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 Al2O3. 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
Highlights
The depth of penetration of X-rays dictates their usefulness for certain applications; megavoltage X-ray beams are useful for reaching deep seated targets, whereas kilovoltage X-rays are more appropriate for shallow targets
These beam qualities are most often used for superficial cancers and keloids with a target depth of only 0.3–1 mm below the outer surface of the skin; the dose to a target from the beam is typically quantified as a surface dose. This surface dose is 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: Dw,z = 0
For ionization chambers there are competing effects: the displaced material in the chamber volume results in a lack of scatter being created within the chamber, and more scatter occurring posterior to the chamber due to lack of attenuation.[2]. There is the concern with these chambers that extra scatter is being produced in the wall due to the physical size of the apparatus
Summary
The depth of penetration of X-rays dictates their usefulness for certain applications; megavoltage X-ray beams are useful for reaching deep seated targets, whereas kilovoltage X-rays are more appropriate for shallow targets. Some of the smallest ionization chambers have a collecting volume of 0.3 cm, which results in an ambiguity of the measurement location This is an important parameter, as the kerma changes rapidly with depth at these energies, and the measurement is assumed to be at the surface. Another detector type is the lithium fluoride thermoluminescent dosimeter (TLD) crystal. This detector can produce incorrect results due to the varied response of the TLD to different incident energies of radiation This energy dependence is due to the effective atomic number of the LiF: approximately 8.14 vs 7.42 for tissue. EBT film exhibits less accuracy, but an even smaller volume, a lack of energy dependence and rereadability for a possibly ideal solution to making this measurement
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