Monte Carlo-based beam quality and phantom scatter corrections for solid-state detectors in 60Co and 192Ir brachytherapy dosimetry

Monte Carlo-based Investigation of Absorbed-dose Energy Dependence of Thermoluminescent Dosimeters in Therapeutic Proton and Carbon Ion Beams

Background

The present study is aimed at calculating relative absorbed-dose energy response correction (R) of commonly used thermoluminescent dosimeters (TLDs) such as LiF, Li2B4O7, and Al2O3 as a function of depth in water for protons (50-250 MeV/n) and carbon ion (80-480 MeV/n) beams using Monte Carlo-based FLUKA code.

Materials and Methods

On-axis depth-dose profiles in water are calculated for protons (50-250 MeV/n) and carbon ion (80-480 MeV/n) beams using FLUKA code. For the calculation of R, selective depths are chosen based on the depth-dose profiles. In the simulations, the TLDs of dimensions 1 mm × 1 mm × 1 mm are positioned at the flat, dose gradient, and Bragg peak regions of the depth-dose profile. Absorbed dose to detector was calculated within the TLD material. In the second step, TLD voxels were replaced by water voxel of similar dimension and absorbed dose to water was scored.

Results

The study reveals that for both proton and carbon ion beams, the value of R differs from unity significantly at the Bragg peak position and is close to unity at the flat region for the investigated TLDs. The calculated R value is sensitive to depth in water, beam energy, type of ion beam, and type of TLD.

Discussion

For accurate dosimetry of protons and carbon ion beams using TLDs, the response of the TLD should be corrected to account for its absorbed-dose energy dependence.

EGSnrc-based depth-dependent photon energy response and phantom scatter corrections for low-energy brachytherapy sources

In the present study, beam quality correction, [Formula: see text], and phantom scatter correction, k_phan(r), for low-energy brachytherapy sources, ¹³¹Cs, ¹²⁵I, and ¹⁰³Pd, are calculated using the Monte Carlo-based EGSnrc code system as a function of the distance along the transverse axis of the source. The solid-state detectors investigated are diamond, LiF, Li₂B₄O₇, Al₂O₃, and radiochromic films, such as HS, EBT, EBT2, EBT3, RTQA, XRT, and XRQA. The solid phantoms investigated are polystyrene, PMMA, virtual water, solid water, plastic water (LR), A150, RW1, RW3, and WE210. For a given detector and brachytherapy source, [Formula: see text] is independent of distance in the water phantom. Meanwhile, for a given detector, k_phan(r) depends on the distance from the source for the investigated solid phantoms. Moreover, the k_phan(r) values do not change with the detector type for sources ¹³¹Cs, ¹²⁵I, and ¹⁰³Pd at all distances. The LR and A150 phantoms are water equivalent for the investigated distances of 1-5 cm. The phantoms including solid water, virtual water, and WE210 are not water-equivalent for distances beyond 1 cm. Furthermore, PMMA, polystyrene, RW1, and RW3 are not water equivalent.

Energy dependence of a radiophotoluminescent glass dosimeter for HDR 192 Ir brachytherapy source

PURPOSE

We determined correction factors for absorbed dose energy dependence and intrinsic energy dependence for measurements of absorbed dose to water around an ¹⁹² Ir source using a radiophotoluminescent glass dosimeter (RPLD) calibrated with a 4-MV photon beam.

METHODS

The ratio of the absorbed dose to the water and the average absorbed dose to RPLD for the ¹⁹² Ir beam relative to the same ratio in a 4 MV photon beam defines the absorbed dose energy dependence and was determined at distances of 2-10 cm (at intervals of 1 cm) from the ¹⁹² Ir source in a water phantom using the egs_chamber user code. The RPLD was calibrated to measure absorbed dose to water, D_w , in a 4 MV photon beam using an ionization chamber, which was also used to measure absorbed dose to water, D_w , in a water phantom using the ¹⁹² Ir source. The detector response radiophotoluminescence (RPL signal per average absorbed dose in the detector) in the ¹⁹² Ir beam relative to that in the 4 MV photon beam (the relative intrinsic efficiency) was determined experimentally. Finally, the beam quality correction factor was obtained as the quotient between the absorbed dose energy dependence and the relative intrinsic efficiency and corrects for the difference between the beam quality Q₀ used at calibration and the beam quality Q used in the measurements.

RESULTS

The relative dose ratio of the average absorbed dose to water relative to RPLD ranged from 0.930 to 0.746, and the beam quality correction factor ranged from 0.999 to 0.794 for distances of 2-10 cm from the ¹⁹² Ir source. The relative detector response to an ¹⁹² Ir source and a 4-MV photon beam was 0.930, and it did not vary significantly with distance.

CONCLUSIONS

These results demonstrate that corrections for absorbed dose energy dependence and intrinsic energy dependence are required when using an RPLD to measure with sources different from the reference source providing the primary calibration.

Monte Carlo Calculation of Beam Quality and Phantom Scatter Corrections for Lithium Formate Electron Paramagnetic Resonance Dosimeter for High-energy Brachytherapy Dosimetry

Purpose:

To investigate beam quality correction, K_QQ0 (r) and phantom scatter correction, K_phan (r) for lithium formate dosimeter as a function of distance r along the transverse axis of the high-energy brachytherapy sources ⁶⁰Co, ¹³⁷Cs, ¹⁹²Ir and ¹⁶⁹Yb using the Monte Carlo-based EGSnrc code system.

Materials and Methods:

The brachytherapy sources investigated in this study are BEBIG High Dose Rate (HDR) ⁶⁰Co (model Co0.A86), ¹³⁷Cs (model RTR), HDR ¹⁹²Ir (model Microselectron) and HDR ¹⁶⁹Yb (model 4140). The solid phantom materials investigated are PMMA, polystyrene, solid water, virtual water, plastic water, RW1, RW3, A150 and WE210.

Result:

K_QQ0 (r) is about unity and distance independent fo ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources, whereas for the ¹⁶⁹Yb source, K_QQ0 (r) increases gradually to about 4 % larger than unity at a distance of 15 cm along the transverse axis of the source. For ⁶⁰Co source, phantoms such as polystyrene, plastic water, solid water, virtual water, RW1, RW3 and WE210 are water-equivalent but PMMA and A150 phantoms show distance-dependent K_phan (r) values. For ¹³⁷Cs and ¹⁹²Ir sources, phantoms such as solid water, virtual water, RW1, RW3 and WE210 are water-equivalent. However, phantoms such as PMMA, plastic water, polystyrene and A150 showed distance-dependent K_phan (r) values, for these sources. For ¹⁶⁹Yb source, all the investigated phantoms show distance-dependent K_phan (r) values.

Conclusion:

K_QQ0 (r) is about unity and distance independent for ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources. Phantoms such as solid water, virtual water, RW1, RW3 and WE210 are water-equivalent for ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources. For ¹⁶⁹Yb source, all the investigated phantoms show distance-dependent K_phan (r) values.

Phantom scatter corrections of radiochromic films in high-energy brachytherapy dosimetry: a Monte Carlo study

Our aim in this study was to calculate Monte Carlo-based phantom scatter corrections of various radiochromic films for different solid phantoms for high-energy brachytherapy sources. Brachytherapy sources (60)Co, (137)Cs, (192)Ir, and (169)Yb and radiochromic films EBT, EBT2 (lot 020609 and lot 031109), RTQA, XRT, XRQA, and HS were investigated in this study. The solid phantom materials investigated were PMMA (polymethylmethacrylate), polystyrene, solid water, virtual water, plastic water, RW1, RW3, A150, and WE210. Monte Carlo-based user codes DOSRZnrc and FLURZnrc of the EGSnrc code system were employed in the present work. For the (60)Co source, the polystyrene, plastic water, solid water, virtual water, RW1, RW3, and WE210 phantoms were water equivalent for the investigated films, but showed distance-dependent values for XRT and XRQA films. For the (137)Cs and (192)Ir sources, the solid water, virtual water, RW1, RW3, and WE210 phantoms were water equivalent for the investigated films, but showed distance-dependent values for XRT and XRQA films. For these sources, the remaining phantoms showed distance-dependent values for all of the films investigated. For the (169)Yb source, all of the investigated phantoms showed distance-dependent values for the investigated films. This study suggests that radiochromic films demonstrate distance-dependent values, but the degree of dependence is related to the types of solid phantom and film. Hence, for brachytherapy dosimetry involving radiochromic films and solid phantom materials, phantom scatter corrections need to be applied.