Experimental validation of Monte Carlo dosimetry for therapeutic beta emitters with radiochromic film in a 3D‐printed phantom

Abstract

Validation of dosimetry software, such as Monte Carlo (MC) radiation transport codes used for patient-specific absorbed dose estimation, is critical prior to their use in clinical decision making. However, direct experimental validation in the clinic is generally not performed for low/medium-energy beta emitters used in radiopharmaceutical therapy (RPT) due to the challenges of measuring energy deposited by short-range particles. Our objective was to design a practical phantom geometry for radiochromic film (RF)-based absorbed dose measurements of beta-emitting radionuclides and perform experiments to directly validate our in-house developed Dose Planning Method (DPM) MC code dedicated to internal dosimetry. Methods: The experimental setup was designed for measuring absorbed dose from beta emitters that have a range sufficiently penetrating to ∼200 µm in water as well as to capture any photon contributions to absorbed dose. Assayed 177 Lu and 90 Y liquid sources, 13-450 MBq estimated to deliver 0.5-10 Gy to the sensitive layer of the RF, were injected into the cavity of two 3D-printed half -cylinders that had been sealed with 12.7 µm or 25.4 µm thick Kapton Tape. A 3.8 × 6 cm strip of GafChromic EBT3 RF was sandwiched between the two taped half -cylinders. After 2-48 h exposures, films were retrieved and wipe tested for contamination. Absorbed dose to the RF was measured using a commercial triple-channel dosimetry optimization method and a calibration generated via 6 MV photon beam. Profiles were analyzed across the central 1 cm 2 area of the RF for validation. Eleven experiments were completed with 177 Lu and nine with 90 Y both in saline and a bone equivalent solution. Depth dose curves were generated for 177 Lu and 90 Y stacking multiple RF strips between a single filled half -cylinder and an acrylic backing. All experiments were modeled in DPM to generate voxelized MC absorbed dose estimates. We extended our study to benchmark general purpose MC codes MCNP6 and EGSnrc against the experimental results as well. Results: A total of 20 experiments showed that both the 3D-printed phantoms and the final absorbed dose values were reproducible. The agreement between the absorbed dose estimates from the RF measurements and DPM was on average −4.0% (range −10.9% to 3.2%) for all single film 177 Lu experiments and was on average −1.0% (range −2.7% to 0.7%) for all single film 90 Y experiments. Absorbed depth dose estimates by DPM agreed with RF on averageThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Active and passive dosimetry for beta-emitting radiopharmaceutical therapy agents in a custom SPECT/CT compatible phantom

“…Unlaminated EBT3 film EBT3 film (Ashland Global Specialty Chemicals Inc., Wilmington, DE) was chosen for its relative energy independence (>100 keV) (Villarreal-Barajas et al 1999, Brown et al 2012, Massillon-JL et al 2012, Sipilä et al 2016, Hammer et al 2018, Tiwari et al 2020, historical use in beta dosimetry (Tiwari et al 2020, Mulet et al 2022, Van et al 2022, sensitivity, minimal volume averaging effects, and high spatial resolution (which allowed for a partial evaluation of the uniformity of the dose distribution near the source window) (McLaughlin et al 1991). The unlaminated version of EBT3 was used for this work to eliminate any additional attenuation caused by the external polyester base.…”

“…Alternatives to full simulations generally employ a more efficient kernel-based method, such as Dose Point Kernel (DPK) (O'Donoghue et al 2022). While a few studies have attempted to benchmark common MC-based RPT dosimetry methods with film measurements, these have been done using small phantoms with passive dosimeters, namely radiochromic film (Tiwari et al 2020, Van et al 2022). The development of a method for comparing image-based RPT absorbed dose calculation software against dosimeter-based measurements may be of interest to RPT clinics and vendors.…”

“…The best approaches to beta dosimetry often use dosimeters with thin entrance windows and active regions that minimally attenuate beta particles. For example, Van et al (Van et al 2022). performed measurements in a custom phantom with a 25.4 μm-thick Kapton window to separate a 90 Y or 177 Lu source solution from a piece of radiochromic film with an equally thick active layer.…”

Validation of Monte Carlo simulated absorbed-dose-to-water inside a custom SPECT/CT phantom using active and passive dosimeters: a feasibility study using 99mTc

“…Previous work in measuring absorbed dose from radionuclides has primarily employed the use of passive dosimeters such as TLDs, radiochromic film, and polymer gel with varying levels of success (Johnson et al 1995, Villarreal-Barajas et al 1999, Braun et al 2009, Tiwari et al 2020, Van et al 2022. There are several challenges that arise when performing dosimetric measurements of liquid beta-emitting radionuclide sources, including their rapid attenuation through material leading to low signal and volume averaging effects across the active layer of the detector, potential contamination of or damage to the dosimeter, and the change in response of the detector due to the positional variation in energy spectrum.…”

“…This is often achieved using small detectors with thin entrance windows. For example, (Van et al 2022) performed measurements in a 3D printed phantom containing a 25.4 μm thick Kapton window to separate a source solution of either 90 Y or 177 Lu from a piece of radiochromic film with an equally thick active layer. The obtained measurements showed agreement generally within 5% of dose estimates simulated using EGSnrc, MCNP6, and an in-house Dose Planning Method (DPM).…”