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Title: SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields

Abstract

Purpose: It is common practice to tabulate dosimetric data like output factors, scatter factors and detector signal correction factors for a set of square fields. In order to get the data for an arbitrary field, it is mapped to an equivalent square, having the same scatter as the field of interest. For rectangular fields both, tabulated data and empiric formula exist. We tested the applicability of such rules for very small fields. Methods: Using the Monte-Carlo method (EGSnrc-doseRZ), the dose to a point in 10cm depth in water was calculated for cylindrical impinging fluence distributions. Radii were from 0.5mm to 11.5mm with 1mm thickness of the rings. Different photon energies were investigated. With these data a matrix was constructed assigning the amount of dose to the field center to each matrix element. By summing up the elements belonging to a certain field, the dose for an arbitrary point in 10cm depth could be determined. This was done for rectangles up to 21mm side length. Comparing the dose to square field results, equivalent squares could be assigned. The results were compared to using the geometrical mean and the 4Xperimeter/area rule. Results: For side length differences less than 2mm, the difference betweenmore » all methods was in general less than 0.2mm. For more elongated fields, relevant differences of more than 1mm and up to 3mm for the fields investigated occurred. The mean square side length calculated from both empiric formulas fitted much better, deviating hardly more than 1mm and for the very elongated fields only. Conclusion: For small rectangular photon fields, deviating only moderately from square both investigated empiric methods are sufficiently accurate. As the deviations often differ regarding their sign, using the mean improves the accuracy and the useable elongation range. For ratios larger than 2, Monte-Carlo generated data are recommended. SW is funded by Deutsche Forschungsgemeinschaft (SA481/10-1)« less

Authors:
; ;  [1]
  1. University of Wuerzburg, Wuerzburg (Germany)
Publication Date:
OSTI Identifier:
22649004
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; CYLINDRICAL CONFIGURATION; MATRICES; MONTE CARLO METHOD; PHOTONS; RADIATION DOSES; RADIOTHERAPY

Citation Formats

Sauer, OA, Wegener, S, and Exner, F. SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields. United States: N. p., 2016. Web. doi:10.1118/1.4956593.
Sauer, OA, Wegener, S, & Exner, F. SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields. United States. doi:10.1118/1.4956593.
Sauer, OA, Wegener, S, and Exner, F. 2016. "SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields". United States. doi:10.1118/1.4956593.
@article{osti_22649004,
title = {SU-F-T-408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields},
author = {Sauer, OA and Wegener, S and Exner, F},
abstractNote = {Purpose: It is common practice to tabulate dosimetric data like output factors, scatter factors and detector signal correction factors for a set of square fields. In order to get the data for an arbitrary field, it is mapped to an equivalent square, having the same scatter as the field of interest. For rectangular fields both, tabulated data and empiric formula exist. We tested the applicability of such rules for very small fields. Methods: Using the Monte-Carlo method (EGSnrc-doseRZ), the dose to a point in 10cm depth in water was calculated for cylindrical impinging fluence distributions. Radii were from 0.5mm to 11.5mm with 1mm thickness of the rings. Different photon energies were investigated. With these data a matrix was constructed assigning the amount of dose to the field center to each matrix element. By summing up the elements belonging to a certain field, the dose for an arbitrary point in 10cm depth could be determined. This was done for rectangles up to 21mm side length. Comparing the dose to square field results, equivalent squares could be assigned. The results were compared to using the geometrical mean and the 4Xperimeter/area rule. Results: For side length differences less than 2mm, the difference between all methods was in general less than 0.2mm. For more elongated fields, relevant differences of more than 1mm and up to 3mm for the fields investigated occurred. The mean square side length calculated from both empiric formulas fitted much better, deviating hardly more than 1mm and for the very elongated fields only. Conclusion: For small rectangular photon fields, deviating only moderately from square both investigated empiric methods are sufficiently accurate. As the deviations often differ regarding their sign, using the mean improves the accuracy and the useable elongation range. For ratios larger than 2, Monte-Carlo generated data are recommended. SW is funded by Deutsche Forschungsgemeinschaft (SA481/10-1)},
doi = {10.1118/1.4956593},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • In an attempt to find the most suitable method for determining the equivalent square for two commonly employed blocked field arrangements, a comparison of measured outputs was made for four different treatment machines. The machines varied by manufacturer, design, and energy. The fields in question were a midline-blocked field and a half-blocked field. The data obtained were used to streamline calculations made by technologists for the original treatment setting. It was further used to enhance the accuracy of computer-generated verification of the original setting. The authors recommend that each institution employing such fields consider obtaining similar data for their ownmore » treatment machines.« less
  • Small field dosimetry measurements including output factors are difficult due to lack of charged-particle equilibrium, occlusion of the radiation source, the finite size of detectors, and non-water equivalence of detector components. With available detectors significant variations could be measured that will lead to incorrect delivered dose to patients. IAEA/AAPM have provided a framework and formulation to correct the detector response in small photon fields. Monte Carlo derived correction factors for some commonly used small field detectors are now available, however validation has not been performed prior to this study. An Exradin A16 chamber, EDGE detector and SFD detector were usedmore » to perform the output factor measurement for a series of conical fields (5–30mm) on a Varian iX linear accelerator. Discrepancies up to 20%, 10% and 6% were observed for 5, 7.5 and 10 mm cones between the initial output factors measured by the EDGE detector and the A16 ion chamber, while the discrepancies for the conical fields larger than 10 mm were less than 4%. After the application of the correction, the output factors agree well with each other to within 1%. Caution is needed when determining the output factors for small photon fields, especially for fields 10 mm in diameter or smaller. More than one type of detector should be used, each with proper corrections applied to the measurement results. It is concluded that with the application of correction factors to appropriately chosen detectors, output can be measured accurately for small fields.« less
  • Purpose: To calculate using Monte-Carlo the intermediate and total correction factors (CFs) for two microchambers and a plastic scintillator for composite fields delivered by the CyberKnife system. Methods: A linac model was created in BEAMnrc by matching percentage depth dose (PDD) curves and output factors (OFs) measured using an A16 microchamber with Monte Carlo calculations performed in egs-chamber to explicitly model detector response. Intermediate CFs were determined for the A16 and A26 microchambers and the W1 plastic scintillator in fourteen different composite fields inside a solid water phantom. Seven of these fields used a 5 mm diameter collimator; the remainingmore » fields employed a 7.5 mm collimator but were otherwise identical to the first seven. Intermediate CFs are reported relative to the respective CF for a 60 mm collimator (800 mm source to detector distance and 100 mm depth in water). Results: For microchambers in composite fields, the intermediate CFs that account for detector density and volume were the largest contributors to total CFs. The total CFs for the A26 were larger than those for the A16, especially for the 5 mm cone (1.227±0.003 to 1.144±0.004 versus 1.142±0.003 to 1.099±0.004), due to the A26’s larger active volume (0.015 cc) relative to the A16 (0.007 cc), despite the A26 using similar wall and electrode material. The W1 total and intermediate CFs are closer to unity, due to its smaller active volume and near water-equivalent composition, however, 3–4% detector volume corrections are required for 5 mm collimator fields. In fields using the 7.5 mm collimator, the correction is nearly eliminated for the W1 except for a non-isocentric field. Conclusion: Large and variable CFs are required for microchambers in small composite fields primarily due to density and volume effects. Corrections are reduced but not eliminated for a plastic scintillator in the same fields.« less
  • 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, themore » 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 18 MV. 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.« less