Determining the Effect of Source Surface Distance Variations on Percentage Depth Dose in Isocentric Radiation Therapy with 6MV Photons

Document Type : Original Article

Authors

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

2 Professor of Radiotherapy and Oncology.Department of Radiotherapy and Oncology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

3 Ph. D Student of Medical Physics.Department of Radiology Technology, Jundishapur University of Medical Sciences, Ahvaz, Iran.

4 MSc Student of Medical Physics.Department of Medical Physics, Medical School, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

5 Assistant Professor of Radiotherapy and Oncology. Department of Radiotherapy and Oncology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

Abstract

Background and Objective: The percent depth dose (PDD) is used as a dose calculation method in radiation therapy. PDD varies with source surface distance (SSD) according to inverse square law. In most clinical situations it is necessary to convert standard SSD to that must be used in practice. Therefore; this research proposes a new factor to calculate PDD corresponding to SSD.
Materials and Methods: Electa compact accelerator for 6MV photon beam, Scanditronix blue phantom the size of 50 ×50 ×50cm3 and two ionization chamber with sensitive volume of 0.13 cc were used.  Dosimetry was done for field sizes: 8×8, 10×10، 6.4×6.4 cm2 at SSD=80 and 100cm. Ks0 and Ks taking into account the collimator scatter, for fields in the standard SSD and the practical SSD, respectively. The ratio of Ks0 and Ks was applied to correct the Maynord F factor.
Results: This corrected Maynord F factor revealed better results for 8×8 and 10×10 cm2 fields further more the corrected Maynord F factor, reached the more precision in buildup region for the 6.4×6.4 cm2 field for PDD calculations.
Conclusion: For small fields with less collimator scatter, Maynord F factor method is more accurate to calculate PDD changes with varying SSD. Although, for large depths or SSDs, using the corrected Maynord F factor will show better results for calculating PDD variations with SSD changes

Keywords

References

1-Boles M. Central axis depth dose data for use in radiotherapy. The Radiological Society of North America; 1972.
2-Khan FM, Sewchand W, Lee J, Williamson JF. Revision of tissue‐maximum ratio and scatter‐maximum ratio concepts for cobalt 60 and higher energy x‐ray beams. Medical physics. 1980;7(3):230-7.
3-Cheng CW, Hyun Cho S, Taylor M, Das IJ. Determination of zero‐field size percent depth doses and tissue maximum ratios for stereotactic radiosurgery and IMRT dosimetry: Comparison between experimental measurements and Monte Carlo simulation. Medical physics. 2007;34(8):3149-57.
4-Arjomandy B, Tailor R, Zhao L, Devic S. EBT2 film as a depth‐dose measurement tool for radiotherapy beams over a wide range of energies and modalities. Medical physics. 2012;39(2):912-21.
5-International Commission on Radiation Units and Measurments. Radiation quantities and units.  . Washington,DC:U.S: 1980.
6-Khan FM. The physics of radiation therapy: Lippincott Williams & Wilkins; 2010.
7-Almond PR, Biggs PJ, Coursey BM, Hanson W, Huq MS, Nath R, et al. AAPM's TG‐51 protocol for clinical reference dosimetry of high‐energy photon and electron beams. Medical physics. 1999;26(9):1847-70.
8-Venselaar J, Welleweerd H, Mijnheer B. Tolerances for the accuracy of photon beam dose calculations of treatment planning systems. Radiotherapy and oncology. 2001;60(2):191-201.
9-Fontenla DP, Napoli JJ, Hunt M, Fass D, McCormick B, Kutcher GJ. Effects of beam modifiers and immobilization devices on the dose in the build-up region. International Journal of Radiation Oncology* Biology* Physics. 1994;30(1):211-9.
10-Mayneord WV, LF L. A survey of depth dose data. . Br J Radiol. 1944;14:255.
11-Mayneord WV, LF L. A survey of depth dose data. . Br J Radiol. 1941;164:10.
12-Podgorsak E, Pla C, Evans M, Pla M. The influence of phantom size on output, peak scatter factor, and percentage depth dose in large‐field photon irradiation. Medical physics. 1985;12(5):639-45.
13-Deng J, Jiang SB, Kapur A, Li J, Pawlicki T, Ma C. Photon beam characterization and modelling for Monte Carlo treatment planning. Physics in medicine and biology. 2000;45(2):411.
14-Kase KR, Svensson GK, Wolbarst AB, Marks MA. Measurements of dose from secondary radiation outside a treatment field. International Journal of Radiation Oncology* Biology* Physics. 1983;9(8):1177-83.
15-Knöös T, Johnsson SA, Ceberg CP, Tomaszewicz A, Nilsson P. Independent checking of the delivered dose for high-energy X-rays using a hand-held PC. Radiotherapy and Oncology. 2001;58(2):201-8.
16Calandrino R, Cattaneo GM, Fiorino C, Longobardi B, Mangili P, Signorotto P. Detection of systematic errors in external radiotherapy before treatment delivery. Radiotherapy and Oncology. 1997;45(3):271-4.
Jundishapur Scientific Medical Journal
Volume 16, Issue 3 - Serial Number 108
September and October 2017
Pages 317-326

Files

History

  • Receive Date: 02 September 2017
  • Accept Date: 02 September 2017

Share

How to cite

Statistics