Abstract
Lack of underlying material can lead to dose reduction in kilovoltage radiotherapy treatments because of backscatter reduction. Conversely, the use of lead shielding can lead to large dose enhancement close to the lead interface. GAFCHROMIC film has been shown to be of use in verification of local backscatter factors compared to reference data in codes of practice, but careful handling and multiple readings are required to reduce systematic uncertainties to between 3% and 4%. Monte Carlo modeling of the specific treatment unit should be performed in cases which are found to differ from reference values before alternative values are adopted clinically, but these cases are expected to be few. GAFCHROMIC film may also be used to estimate backscatter reduction more readily than customized ionization chambers, for a range of beam qualities, applicator sizes and depth, with and without lead shielding. Differences were found between different studies, and it is not clear to what extent these are due to variation in equipment and/or technique. However, a layer of wax around lead shielding of 1 mm thickness should be sufficient to eliminate lead enhancement effects for all kilovoltage energies from 40 kV to 300 kVPACS numbers: 87.55.Qr, 87.56.jk
Highlights
Superficial tumors are commonly treated using kilovoltage X-ray units with tube potentials in the region of [40–300] kV
Reproducibility between different sessions was found to be within 3%, a few readings were discarded that were much lower than repeated values
GAFCHROMIC film offers a practical and readily available method for verification of backscatter factors and estimation of perturbation effects in a local center, careful handling and repeated measurements are needed to reduce uncertainties to between 3% and 4%
Summary
Superficial tumors are commonly treated using kilovoltage X-ray units with tube potentials in the region of [40–300] kV. Calibration is typically performed using air ionization chambers, and converted to dose in water using the backscatter factor (BSF), defined as the ratio between the dose at the surface of a full scatter water phantom and the dose with no phantom Measurement of these values has been challenging because of detector size and energy response. Dosimetric codes of practice contain data for a range of beam qualities, source-to-surface distances (SSD), and field sizes, which may be used by a center following spot checks against their own equipment These data are based on Monte Carlo simulations verified by measurements with a custom parallel plate ionization chamber.(1,2) More recently, GAFCHROMIC EBT film(3,4) and its replacement EBT2(5,6) (International Specialty Products, Wayne, NJ) have been used for local verification of BSF, but with varying degrees of success. Radiochromic film has the advantages of small thickness, ease of handling, and tissue equivalence, EBT2 may have greater energy dependence, batch variation, and nonuniformity
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