Erratum: Impact of backscatter material thickness on the depth dose of orthovoltage irradiators for radiobiology research (2019 Phys. Med. Biol. 64 055001)

Abstract

The orthovoltage x-Ray energy frequently used in radiation research is prone to dosimetry errors due to insufficient backscatter conditions. In many radiobiology studies, especially for cell irradiations, precise dose calculation algorithms such as Convolution-Superposition or Monte Carlo are impractical and as such, less accurate hand calculation methods are used for dose estimation. These dose estimation methods typically assume full backscatter conditions. The purpose of this study is to demonstrate the magnitude of the dose error that results from insufficient backscatter, and to provide lookup tables to account this issue. The beam spectra of several widely used commercial systems (XRAD-225, XRAD-320, SARRP) were used in Monte Carlo (MC) simulations on a series of phantom setups to investigate the impact of varying backscatter conditions on dosimetry. The depth dose curves for different field sizes, water phantom thicknesses and beam qualities were generated. In addition, depth dependent backscatter factors for different field sizes and different beam qualities were calculated. It is demonstrated that as much as a 50% dose difference exists for different backscatter conditions at the beam qualities studied. The choice of cell dish size as well as other changes in the experiment setup can have more than 10% impact on the dose. The impact of backscatter is reduced with a decrease in field size. Further, the thickness needed to provide full backscatter can be approximated as being equal to the field size. It is imperative to ensure full backscatter conditions during system and dosimeter calibration, or to use the look-up table provided in this study.

A review of small animal dosimetry techniques: image-guided and spatially fractionated therapy

“…It is recommended to not use the reference dosimetry dose rate as a means to establish the irradiation time for therapy treatments for mice or cell irradiations if the irradiation geometry and mouse/cell-thickness is not identical to the reference dosimetry. The different scattering conditions can misrepresent the dose in mice by as much as 15% due to large differences in the lateral scatter, backscatter, and field size differences within the full-scattering conditions recommended by the AAPM TG-61 compared to the relatively small thicknesses of mice or irradiated cells [6,152,153]. Instead, it is recommended to use a cylindrical solid water mouse phantom that houses a PinPoint ionization chamber to better model the realistic scattering conditions of mice treatments [114,115].…”

“…Instead, it is recommended to use a cylindrical solid water mouse phantom that houses a PinPoint ionization chamber to better model the realistic scattering conditions of mice treatments [114,115]. If a mouse phantom is not available, the backscatter factors from Chen et al [153] that report scatter factors with varying phantom thickness can be used with reference dosimetry measurements, and likewise, could be used for quantifying the mm-thick backscatter conditions for cell irradiations. Beam quality for low kilovoltage x-rays is quantified by using both the first half-value layer (HVL) and kVp and are required properties for reference dosimetry [39,124,154].…”

“…This includes the source-to-surface distance (SSD), the attenuation and material above the point of measurement, the backscatter thickness and material below the point of measurement, field size, and beam quality (kVp and HVL). In conformal small animal irradiators, the dose can change by 10% with the following: 2 cm change in SSD, 10% per cm of attenuating material [48,155], 0.05 mm change in Cu external filtration [154], or 1 cm of backscatter (field sizes > 1 cm) [153,156]. Changes in dose from backscatter thickness become negligible at backscatter thicknesses of approximately 5 cm and thicker [153].…”

“…In conformal small animal irradiators, the dose can change by 10% with the following: 2 cm change in SSD, 10% per cm of attenuating material [48,155], 0.05 mm change in Cu external filtration [154], or 1 cm of backscatter (field sizes > 1 cm) [153,156]. Changes in dose from backscatter thickness become negligible at backscatter thicknesses of approximately 5 cm and thicker [153]. A 5% change in dose can occur with a change in field size from 5 × 5 mm 2 to 3 × 3 mm 2 and to 10 × 10 mm 2 [24].…”

“…A 5% change in dose can occur with a change in field size from 5 × 5 mm 2 to 3 × 3 mm 2 and to 10 × 10 mm 2 [24]. An increase in field size from 1 to 4 cm increases the backscatter factor for mice irradiations by 10% [153]. Changes in field size or beam quality affect the attenuation, backscatter, and overall dose [39,48,124,153,155,156], and changes in beam quality can affect the relative biological effectiveness (RBE) [154].…”