The Impact of Tissue Type and Density on Dose Point Kernels for Patient-Specific Voxel-Wise Dosimetry: A Monte Carlo Investigation

Optimizing Cancer Treatment: Exploring the Role of AI in Radioimmunotherapy

Radioimmunotherapy (RIT) is a novel cancer treatment that combines radiotherapy and immunotherapy to precisely target tumor antigens using monoclonal antibodies conjugated with radioactive isotopes. This approach offers personalized, systemic, and durable treatment, making it effective in cancers resistant to conventional therapies. Advances in artificial intelligence (AI) present opportunities to enhance RIT by improving precision, efficiency, and personalization. AI plays a critical role in patient selection, treatment planning, dosimetry, and response assessment, while also contributing to drug design and tumor classification. This review explores the integration of AI into RIT, emphasizing its potential to optimize the entire treatment process and advance personalized cancer care.

Dosimetric and biological impact of activity extravasation of radiopharmaceuticals in PET imaging

BACKGROUND

The increasing use of nuclear medicine and PET imaging has intensified scrutiny of radiotracer extravasation. To our knowledge, this topic is understudied but holds great potential for enhancing our understanding of extravasation in clinical PET imaging.

PURPOSE

This work aims to (1) quantify the absorbed doses from radiotracer extravasation in PET imaging, both locally at the site of extravasation and with the extravasation location as a source of exposure to bodily organs and (2) assess the biological ramifications within the injection site at the cellular level.

METHODS

A radiation dosimetry simulation was performed using a whole-body 4D Extended Cardiac-Torso (XCAT) phantom embedded in the GATE Monte Carlo platform. A 10-mCi dose of 18F-FDG was chosen to simulate a typical clinical PET scan scenario, with 10% of the activity extravasated in the antecubital fossa of the right arm of the phantom. The extravasation volume was modeled as a 5.5 mL rectangle in the hypodermal layer of skin. Absorbed dose contributions were calculated for the first two half-lives, assuming biological clearance thereafter. Dose calculations were performed as absorbed doses at the organ and skin levels. Energy deposition was simulated both at the local extravasation site and in multiple organs of interest and converted to absorbed doses based on their respective masses. Each simulation was repeated ten times to estimate Monte Carlo uncertainties. Biological impacts on cells within the extravasated volume were evaluated by randomizing cells and exposing them to a uniform radiation source of 18F and 68Ga. Particle types, their energies, and direction cosines were recorded in phase space files using a separate Geant4 simulation to characterize their entry into the nucleus of the cellular volume. Subsequently, the phase space files were imported into the TOPAS-nBio simulation to assess the extent of DNA damage, including double-strand breaks (DSBs) and single-strand breaks (SSBs).

RESULTS

Organ-level dosimetric estimations are presented for 18F and 68Ga radionuclides in various organs of interest. With 10% extravasation, the hypodermal layer of the skin received the highest absorbed dose of 1.32 ± 0.01 Gy for 18F and 0.99 ± 0.01 Gy for 68Ga. The epidermal and dermal layers received absorbed doses of 0.07 ± 0.01 Gy and 0.13 ± 0.01 Gy for 18F, and 0.14 ± 0.01 Gy and 0.29 ± 0.01 Gy for 68Ga, respectively. In the extravasated volume, 18F caused an average absorbed dose per nucleus of 0.17 ± 0.01 Gy, estimated to result in 10.58 ± 0.50 DSBs and 268.11 ± 12.43 SSBs per nucleus. For 68Ga, the absorbed dose per nucleus was 0.11 ± 0.01 Gy, leading to an estimated 6.49 ± 0.34 DSBs and 161.24 ± 8.12 SSBs per nucleus. Absorbed doses in other organs were on the order of micro-gray (µGy).

CONCLUSION

The likelihood of epidermal erythema resulting from extravasation during PET imaging is low, as the simulated absorbed doses to the epidermis remain below the thresholds that trigger such effects. Moreover, the organ-level absorbed doses were found to be clinically insignificant across various simulated organs. The minimal DNA damage at the extravasation site suggests that long-term harm, such as radiation-induced carcinogenesis, is highly unlikely.

A diffusion-leakage model coupled with dose point kernels (DPK) for dosimetry of diffusing alpha-emitters radiation therapy (DaRT)

BACKGROUND

Diffusing alpha-emitters radiation therapy (DaRT) is a novel brachytherapy technique that leverages the diffusive flow of 224Ra progeny within the tumor volume over the course of the treatment. Cell killing is achieved by the emitted alpha particles that have a short range in tissue and high linear energy transfer. The current proposed absorbed dose calculation method for DaRT is based on a diffusion-leakage (DL) model that neglects absorbed dose from beta particles.

PURPOSE

This work aimed to couple the DL model with dose point kernels (DPKs) to account for dose from beta particles as well as to consider the non-local deposition of energy.

METHODS

The DaRT seed was modeled using COMSOL multiphysics and the DL model was implemented to extract the spatial information of the diffusing daughters. Using Monte-Carlo (MC) methods, DPKs were generated for 212Pb, 212Bi, and their progenies since they were considered to be the dominant beta emitters in the 224Ra radioactive decay chain. A convolution operation was performed between the integrated number densities of the diffusing daughters and DPKs to calculate the total absorbed dose over a 30-day treatment period. Both high-diffusion and low-diffusion cases were considered.

RESULTS

The calculated DPKs showed non-negligible energy deposition over several millimeters from the source location. An absorbed dose >10 Gy was deposited within a 1.8 mm radial distance for the low diffusion case and a 2.2 mm radial distance for the high diffusion case. When the DPK method was compared with the local energy deposition method that solely considered dose from alpha particles, differences above 1 Gy were found within 1.3 and 1.8 mm radial distances from the surface of the source for the low diffusion and high diffusion cases, respectively.

CONCLUSIONS

The proposed method enhances the accuracy of the dose calculation method used for the DaRT technique.

Development of a stand-alone precalculated Monte Carlo code to calculate the dose by alpha and beta emitters from the Ra-224 decay chain

BACKGROUND

Recent developments in alpha and beta emitting radionuclide therapy highlight the importance of developing efficient methods for patient-specific dosimetry. Traditional tabulated methods such as Medical Internal Radiation Dose (MIRD) estimate the dose at the organ level while more recent numerical methods based on Monte Carlo (MC) simulations are able to calculate dose at the voxel level. A precalculated MC (PMC) approach was developed in this work as an alternative to time-consuming fully simulated MC. Once the spatial distribution of alpha and beta emitters is determined using imaging and/or numerical methods, the PMC code can be used to achieve an accurate voxelized 3D distribution of the deposited energy without relying on full MC calculations.

PURPOSE

To implement the PMC method to calculate energy deposited by alpha and beta particles emitted from the Ra-224 decay chain.

METHODS

The GEANT4 (version 10.7) MC toolkit was used to generate databases of precalculated tracks to be integrated in the PMC code as well as to benchmark its output. In this regard, energy spectra of alpha and beta particles emitted by the Ra-224 decay chain were generated using GAMOS (version 6.2.0) and imported into GEANT4 macro files. Either alpha or beta emitting sources were defined at the center of a homogeneous phantom filled with various materials such as soft tissue, bone, and lung where particles were emitted either mono-directionally (for database generation) or isotropically (for benchmarking). Two heterogeneous phantoms were used to demonstrate PMC code compatibility with boundary crossing events. Each precalculated database was generated step-by-step by storing particle track information from GEANT4 simulations followed by its integration in a PMC code developed in MATLAB. For a user-defined number of histories, one of the tracks in a given database was selected randomly and rotated randomly to reflect an isotropic emission. Afterward, deposited energy was divided between voxels based on step length in each voxel using a ray-tracing approach. The radial distribution of deposited energy was benchmarked against fully simulated MC calculations using GEANT4. The effect of the GEANT4 parameter StepMax on the accuracy and speed of the code was also investigated.

RESULTS

In the case of alpha decay, primary alpha particles show the highest contribution (>99%) in deposited energy compared to their secondary particles. In most cases, protons act as the main secondary particles in the deposition of energy. However, for a lung phantom, using a range cutoff parameter of 10 µm on primary alpha particles yields a higher contribution of secondary electrons than protons. Differences between deposited energy calculated by PMC and fully simulated MC are within 2% for all alpha and beta emitters in homogeneous and heterogeneous phantoms. Additionally, statistical uncertainties are less than 1% for voxels with doses higher than 5% of the maximum dose. Moreover, optimization of the parameter StepMax is necessary to achieve the best tradeoff between code accuracy and speed.

CONCLUSIONS

The PMC code shows good performance for dose calculations deposited by alpha and beta emitters. As a stand-alone algorithm, it is suitable to be integrated into clinical treatment planning systems.

Technical note: The contribution of internal bremsstrahlung to the 90 Y dose point kernel

BACKGROUND

Internal dosimetry has an increasing role in the planning and verification of nuclear medicine therapies with radiopharmaceuticals. Dose Point Kernels (DPKs), quantifying the energy deposition all around a point source, in a homogenous medium, are extensively used for 3D dosimetry and nowadays are mostly evaluated by Monte Carlo (MC) simulation. To our knowledge, DPK for beta emitters is estimated neglecting the continuous photon emission due to the Internal Bremsstrahlung (IB), whose contribution to the absorbed dose can be relevant beyond the maximum range of betas, as evidenced in recent works.

PURPOSE

Aim of this study was to investigate and quantify, by means of MC simulations, the contribution of IB photons to DPK calculated for ⁹⁰ Y and provide the updated ⁹⁰ Y DPK.

METHODS

The overall radiation due to the decay of a ⁹⁰ Y point source, placed at the centre of concentric water shells of increasing radii from 0.02 cm to 20 cm, was simulated with GAMOS, including the IB source term whose spectral distribution was described by an analytical model. Energy deposition was scored in the shells as a function of the distance from the source, R, and DPK was estimated in terms of the scaled absorbed dose fraction, F(R/X₉₀ ), where X₉₀ is the range within which the beta particles deposit 90% of their energy.

RESULTS

A comparison between the two simulated absorbed dose distributions, calculated with or without IB, clearly shows that the latter (incomplete) choice is consistent with the findings of other Authors and systematically underestimates the absorbed dose imparted to the tissue. ⁹⁰ Y DPK values currently used are underestimated by 20%-34% for R>2X₉₀ .

CONCLUSIONS

The revised values provided in this work suggest that the inclusion of IB emission in DPK evaluations is advisable for pure beta emitters.

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

PURPOSE

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 cm² 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 average 1.2% (range -8.0% to 15.2%) across all depths for ¹⁷⁷ Lu and on average 4.0% (range -5.0% to 9.3%) across all depths for ⁹⁰ Y. DPM absorbed dose estimates agreed with estimates from EGSnrc and MCNP across the board, within 4.7% and within 3.4% for ¹⁷⁷ Lu and ⁹⁰ Y respectively, for all geometries and across all depths. MC showed that absorbed dose to RF from betas was greater than 92% of the total (betas + other radiations) for ¹⁷⁷ Lu, indicating measurement of dominant beta contribution with our design.

CONCLUSIONS

The reproducible results with a RF insert in a simple phantom designed for liquid sources demonstrate that this is a reliable setup for experimentally validating dosimetry algorithms used in therapies with beta-emitting unsealed sources. Absorbed doses estimated with the DPM MC code showed close agreement with RF measurement and with results from two general purpose MC codes, thereby validating the use of this algorithms for clinical RPT dosimetry.

Monte Carlo evaluation of hypothetical long axial field-of-view PET scanner using GE discovery MI PET front-end architecture

PURPOSE

The development of total-body PET scanners is of growing interest in the PET community. Investigation into the imaging properties of a hypothetical extended axial field-of-view (AFOV) GE Healthcare SiPM-based Discovery MI (DMI) system architecture has not yet been performed. In this work, we assessed its potential as a whole-body scanner using Monte Carlo simulations. The aim of this work was to (1) develop and validate a Monte Carlo model of a 4-ring scanner and (2) extend its AFOV up to 2 m to evaluate performance gain through NEMA-based evaluation.

METHODS

The DMI 4-ring geometry and its pulse digitization scheme were modeled within the GATE Monte Carlo platform using published literature. The GATE scanner model was validated by comparing results against published NEMA performance measurements. Following the validation of the 4-ring model, the model was extended to simulate 8, 20, 30, and 40-ring systems. Spatial resolution, sensitivity, NECR, and scatter fraction were characterized with modified NEMA NU-2 2018 standards; however, the image quality measurements were not acquired due to computational limitations. Spatial resolutions were simulated for all scanner ring configurations using point sources to examine the effects of parallax errors. NEMA count rates were estimated using a standard 70 cm scatter phantom and an extended version of scatter phantom of length 200 cm with (1-800) MBq of ¹⁸ F for all scanners. Sensitivity was evaluated using NEMA methods with a 70 cm standard and a 200 cm long line source.

RESULTS

The average FWHM of the radial/tangential/axial spatial resolution reconstructed with filtered back-projection at 1 and 10 cm from the scanner center were 3.94/4.10/4.41 mm and 5.29/4.89/5.90 mm for the 4-ring scanner. Sensitivity was determined to be 14.86 cps/kBq at the center of the FOV for the 4-ring scanner using a 70 cm line source. Sensitivity enhancement up to 21-fold and 60-fold were observed for 1 m and 2 m AFOV scanners compared to 4-ring scanner using a 200 cm long line source. Spatial resolution simulations in a 2 m AFOV scanner suggest a maximum degradation of ∼23.8% in the axial resolution compared to the 4-ring scanner. However, the transverse resolution was found to be relatively constant when increasing the axial acceptance angle up to ±70°. The peak NECR was 212.92 kcps at 22.70 kBq/mL with a scatter fraction of 38.9% for a 4-ring scanner with a 70 cm scatter phantom. Comparison of peak NECR using the 200 cm long scatter phantom relative to the 4-ring scanner resulted in a NECR gain of 15 for the 20-ring and 28 for the 40-ring geometry. Spatial resolution, sensitivity, and scatter fraction showed an agreement within ∼7% compared with published measured values.

CONCLUSIONS

The 4-ring DMI scanner simulation was successfully validated against published NEMA measurements. Sensitivity and NECR performance of extended 1 and 2 meters AFOV scanners based upon the DMI architecture were subsequently simulated. Increases in sensitivity and count-rate performance are consistent with prior simulation studies utilizing extensions of the Siemens mCT architecture and published NEMA measurements with the uEXPLORER system. This article is protected by copyright. All rights reserved.

Implementation of dose point kernel (DPK) for dose optimization of 177Lu/90Y cocktail radionuclides in internal dosimetry

BACKGROUND

Due to the importance of choosing the applicable dosimetry method in radionuclide therapy, the present study was conducted to investigate the efficiency of the implementation of Dose Point Kernel (DPK) for dose optimization of ¹⁷⁷Lu/⁹⁰Y Cocktail Radionuclides in internal Dosimetry.

METHODS

In this study, simulations and calculations of DPK were performed using the GATE/GEANT4 Monte Carlo code. For specific liver dosimetry, the NCAT phantom and convolution algorithm-based Fast Fourier Transform method was used by MATLAB software.

RESULTS

The self-dose of ¹⁷⁷Lu and ⁹⁰Y radionuclides in the liver of NCAT phantom were 1.1708E-13, and 4.8420E-11 (Gy/Bq), respectively, and the cross-dose of ¹⁷⁷Lu and ⁹⁰Y radionuclides out of the liver of NCAT phantom were 2.03615E-16, and 0.8422E-13 (Gy/Bq) respectively. Overall results showed that with an increase the value of ⁹⁰Y with quarter steps in a cocktail, the amount of the self-dose increase 1.5, 6, and 29 times respectively, and with an increase the value of ¹⁷⁷Lu in quarter step in a cocktail, the amount of the cross dose decrease 3, 15 and 68 percent respectively.

CONCLUSION

Generally, the present results indicate that the calculated DPK functions of ¹⁷⁷Lu and ⁹⁰Y cocktails can play an important role in choosing the best combination of radionuclide to optimize treatment planning in cocktail radionuclide therapy.

A Monte Carlo Investigation of Dose Point Kernel Scaling for α-Emitting Radionuclides

Background: Density-based dose point kernel (DPK) scaling accuracy was investigated in various homogeneous tissue media. Methods: Using GEometry ANd Tracking 4 Monte Carlo code, DPKs were generated for 5, 8 MeV monoenergetic α particles and ²²³Ra, ²²⁵Ac, and ²²⁷Th. Dose was scored in 1 μm thick concentric shells and DPKs were scaled based on the tissue's mass density and compared with the water DPK. Results: Scaled kernels agreed within ±5% except near the Bragg peaks, where they differed up to 25%. Conclusions: The authors conclude that kernel scaling based on mass density of the transport medium can be utilized accurately up to 5%, excluding Bragg peak regions.

Absorbed dose distributions from beta-decaying radionuclides: Experimental validation of Monte Carlo tools for radiopharmaceutical dosimetry

PURPOSE

This study aims to experimentally validate the Monte Carlo generated absorbed doses from the beta particles emitted by ⁹⁰ Y and ¹⁷⁷ Lu using radiochromic EBT3 film-based dosimetry.

METHODS

Line sources of ⁹⁰ Y and ¹⁷⁷ Lu were inserted longitudinally through blocks of low-density polyethylene and tissue-equivalent slabs of cortical bone and lung equivalent plastics. Radiochromic film (Gafchromic EBT3) was laser cut to accommodate orthogonal line sources of radioactivity, and the film was sandwiched intimately between the rectangular blocks to achieve charged particle equilibrium. Line sources consisted of plastic capillary tube of length (13 ± 0.1) cm, with 0.42-mm inner diameter and a wall thickness of 0.21 mm. ⁹⁰ Y line sources were prepared from a solution of dissolved ⁹⁰ Y resin microspheres. ¹⁷⁷ Lu line sources were prepared from an aliquot of ¹⁷⁷ Lu-DOTATATE. Film exposures were conducted for durations ranging from 10 min to 38 h. Radiochromic film calibration was performed by irradiation with 6-MV-bremsstrahlung x rays from a calibrated linear accelerator, in accordance with literature recommendations. Experimental geometries were precisely simulated within the GATE Monte Carlo toolkit, which has previously been used for the generation of dose point kernels.

RESULTS

The mean percentage difference between measured and simulated absorbed doses were 5.04% and 7.21% for ⁹⁰ Y and ¹⁷⁷ Lu beta absorbed dose in the range of (0.1-10) Gy. Additionally, 1D gamma analysis using a local 10%/1 mm gamma criterion was performed to compare the absorbed dose distributions. The percentage of measurement points passing the gamma criterion, averaged over all tests, was 93.5%.

CONCLUSIONS

We report the experimental validation of Monte Carlo derived beta absorbed dose distributions for ⁹⁰ Y and ¹⁷⁷ Lu, solidifying the validity of using Monte Carlo-based methods for estimating absorbed dose from beta emitters. Overall, excellent agreement was observed between the experimental beta absorbed doses in the linear region of the radiochromic film and the GATE Monte Carlo simulations demonstrating that radiochromic film dosimetry has sufficient sensitivity and spatial resolution to be used as a tool for measuring beta decay absorbed dose distributions.