Variations in photon energy spectra of a 6 MV beam and their impact on TLD response

Abstract

Purpose: Measurement of the absorbed dose from radiotherapy beams is an essential component of providing safe and reproducible treatment. For an energy-dependent dosimeter such as thermoluminescent dosimeters (TLDs), it is generally assumed that the energy spectrum is constant throughout the treatment field and is unperturbed by field size, depth, field modulation, or heterogeneities. However, this does not reflect reality and introduces error into clinical dose measurements. The purpose of this study was to evaluate the variability in the energy spectrum of a Varian 6 MV beam and to evaluate the impact of these variations in photon energy spectra on the response of a common energy-dependent dosimeter, TLD. Methods: Using Monte Carlo methods, we calculated variations in the photon energy spectra of a 6 MV beam as a result of variations of treatment parameters, including field size, measurement location, the presence of heterogeneities, and field modulation. The impact of these spectral variations on the response of the TLD is largely based on increased photoelectric effect in the dosimeter, and this impact was calculated using Burlin cavity theory. Measurements of the energy response were also made to determine the additional energy response due to all intrinsic and secondary effects. Results: For most in-field measurements, regardless of treatment parameter, the dosimeter response was not significantly affected by the spectral variations (<1% effect). For measurement points outside of the treatment field, where the spectrum is softer, the TLD over-responded by up to 12% due to an increased probability of photoelectric effect in the TLD material as well as inherent ionization density effects that play a role at low photon energies. Conclusions: It is generally acceptable to ignore the impact of variations in the photon spectrum on the measured dose for locations within the treatment field. However, outside the treatment field, the spectra are much softer, and a correction factor is generally appropriate. The results of this work have determined values for this factor, which range from 0.88 to 0.99 depending on the specific irradiation conditions.

The effect of energy spectrum change on DNA damage in and out of field in 10-MV clinical photon beams

“…These differences in photon spectra are in conceptual agreement with other published data [10][11][12][13]23]. For shallow depths, the photon spectrum peak is lower in the out-of-field regions, but for deeper depths, the peak energy is independent of location in or out of the primary beam.…”

Accuracy and sources of error of out‐of field dose calculations by a commercial treatment planning system for intensity‐modulated radiation therapy treatments

“…( 17 , 18 ) For this study, standards were irradiated with a Co‐60 unit in order to obtain the system calibration factor ( C D,w ). In addition to the standard energy response correction factor that accounts for the difference in TLD response to a 6 MV spectrum relative to Co‐60 photons ($k_E$), an additional energy correction factor was used to account for the softer beam spectra observed outside of the treatment field ($k_{NR}$); these nonreference (out‐of‐field) energy correction factors were taken from Scarboro et al ( 19 ) Thus, the TLD absorbed dose was determined by Eq. (1), where D is the absorbed dose, M is the raw TLD signal per unit mass of TLD powder, $k_L$ is the linearity correction factor, and $k_F$ is the fading correction factor: $\mathrm{D}=\mathrm{M}\times {\mathrm{normalC}}_{\text{D,W}}\times {\mathrm{normalk}}_{\mathrm{normalL}}\times {\mathrm{normalk}}_{\mathrm{normalF}}\times {\mathrm{normalk}}_{\mathrm{normalE}}\times {\mathrm{normalk}}_{\text{NR}}$ …”

Involved field radiation for Hodgkin's lymphoma: The actual dose to breasts in close proximity

“…10 TLDs in or at the field edge were not reported on because they were calibrated for lower dose measurements. Based on a recently published work by Scarboro et al, and to account for the variations in photon energy spectra of a 6-MV beam and their impact on TLDs response, 11 we have used a correction factor of 0.95 to adequately account for the dose overestimation in TLDs. We also determined the dose corresponding to each TLD's position from the Pinnacle plan to compare it with the dose measured by the TLDs.…”