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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. Wed . "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 = {Wed Jun 15 00:00:00 EDT 2016},
month = {Wed Jun 15 00:00:00 EDT 2016}
}
  • Purpose: Different detector properties influence their responses especially in field sizes below the lateral electron range. Due to the finite active volume, the detector density and electron perturbation at other structural parts, the response factor is in general field size dependent. We aimed to visualize and separate the main effects contributing to detector behavior for a variety of detector types. This was achieved in an experimental setup, shielding the field center. Thus, effects caused by scattered radiation could be examined separately. Methods: Signal ratios for field sizes down to 8 mm (SSD 90 cm, water depth 10 cm) of amore » 6MV beam from a Siemens Primus LINAC were recorded with several detectors: PTW microDiamond and PinPoint ionization chamber, shielded diodes (PTW P-60008, IBA PFD and SNC Edge) and unshielded diodes (PTW E-60012 and IBA SFD). Measurements were carried out in open fields and with an aluminum pole of 4 mm diameter as a central block. The geometric volume effect was calculated from profiles obtained with Gafchromic EBT3 film, evaluated using FilmQA Pro software (Ashland, USA). Results: Volume corrections were 1.7% at maximum. After correction, in small open fields, unshielded diodes showed a lower response than the diamond, i.e. diamond detector over-response seems to be higher than that for unshielded diodes. Beneath the block, this behavior was amplified by a factor of 2. For the shielded diodes, the overresponse for small open fields could be confirmed. However their lateral response behavior was strongly type dependent, e.g. the signal ratio dropped from 1.02 to 0.98 for the P-60008 diode. Conclusion: The lateral detector response was experimentally examined. Detector volume and density alone do not fully account for the field size dependence of detector response. Detector construction details play a major role, especially for shielded diodes.« less
  • Purpose: To measure the output factor (OF) and output correction factors (OCF) of small circular cones using various active and passive detectors. Methods: The OF of BrainLab circular cones of 4, 6, 7.5, 10, 12.5 and 15mm diameter were measured isocentrically at 15mm depth for 6 MV X-rays at 1000 MU/min from Novalis-Tx Linac using high resolution diodes (SFD, Edge, PTW60008), plastic scintillator (ExtradinW1), small ionization chamber (CC01), EBT3 film and Al2O3:C OSLDs (nanoDot). OCF was calculated as a ratio of the OF of each detector relative to EBT3. Long term overall measurement uncertainty were estimated from the 15 repeatmore » measurement performed for each detector over a period of 1year Results: The mean OF measured using various detectors are shown in the figure. In comparison to EDR3 measurement, nanoDot, CC01 and ExtradinW1 showed consistently lower OF upto −25.7% (nanoDot) which gradually improves with cone size (Table). The variation in OF measure with CC01 and ExtradinW1 for cone size >10mm is within −1.4% and −1.9%. Whereas, Edge and PTW60008 measured consistently larger OF except for 4mm cone. All diodes showed better agreement with EDR3 with maximum variation within ±1.7% (SFD), 2.5% (Edge) and 4.4% (PTW60008). The OCF was largest for nanoDot (range 1.04–1.35) and least for the diodes (range 0.97–1.04). The estimated overall long term standard uncertainty (k=2, 95% confidence) were highest for nanoDot at 3% followed by 2.43% for EDR3, 1% for SFD, 0.49% for ExtradinW1, 0.42% for CC01 and PTW60008, 0.16% for edge. Conclusion: Largest OF for the smallest cone was observed from EDR3. Although correction factor needed for all diodes were very similar, Edge showed most consistent result. nanoDot in its present form is not suitable for OF measurement from small fields. ExtradinW1 with its stem oriented perpendicular to beam axis underestimate OF for field size ≤10mm and need further investigation.« less
  • Purpose: This study was aimed to study the effects oflow- and high-energy intensity-modulated (IMRT)photon beams on the planning of target volume and thecritical organs in cases of prostate cancer. Methods: Thirty plans were generated using either 6 MV or 15 MValone, and both 6 and 15 MV beams. For each plangenerated using suitable planning objectives and doseconstraints, which was set to be identical except thebeam energy. The plans were analyzed in terms of targetcoverage, conformity, and homogeneity regardless ofbeam energy. Results: Mean percentage of V 70Gy ofrectal wall in 6 MV, 15 MV and mixed-energy plans was16.9%, 17.8%, and 16.4%,more » respectively, while the meanpercentage of V 40Gy was 53.6%, 52.3%, and 50.4%.The mean dose to femoral heads in 6 MV, 15 MV, andmixed-energy plans were 30.1 Gy, 25.5 Gy, and 25.4 Gy,respectively. The integral dose of 6 MV plans was 10%(on average) larger than those of 15 MV or mixed-energyplans. Conclusion: The preliminary results suggested thatmixed-energy IMRT plans may take an advantage of thedosimetric characteristics of low- and high-energybeams. Although the reduction of dose to the organs atrisk may not be clinically relevant, in this study, mixingenergy in an IMRT plans showed better OAR sparingand for deep-seated tumors, the overall plan quality wasimproved.« less
  • Purpose: Indigenous Farmer type chamber FAR 65 GB is a reference class 0.6 cc ion chamber. It can be used for dosimetric evaluation of photon and high energy electron beams. We studied dosimetric characteristics of the chamber for 6MV and 10MV Flattening filter free FFF photon beams available on trueBEAM STx Linac. Methods: The study was carried out on trueBEAM STx Linac having 6 and 10 MV FFF photon beam with maximum dose rate 1400 and 2400 MU per min respectively. The dosimetric device to be evaluated is Rosalina Instruments FAR 65-GB Ion Chamber with active volume 0.65 cc, totalmore » active length 23.1cm, inner diameter of cylinder 6.2mm, wall thickness 0.4mm, inner electrode diameter 1mm. Inner and outer electrodes are made from Aluminium 2.7 gm per cc and graphite 1.82 gm per cc respectively. The ion chamber was placed along central axis of beam at 10cm depth and irradiated for 10cm × 10cm field size at SAD of 100 cm in plastic phantom. We studied Precision, Dose Linearity, Dose Rate dependence, directional dependence, Recombination effect. Recombination effect was determined using standard two-voltage method. Results: 1. Measurements were reproducible std deviation of 0.0105 and type A uncertainty 0.003265 under same set of reference conditions 2. Chamber exhibit dose linearity over a wider dose range. 3. Chamber shows dose rate independence for all available dose rate range. 4. Response of chamber with the angle of incidence of radiation is constant. 5. Recombination correction factors were 1.01848 and 1.02537 for dose rate 1400 and 2400 MU per min resp. Conclusion: Our study reveals that the chamber is prone to saturation effect at dose rate of 2400 MU per min. FAR 65-GB can be used for reference dosimetry of FFF MV photon beam with proper calculation of recombination effect.« less
  • Purpose: To investigate the influence of photon field size on the electron and photon fluence spectra in the active volume of small field detectors. Methods: The PENELOPE MC system based usercode PenEasy was used to calculate the material influence on the spectra by scoring the differential fluence in inserts of silicon, carbon, phosphorus and aluminium having 3 mm diameter and height. The spectra were then calculated inside the active volume of eleven detectors (ion chambers and solid-state detectors) whose geometry was simulated with great detail. The inserts/detectors were placed at 10 cm depth in a 30 cm x 30 cmmore » x 30 cm water phantom and irradiated with 2.5 MeV photons and Varian Clinac 6 MV beams of small, medium and large size. Results: For all configurations, photon spectra in the scoring volume were similar to that in a small water volume except for additional characteristic x-ray peaks resulting from the material itself and from the materials surrounding the detectors (i.e. high-Z shielding the silicon). Electron fluence calculated in the inserts were up to 60% larger than in water; the difference increased with material density and decreasing field size. MC-calculated doses were compared to analytically determined collision kerma and restricted cema (cut-off=15keV). For the inserts, with large and medium fields K-col agreed with MC-dose, but K-col overestimated the dose for small fields due to lack of lateral CPE. For the detectors, up to 15% differences between K-col and the MC-dose were found. For all configurations the C-delta and MC-dose agreed within ±2%. Conclusion: The most relevant findings were that shielding affects substantially the photon spectra and material conditions the electron spectra, their field size dependence varying with the geometry configuration. These affect the values of factors entering into relative dosimetry.« less