An analytical method to calculate equivalent fields to irregular symmetric and asymmetric photon fields.
Equivalent field is frequently used for central axis depth-dose calculations of rectangular- and irregular-shaped photon beams. As most of the proposed models to calculate the equivalent square field are dosimetry based, a simple physical-based method to calculate the equivalent square field size was used as the basis of this study. The table of the sides of the equivalent square or rectangular fields was constructed and then compared with the well-known tables by BJR and Venselaar, et al. with the average relative error percentage of 2.5 ± 2.5% and 1.5 ± 1.5%, respectively. To evaluate the accuracy of this method, the percentage depth doses (PDDs) were measured for some special irregular symmetric and asymmetric treatment fields and their equivalent squares for Siemens Primus Plus linear accelerator for both energies, 6 and 18MV. The mean relative differences of PDDs measurement for these fields and their equivalent square was approximately 1% or less. As a result, this method can be employed to calculate equivalent field not only for rectangular fields but also for any irregular symmetric or asymmetric field.
Department of Radiation Therapy, Golestan Hospital, JondiShapour University of Medical Science, Ahvaz, Iran; Department of Medical Physics, JondiShapour University of Medical Sciences, Ahvaz, Iran.
Mohamad J
MJ
Tahmasebi Birgani
Department of Medical Physics, JondiShapour University of Medical Sciences, Ahvaz, Iran. Electronic address: nchegen@yahoo.com.
Nahid
N
Chegeni
Department of Medical Physics, JondiShapour University of Medical Sciences, Ahvaz, Iran.
Mansoor
M
Zabihzadeh
Department of Medical Physics, JondiShapour University of Medical Sciences, Ahvaz, Iran.
Nima
N
Hamzian
D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D009369:Neoplasms / Q000503:physiopathology* / Q000532:radiotherapy*; D017785:Photons / Q000627:therapeutic use; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
Asymmetric field; Equivalent field; Irregular field; Multileaf collimators