Determination of the Collimator Scatter Factor by the Correction Sterling Formula for Asymmetrical Field Radiation Therapy with Megavoltage Photon beam

Document Type : Original Article

Authors

Department of Medical Physics, Medical School, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran

Abstract

Background and Objective:  The output factor is an important factor in calculating the dose in the treatment of cancer patients with Megavoltage photon beams. Since in the most cases, the treatment field is asymmetric, therefore the calculation of this factor is very important.
Subjects and Methods: Measurements were done for Siemens Primus Plus accelerator, Golestan Hospital using Farmer ionization chamber 6cc. The output factors was measured for the symmetric fields and calibration curves were plotted for collimator scatter factor. For each symmetric square field, many asymmetrical fields were defined. Using analysis and comparison with measured values for Siemens Linac, a tuning parameter was calculated for each symmetric square field. Then, the tuning parameter was applied to calculate collimator scatter factor for the corresponding asymmetric field. Finally the calculation results were compared with experimental values.
Results: The tuning parameter for the two symmetric fields 30 × 30 and 25 × 25 were 0.452±0.001 and 0.481 ± 0.001 respectively. Taking into account this tuning parameter for asymmetric fields, the maximum percentage difference between calculated and measured results, was 0.7%.
Conclusions: Results indicate a reduction in the asymmetric collimator scatter factor relative to the symmetrical field. In addition, with increase in distance from center of symmetry, it becomes more significant decline. Also according to the calibration curve, the equivalent square field side for every asymmetric field can be obtained.
►Please cite this paper as:
JavadTahmasebi Birgani M, Chegeni N, Behrooz MA, Zabih Zadeh M, Habibi A. Determination of the Collimator Scatter Factor by the Correction Sterling Formula for Asymmetrical Field Radiation Therapy with Megavoltage Photon beam.Jundishapur Sci Med J 2017;16(1):45-51.

Keywords

References

1-Khan FM, Gibbons JP. Khan's the physics of radiation therapy: Lippincott Williams & Wilkins; 2014.
2-Kim S, Zhu TC, Palta JR. An equivalent square field formula for determining head scatter factors of rectangular fields. Medical physics. 1997;24(11):1.4-77.
3-Vadash P, Bjärngard B. An equivalent‐square formula for head‐scatter factors. Medical physics.1993;20(3):733-4.
4-Day M, Aird E. The equivalent-field method for dose determinations in rectangular fields. British journal of radiology Supplement. 17:105;1983.
5-Sterling T, Katz L, Perry H. Derivation of a Mathematical Expression for the Per Cent Depth Dose Surface of Cobalt 60 Beams and Visualisation of Multiple Field Dose Distributions1964.
6-Boyer A, Biggs P, Galvin J, Klein E, LoSasso T, Low D, et al. Basic applications of multileaf collimators. report of the aapm radiation therapy committee task group no. 50. Med Phys. 2001.
7-Lutz WR, Larsen RD, Bjärngard BE. Beam alignment tests for therapy accelerators. International Journal of Radiation Oncology* Biology* Physics. 1981;7(12):1727-31.
8-Dunscombe P, Nieminen J. On the field‐size dependence of relative output from a linear accelerator. Medical physics. 1992;19(6):1441-4.
9-Tatcher M, Bjärngard BE. Equivalent squares of irregular photon fields. Medical physics. 1993;20(4):1229-32.
10-Venselaar JL, Beckers N. A single-variable method for the derivation of collimator scatter correction factors in symmetrical and asymmetrical X-ray beams. Radiotherapy and oncology. 1999;50(3):319-24.
11-Kwa W, Tsang V, Fairey RN, Jackson SM, El-Khatib E, Harrison RW, et al. Clinical use of asymmetric collimators. International Journal of Radiation Oncology* Biology* Physics. 1997;37(3):705-10.
12-Tahmasebi Birgani M, Behrooz M, Hosseini M, Ghahremani F, Seif F. Determination of Correction Coefficient of the Sterlling's Formula (Vadash̒s factor) in Megavoltage Photon therapy. Jundishapur Sci Med J. 2013;11(6):619-24.
13-Stern RL, Rosenthal SA, Doggett EC, Mangat JK, Phillips TL, Ryu JK. Applications of asymmetric collimation on linear accelerators. Medical Dosimetry. 1995;20(2):95-8.
Jundishapur Scientific Medical Journal
Volume 16, Issue 1 - Serial Number 106
March and April 2017
Pages 45-51

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History

  • Receive Date: 06 August 2016
  • Revise Date: 11 March 2017
  • Accept Date: 13 March 2017

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