Measurement of low‐energy backscatter factors using GAFCHROMIC film and OSLDs

Abstract

Some of the lowest voltages used in radiotherapy are termed Grenz and superficial X‐rays of ~ 20 and ~ 100 kVp, respectively. Dosimetrically, the surface doses from these beams are calculated with the use of a free in‐air air kerma measurement combined with a backscatter factor and the appropriate ratio of mass energy absorption coefficients from the measurement material to water. Alternative tools to the standard ion chamber for measuring the BSF are GAFCHROMIC EBT2 film and optically stimulated luminescent dosimeter (OSLD) crystals made from Al2normalO3. The scope of this project included making three different backscatter measurements with an Xstrahl‐D3100 X‐ray unit on the Grenz ray and superficial settings. These measurements were with OSLDs, GAFCHROMIC EBT2 film, and a PTW ionization chamber. The varied measurement methods allowed for intercomparison to determine the accuracy of the results. The ion chamber measurement was the least accurate, as expected from previous experimental findings. GAFCHROMIC EBT2 film proved to be a useful tool which gave reasonable results, and Landauer OSLDs showed good results for smaller field sizes and an increasing overresponse with larger fields. The specific backscatter factors for this machine demonstrated values about 5% higher than the universal values suggested by the AAPM and IPEMB codes of practice for the 100 kVp setting. The 20 kvp measured data from both techniques showed general agreement with those found in the BJR Supplement No. 10, indicating that this unit's Grenz ray spectrum is similar to those used in previous experimental work.PACS number: 87.53.Bn

Review of backscatter measurement in kilovoltage radiotherapy using novel detectors and reduction from lack of underlying scattering material

“…They suggested that TLDs can be used to measure BSFs in cases where data are not available, but for their clinical system, IPEM data was valid. Mart et al (6) used both EBT2 film and OSLDs to measure BSFs for two energies, including one with a very low beam quality . They found differences of greater than 10% both between measured values with the two dosimeters and between measured values and reference data.…”

“…Several authors ( 3 , 6 , 17 ) have ascribed the differences between measured and code of practice values to differences in spectral distribution of local beams. However, in all of these cases, the differences are of the same order as the measurement uncertainties, so a more reasonable conclusion would be that the local factors agree with reference data within inherent uncertainties.…”

“…Local measurement of these factors is confounded by the finite size of many common detectors, leading to perturbation and partial volume effects, as well as the potential lack of water‐equivalence of detectors and solid phantom materials at kilovoltage energies. Recently, a number of novel detectors have been investigated to see if they can be used for local verification, including GAFCHROMIC film, ( 3 , 4 , 5 , 6 ) thermoluminescent dosimeters (TLDs), (17) and optically stimulated luminescent dosimeters (OSLDs) (6) . Results of these studies are discussed below, alongside data from this current work.…”

Characterization of XR‐RV3 GafChromic® films in standard laboratory and in clinical conditions and means to evaluate uncertainties and reduce errors

“…Additionally, since the characterization was done free-in-air, the dose to the skin will be slightly overestimated since these irradiations do not account for low-energy backscattered photons present in the case of on-patient film exposures. 28 Backscatter factor converting the air kerma measured free-in-air to air kerma on the surface of the patient is however strongly dependent on the geometrical setup, field size, and beam energy and is expected T II. X-ray equipment, dosimetry devices, radiation qualities, and dose uncertainties used for film characterization in standard laboratory conditions as well as in clinical conditions.…”

Investigation of backscatter factor in medical radiography using anthropomorphic phantom by optically stimulated luminescence dosimeter

“…Previous studies have included actual measurements to determine BSF using an ionization chamber dosimeter [10], thermoluminescence dosimeters (TLDs) [10,11], radiochromic film [12,13], and other means, but for an ionization chamber dosimeter, the physical size of chambers raises concern about the possibility of excessive dispersion, and for TLDs, reproducibility is low because of error and other problems in reading elements, and studies with radiochromic film have generally applied to radiotherapy.…”