Exit Dose Measurement in Therapeutic High Energy Photon Beams and Cobalt-60 Gamma Rays
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To estimate the skin dose to the patient from the treatment planning, the knowledge about exit dose is essential, which is calculated from the percentage depth dose. In this study 6 MV and 18 MV beams from linear accelerator and cobalt-60 beams were used. The ionometric measurements were carried out with parallel plate chamber of sensitive volume 0.16 cc. Parallel plate chamber was fitted in to 30 x 30 cm2 polystyrene phantom at a fixed FSD with the measuring entrance window facing farther from the source. The field size for this measuring condition was maintained at 10 x 10 cm2. The ionization measurements were also carried out by changing the thickness of the polystyrene phantom at the entrance side of the point of measurement. In order to find out the variation of relative exit dose (RED) with field size the measurements were carried out without and with the full back-scattering material (27.2 gm/cm2) placed beyond the entrance window of the chamber. The measurements were also done for the entrance polystyrene phantom thicknesses of 10, 20 and 30 cm for the field size ranging from 5 x 5 cm2 to 30 x 30 cm2. The dose at the exit surface with no backscatter material is about 4.4%, 3.7% and 5.8% less than the dose with the full backscatter material present beyond the point of measurement for 6 MV, 18 MV X-rays and cobalt-60 gamma rays. The reduction in exit dose does not depend much of the phantom thickness through which the beam traverses before exiting at the chamber side. Dose enhancements of about 1.03 times were observed for a field size of 5 x 5 cm2 for 6 MV, 18 MV X-rays and cobalt-60 gamma rays. The dose enhancement factor (DEF) values were noticed to vary with field size beyond 15 x 15 cm2 for all the energies studied. Also it can be observed that the dose enhancement factor (DEF) values do not depend on the thickness of the phantom material through which the beam has traversed. The DEF values were found to vary marginally for different phantom material thickness for the particular field size. The study indicates that a reduction of 4.4% and 3.7% in relative exit dose when there is no backscatter material present for 6 and 18 MV X-rays for most of the clinically used radiotherapy portals. The measured exit dose was found to be mostly independent of field size and the thickness of the phantom material through which the beam gets transmitted at the entrance side. An addition of backscatter material of thickness equal to two-thirds of the dmax depth of the radiation beam concerned results in full dose at the exit side.
1 - 1 - 2007
30 - 12 - 2008
- Biggs PJ, Ling CC. Electrons as the cause of the observed Dmax shift with field size in high energy photon beams. Med Phys 1979; 6: 291-295.[PubMed]
- Clarkson JR, Herbert RJT. Surface and emergent doses in radiotherapy. Br J Radiol 1948; 21: 494-500.[PubMed][Crossref]
- Gagnon WF, Grant WG. Surface dose from megavoltage therapy machines Radiology 1973; 117: 705-708.
- Geode MR, Anderson DW, McCray KL. Corrections to megavoltage depth dose values due to reduced backscatter thickness. Med Phys 1977; 2: 123-126.[Crossref]
- Gagnon WF, Hortan JL. Physical factors affecting absorbed dose from cobalt-60 gamma rays and 25 MV X-rays. Med Phys 1979; 6: 285-290.
- Gerbi B, Khan F. Measurement of dose in the buildup region using fixed separation plan-parallel ionization chambers. Med Phys 1990; 17: 17-26.[Crossref]
- International Commission on Radiation Units and Measurements. Determination of absorbed dose in patients irradiated by beams of X and gamma rays in radiotherapy procedures. 1976 ICRU Report 24 (Washington DC, USA).
- Johns HE Cunningham R. The physics of Radiology (Thomas Springfield, Illinois 1969) 316-317.
- Krithivas G, Rao SH. A study of the characteristics of radiation contaminants within a clinically useful photon beam. Med Phys 1985;12: 764-768.[PubMed][Crossref]
- Klein EE, Purdy JA. Entrance and exit dose regions for a Clinac-2100C. Int J Radiat Oncol Biol Phys 1993; 9: 429-435.
- Legare JM. Exit-surface-dose correction factors for X-rays of 1.5 to 4.0 mm Cu half value layer. Radiology 1964; 82: 272-276.
- Lambert GD, Liversage WE, Hirst AM, Doughty D. Exit dose studies in megavoltage photon therapy. Br J Radiol 1983; 56: 329-334.[PubMed][Crossref]
- Meredith WJ, Massey JB. Fundamental Physics of Radiology (Williams and Wilkins, Baltimore 1972) 413.
- Petti PL, Goodman MS, Gabriel TA, Mohan R. Investigation of buildup dose from electron contamination of clinical photon beams. Med Phys 1983; 10: 18-24.[Crossref][PubMed]
- Purdy JA. Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator. Med Phys 1986; 13: 259-262.
- Ravikumar M, Ravichandran R. Dose measurements in the build-up region for the photon beams from Clinac-1800 dual energy medical linear accelerator Strahlenther. Onkol 2000; 5: 223-228.
- Tannous NBJ, Gagnon WF, Almond PR. Buildup region and skin dose measurements for the Therac 6 linear accelerator for radiation therapy. Med Phys 1981; 8: 378-381.[Crossref]
- Velkley DE, Manson DJ, Purdy JA, Oliver GD. Build-up region of megavoltage photon radiation sources. Med Phys 1975; 2: 14-19.[PubMed][Crossref]
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