SPR) and tissue phantom ratio (TPR) respectively, were used instead. SMR and TMR can be calculated from depth dose (PDD) curves by method described in British Journal of Radiology, 1996, Supplement No. 25 [2]. Calculation is done by using peak scatter factor (PSF). Total SMR of irregularly shaped field can be determined by following steps:
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