Treatment planning for radio-immunotherapy

Application of machine learning to pretherapeutically estimate dosimetry in men with advanced prostate cancer treated with 177Lu-PSMA I&T therapy

Purpose

Although treatment planning and individualized dose application for emerging prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) are generally recommended, it is still difficult to implement in practice at the moment. In this study, we aimed to prove the concept of pretherapeutic prediction of dosimetry based on imaging and laboratory measurements before the RLT treatment.

Methods

Twenty-three patients with metastatic castration-resistant prostate cancer (mCRPC) treated with ¹⁷⁷Lu-PSMA I&T RLT were included retrospectively. They had available pre-therapy ⁶⁸ Ga-PSMA-HEBD-CC PET/CT and at least 3 planar and 1 SPECT/CT imaging for dosimetry. Overall, 43 cycles of ¹⁷⁷Lu-PSMA I&T RLT were applied. Organ-based standard uptake values (SUVs) were obtained from pre-therapy PET/CT scans. Patient dosimetry was calculated for the kidney, liver, spleen, and salivary glands using Hermes Hybrid Dosimetry 4.0 from the planar and SPECT/CT images. Machine learning methods were explored for dose prediction from organ SUVs and laboratory measurements. The uncertainty of these dose predictions was compared with the population-based dosimetry estimates. Mean absolute percentage error (MAPE) was used to assess the prediction uncertainty of estimated dosimetry.

Results

An optimal machine learning method achieved a dosimetry prediction MAPE of 15.8 ± 13.2% for the kidney, 29.6% ± 13.7% for the liver, 23.8% ± 13.1% for the salivary glands, and 32.1 ± 31.4% for the spleen. In contrast, the prediction based on literature population mean has significantly larger MAPE (p < 0.01), 25.5 ± 17.3% for the kidney, 139.1% ± 111.5% for the liver, 67.0 ± 58.3% for the salivary glands, and 54.1 ± 215.3% for the spleen.

Conclusion

The preliminary results confirmed the feasibility of pretherapeutic estimation of treatment dosimetry and its added value to empirical population-based estimation. The exploration of dose prediction may support the implementation of treatment planning for RLT.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05883-w.

Evaluation of Dosimetric Parameters for Tumor Therapy with 177Lu and 90Y Radionuclides in Gate Monte Carlo Code

BACKGROUND

⁹⁰Y and ¹⁷⁷Lu are two well-known radionuclides used in radionuclide therapy to treat neuroendocrine tumors.

OBJECTIVE

This current study aims to evaluate, compare and optimize tumor therapy with ⁹⁰Y and ¹⁷⁷Lu for different volumes of the tumor using the criterion of self-absorbed dose, cross-absorbed dose, absorbed dose profile, absorbed dose uniformity, and dose-volume histogram (DVH) curve using Gate Monte Carlo simulation code.

MATERIAL AND METHODS

In our analytical study, Gate Monte Carlo simulation code has been used to model tumors and simulate particle transport. Spherical tumors were modeled from radius 0.5 to 20 mm. Tumors were uniformly designed from water (soft tissue reagent). The full energy spectrum of each radionuclide of ¹⁷⁷Lu and ⁹⁰Y was used in the total volume of tumors with isotropic radiation, homogeneously. Self-absorbed dose, cross-absorbed dose, absorbed dose profile, absorbed dose uniformity, and DVH curve parameters were evaluated.

RESULTS

The absorbed dose for ⁹⁰Y is higher than ¹⁷⁷Lu in all tumors (p-value <5%). The uniformity of the absorbed dose for ¹⁷⁷Lu is much greater than ⁹⁰Y. As the tumor size increases, the DVH graph improves for ⁹⁰Y.

CONCLUSION

Based on self-absorbed dose, cross-absorbed dose, absorbed dose uniformity, and DVH diagram, ¹⁷⁷Lu and ⁹⁰Y are appropriate for smaller and larger tumors, respectively. Next, we can evaluate the appropriate cocktail of these radionuclides, in terms of the type of composition, for the treatment of tumors with a specific size.

PET imaging facilitates antibody screening for synergistic radioimmunotherapy with a 177Lu-labeled αPD-L1 antibody

Rationale: The low response rate of immunotherapy, such as anti-PD-L1/PD-1 and anti-CTLA4, has limited its application to a wider population of cancer patients. One widely accepted view is that inflammation within the tumor microenvironment is low or ineffective for inducing the sufficient infiltration and/or activation of lymphocytes. Here, a highly tumor-selective anti-PD-L1 (αPD-L1) antibody was developed through PET imaging screening, and it was radiolabeled with Lu-177 for PD-L1-targeted radioimmunotherapy (RIT) and radiation-synergized immunotherapy. Methods: A series of αPD-L1 antibodies were radiolabeled with zirconium-89 for PET imaging to screen the most suitable antibodies for RIT. Mice were divided into an immunotherapy group, a RIT group and a radiation-synergized immunotherapy group to evaluate the therapeutic effect. Alterations in the tumor microenvironment after treatment were assessed using flow cytometry and immunofluorescence microscopy. Results: Radiation-synergistic RIT can achieve a significantly better therapeutic effect than immunotherapy or RIT alone. The dosages of the radiopharmaceuticals and αPD-L1 antibodies were reduced, the infiltration of CD4⁺ and CD8⁺ T cells in the tumor microenvironment was increased, and no side effects were observed. This radiation-synergistic RIT strategy successfully showed a strong synergistic effect with αPD-L1 checkpoint blockade therapy, at least in the mouse model. Conclusions: PET imaging of ⁸⁹Zr-labeled antibodies is an effective method for antibody screening. RIT with a ¹⁷⁷Lu-labeled αPD-L1 antibody could successfully upregulate antitumor immunity in the tumor microenvironment and turn "cold" tumors "hot" for immunotherapy.

SPECT performance evaluation on image of Yttrium 90 - Bremsstrahlung using Monte Carlo simulation

Yttrium-90 (⁹⁰Y) is one of the most widely used radionuclides in Nuclear Medicine practice. However, characteristic energy of this beta emitter constitutes a difficulty for dose planning using SPECT imaging. This work aimed to study bremsstrahlung X-rays effects produced by ⁹⁰Y beta particles during SPECT image acquisition using Monte Carlo code MCNPX. Several simulations were carried out to evaluate different aspects that could affect SPECT image quality, such as: collimator type, source-collimator distance and composition of each interacting material. Two configurations of ⁹⁰Y sources were simulated: a point source in several spheres of different materials (soft tissue, water, articular cartilage, and bone) and dimensions with radius ranging from 1 to 20 mm; and a uniformly distributed source in a Lucite cylindrical phantom filled with water. It was evaluated the bremsstrahlung photon emission generated inside different materials; for this was considered the number photons that passing through every different sphere's surface for each radii and material. In case of cylindrical phantom filled with water, in order to obtain the energy deposited over NaI (Tl) crystal detector; there was considered Median Energy General Purpose (MEGP) and Low Energy High Resolution (LEHR) collimators. Moreover, using TMESH routine available in the MCNPX Monte Carlo code, energy distribution images according to the collimator type and the source-collimator distance were obtained. The simulation was validated by comparing with the spectral distribution of the ⁹⁰Y bremsstrahlung X-rays obtained experimentally from an acrylic cylindrical phantom. Results corroborated the importance of Monte Carlo simulation method to evaluate a performance of SPECT image acquisition with ⁹⁰Y. The best resolution was obtained with MEGP collimator independent of source-collimator distance.

Comparison between Fractionated Dose and Single Dose of Cu-64 Trastuzumab Therapy in the NCI-N87 Gastric Cancer Mouse Model

For optimum radioimmunotherapy (RIT), deep penetration and uniform distribution into the tumor core is important. The solid tumor microenvironment, consisting of a highly fibrotic or desmoplastic tumor, abnormal tumor vasculature, high fluid pressure, and the absence of fluid lymphatics, limits the distribution of monoclonal antibodies mAbs to the tumor core. To investigate the optimal rationale for therapeutic mAbs administration and the microdistribution of mAbs, single and serial fractional dosage regimens of Cu-64-trastuzumab (TRZ) with paclitaxel were evaluated. Groups of nude mice were inoculated with gastric cancer cell line NCI-N87 tumor cells. When the tumor size reached 200 ± 20 mm³, the mice were divided into two groups for injection of Alexa-647-TRZ. One group (n = 5) was injected with 15 mg/kg in a single dose (SD), and the other group (n = 5) with two doses of 7.5 mg/kg (fractionated dose (FD)). In both cases, the injections were done intravenously in combination with intraperitoneal paclitaxel either as a SD of 70 mg/kg or fractionated into two doses of 40 and 30 mg/kg. Tumors were harvested, flash frozen, and sectioned (8 µm) five days after Alexa-647-TRZ injection. Rhodamine lectin (rhodamine-labeled Ricinus communis agglutinin I, 1 mg in 0.2 mL of phosphate-buffered saline (PBS)) was intravenously injected to delineate the functional vessel for a wait time of 5 min before animal euthanization. Microscopic images were acquired with an IN Cell Analyzer. The amount of TRZ that penetrated the tumor surface and the tumor vessel was calculated by area under the curve (AUC) analysis. For RIT efficacy (n = 21), Cu-64-TRZ was injected following the same dose schedule to observe tumor volume and survival ratio for 30 days. The SD and FD regimens of Alexa-647-TRZ were observed to have no significant difference in penetration of mAbs from the tumor edge and vessel, nor was the total accumulation across the whole tumor tissue significantly different. Additionally, the SD and FD regimens of Cu-64-TRZ were not proven to be significantly efficacious. Our study reveals that SD and FD in a treatment design with Cu-64-TRZ and paclitaxel shows no significant difference in therapeutic efficacy on tumor growth inhibition in vivo in mice bearing human gastric cancer xenografts overexpressing HER2 antigen.

Measurement of Tumor Pressure and Strategies of Imaging Tumor Pressure for Radioimmunotherapy

Tumor interstitial pressure is a fundamental feature of cancer biology. Elevation in tumor pressure affects the efficacy of cancer treatment and results in the heterogenous intratumoral distribution of drugs and macromolecules. Monoclonal antibodies (mAb) play a prominent role in cancer therapy and molecular nuclear imaging. Therapy using mAb labeled with radionuclides—also known as radioimmunotherapy (RIT)—is an effective form of cancer treatment. RIT is clinically effective for the treatment of lymphoma and other blood cancers; however, its clinical use for solid tumor was limited because their high interstitial pressure prevents mAb from penetrating into the tumor. This pressure can be decreased using anti-cancer drugs or additional external therapy. In this paper, we reviewed the intratumoral pressure using direct tumor-pressure measurement strategies, such as the wick-in-needle and pressure catheter transducer method, and indirect tumor-pressure measurement strategies via magnetic resonance.

Quantitative Imaging for Targeted Radionuclide Therapy Dosimetry - Technical Review

Targeted radionuclide therapy (TRT) is a promising technique for cancer therapy. However, in order to deliver the required dose to the tumor, minimize potential toxicity in normal organs, as well as monitor therapeutic effects, it is important to assess the individualized internal dosimetry based on patient-specific data. Advanced imaging techniques, especially radionuclide imaging, can be used to determine the spatial distribution of administered tracers for calculating the organ-absorbed dose. While planar scintigraphy is still the mainstream imaging method, SPECT, PET and bremsstrahlung imaging have promising properties to improve accuracy in quantification. This article reviews the basic principles of TRT and discusses the latest development in radionuclide imaging techniques for different theranostic agents, with emphasis on their potential to improve personalized TRT dosimetry.

Combination Radioimmunotherapy Approaches and Quantification of Immuno-PET

Monoclonal antibodies (mAbs), which play a prominent role in cancer therapy, can interact with specific antigens on cancer cells, thereby enhancing the patient's immune response via various mechanisms, or mAbs can act against cell growth factors and, thereby, arrest the proliferation of tumor cells. Radionuclide-labeled mAbs, which are used in radioimmunotherapy (RIT), are effective for cancer treatment because tumor associated-mAbs linked to cytotoxic radionuclides can selectively bind to tumor antigens and release targeted cytotoxic radiation. Immunological positron emission tomography (immuno-PET), which is the combination of PET with mAb, is an attractive option for improving tumor detection and mAb quantification. However, RIT remains a challenge because of the limited delivery of mAb into tumors. The transport and uptake of mAb into tumors is slow and heterogeneous. The tumor microenvironment contributed to the limited delivery of the mAb. During the delivery process of mAb to tumor, mechanical drug resistance such as collagen distribution or physiological drug resistance such as high intestinal pressure or absence of lymphatic vessel would be the limited factor of mAb delivery to the tumor at a potentially lethal mAb concentration. When α-emitter-labeled mAbs were used, deeper penetration of α-emitter-labeled mAb inside tumors was more important because of the short range of the α emitter. Therefore, combination therapy strategies aimed at improving mAb tumor penetration and accumulation would be beneficial for maximizing their therapeutic efficacy against solid tumors.

Limits of Tumor Detectability in Nuclear Medicine and PET

OBJECTIVE

Nuclear medicine is becoming increasingly important in the early detection of malignancy. The advantage of nuclear medicine over other imaging modalities is the high sensitivity of the gamma camera. Nuclear medicine counting equipment has the capability of detecting levels of radioactivity which exceed background levels by as little as 2.4 to 1. This translates to only a few hundred counts per minute on a regular gamma camera or as few as 3 counts per minute when using coincidence detection on a positron emission tomography (PET) camera.

MATERIAL AND METHODS

We have experimentally measured the limits of detectability using a set of hollow spheres in a Jaszczak phantom at various tumor-to-background ratios. Imaging modalities for this work were (1) planar, (2) SPECT, (3) PET, and (4) planar camera with coincidence detection capability (MCD).

RESULTS

When there is no background (infinite contrast) activity present, the detectability of tumors is similar for PET and planar imaging. With the presence of the background activity , PET can detect objects in an order of magnitude smaller in size than that can be seen by conventional planar imaging especially in the typical clinical low (3:1) T/B ratios. The detection capability of the MCD camera lies between a conventional nuclear medicine (planar / SPECT) scans and the detection capability of a dedicated PET scanner.

CONCLUSION

Among nuclear medicine's armamentarium, PET is the closest modality to CT or MR imaging in terms of limits of detection. Modern clinical PET scanners have a resolution limit of 4 mm, corresponding to the detection of tumors with a volume of 0.2 ml (7 mm diameter) in 5:1 T/B ratio. It is also possible to obtain better resolution limits with dedicated brain and animal scanners. The future holds promise in development of new detector materials, improved camera design, and new reconstruction algorithms which will improve sensitivity, resolution, contrast, and thereby further diminish the limits of tumor detectability.

CONFLICT OF INTEREST

None declared.

Overview of dosimetry for systemic targeted radionuclide therapy (STaRT)

The purposes of systemic targeted radionuclide therapy dosimetry include compiling a database of normal organ radiation-absorbed doses that are carrier- and radionuclide-specific, and assuring that the normal organ radiation doses are within a safe range before therapy. Also of importance is quantitation of radiation delivery to tumors vs. normal tissues to correlate absorbed dose with tumor control. For agents with significant and variable excretion, estimates of individual patient distribution/clearance may be needed to optimize the dose-response relationship.

Validation of the Monte Carlo simulator GATE for indium-111 imaging

Monte Carlo simulations are useful for optimizing and assessing single photon emission computed tomography (SPECT) protocols, especially when aiming at measuring quantitative parameters from SPECT images. Before Monte Carlo simulated data can be trusted, the simulation model must be validated. The purpose of this work was to validate the use of GATE, a new Monte Carlo simulation platform based on GEANT4, for modelling indium-111 SPECT data, the quantification of which is of foremost importance for dosimetric studies. To that end, acquisitions of (111)In line sources in air and in water and of a cylindrical phantom were performed, together with the corresponding simulations. The simulation model included Monte Carlo modelling of the camera collimator and of a back-compartment accounting for photomultiplier tubes and associated electronics. Energy spectra, spatial resolution, sensitivity values, images and count profiles obtained for experimental and simulated data were compared. An excellent agreement was found between experimental and simulated energy spectra. For source-to-collimator distances varying from 0 to 20 cm, simulated and experimental spatial resolution differed by less than 2% in air, while the simulated sensitivity values were within 4% of the experimental values. The simulation of the cylindrical phantom closely reproduced the experimental data. These results suggest that GATE enables accurate simulation of (111)In SPECT acquisitions.

Radiation treatment planning with an integrated positron emission and computer tomography (PET/CT): a feasibility study

PURPOSE

To investigate the usefulness of hardware coregistered PET/CT images for target volume definition.

METHODS AND MATERIALS

Thirty-nine patients presenting with various solid tumors were investigated. CT and a FDG-PET were obtained in treatment position in an integrated PET/CT scanner, and coregistered images were used for treatment planning. First, volume delineation was performed on the CT data. In a second step, the corresponding PET data were used as an overlay to the CT data to define the target volume. Delineation was done independently by two investigators.

RESULTS

Coregistered PET/CT showed good fusion accuracy. The GTV increased by 25% or more because of PET in 17% of cases with head-and-neck (2/12) and lung cancer (1/6), and in 33% (7/21) in cancer of the pelvis. The GTV was reduced > or =25% in 33% of patients with head-and-neck cancer (4/12), in 67% with lung cancer (4/6), and 19% with cancer of the pelvis (4/21). Overall, in 56% (22/39) of cases, GTV delineation was changed significantly if information from metabolic imaging was used in the planning process. The modification of the GTV translated into altered PTV changes exceeding >20% in 46% (18/39) of cases. With PET, volume delineation variability between two independent oncologists decreased from a mean volume difference of 25.7 cm(3) to 9.2 cm(3) associated with a reduction of the standard deviation from 38.3 cm(3) to 13.3 cm(3) (p = 0.02). In 16% of cases, PET/CT revealed distant metastasies, changing the treatment strategy from curative to palliative.

CONCLUSION

Integrated PET/CT for treatment planning for three-dimensional conformal radiation therapy improves the standardization of volume delineation compared with that of CT alone. PET/CT has the potential for reducing the risk for geographic misses, to minimize the dose of ionizing radiation applied to non-target organs, and to change the current practice to three-dimensional conformal radiation therapy planning by taking into account the metabolic and biologic features of cancer. The impact on treatment outcome remains to be demonstrated.

Validation of the EGS usercode DOSE3D for internal beta dose calculation at the cellular and tissue levels

Internal radiotherapy is currently focusing on beta emitters such as 90Y or 131I because of their high-energy emissions. However, conventional dosimetric methods (MIRD) are known to be limited for such applications. They are unable to take into account microscopic radionuclide distribution because standardized anthropomorphic phantoms are used, and absorbed dose is calculated at the organ level. New tools are therefore required for dose assessment at cellular and tissue level (10-100 microm). The purpose of this study was to validate, at this scale, a Monte Carlo usercode (DOSE3D), based on the MORSE combinatorial geometry package and the EGS code system. Dose point-kernel calculations in water were compared to those published by Cross et al and Simpkin and Mackie. They confirm that DOSE3D is a reliable tool for cellular dosimetry in various geometric configurations.

Impact of scatter and attenuation corrections for iodine-131 two-dimensional quantitative imaging in patients

This study assessed the impact of scatter and attenuation corrections on the estimated activity delivered to whole body and liver in five patients included in a radioimmunotherapy clinical trial. Before injection of the radiopharmaceutical, transmission images were acquired with the Transmission Attenuation Correction - Whole-body (SMVi-GEMS) prototype. Emission images were obtained in energy-indexed list mode at least four times after injection. 20% window and scatter-corrected images (Dual Energy Window-DEW and Triple Energy Window-TEW) were generated. Whole-body activity was calculated 1-h after injection (and compared with injected activity). Cumulated activities in whole body and liver were determined according to the geometric mean approach. The mean relative error made in estimations of whole-body activity at 1-h was 6.9+/-10.3% without corrections. Taking scatter into account led to underestimation, but reduced the influence of patient morphotype (-40.0+/-7.6% and -43.3+/-6.2% for DEW and TEW). Attenuation correction led to a large overestimation, whether used alone (155.2+/-39.0%) or associated with scatter correction (39.6+/-10.4% and 35.9+/-10.2% for DEW and TEW). Compared to the geometric mean alone, scatter correction led to a reduction of cumulated activities of around 45% for whole body and less than 30% for liver. Attenuation correction had a more marked impact, particularly for liver where estimated cumulated activity increased from 150 to 300%. Preliminary scatter correction limited the increase to 100% for DEW and 150% for TEW in liver and to 25% for both DEW and TEW in whole body. Although this would probably be different at the organ level, the calculation of whole-body activity without scatter and attenuation correction gave the lowest biases. But from a scientific point of view, this cannot be a satisfactory method. Attenuation correction has a greater impact than scatter correction. The association of both corrections is not sufficient to obtain accurate absolute quantification. Other factors limit planar quantification with iodine-131, notably auto-absorption of sources, septal penetration of high-energy photons through the collimator and superimposition of sources.

Voxeldoes: a computer program for 3-D dose calculation in therapeutic nuclear medicine

A computer program, VoxelDose, was developed to calculate patient specific 3-D-dose maps at the voxel level. The 3-D dose map is derived in three steps: (i) The SPECT acquisitions are reconstructed using a filtered back projection method, with correction for attenuation and scatter; (ii) the 3-D cumulated activity map is generated by integrating the SPECT data; and (iii) a 3-D dose map is computed by convolution (using the Fourier Transform) of the cumulated activity map and corresponding MIRD voxel S values. To validate the VoxelDose software, a Liqui-Phil abdominal phantom with four simulated organ inserts and one spherical tumor (radius 4.2 cm) was filled with known activity concentrations of 111In. Four cylindrical calibration tubes (from 3.7 to 102 kBq/mL) were placed on the phantom. Thermoluminescent mini-dosimeters (mini-TLDs) were positioned on the surface of the organ inserts. Percent differences between the known and measured activity concentrations were determined to be 12.1 (tumor), 1.8 (spleen), 1.4, 8.1 (right and left kidneys), and 38.2% (liver), leading to percent differences between the calculated and TLD measured doses of 41, 16, 3, 5, and 62%. Large differences between the measured and calculated dose in the tumor and the liver may be attributed to several reasons, such as the difficulty in precisely associating the position of the TLD to a voxel and limits of the quantification method (mainly the scatter correction and partial volume effect). Further investigations should be performed to better understand the impact of each effect on the results and to improve absolute quantification. For all other organs, activity concentration measurements and dose calculations agree well with the known activity concentrations.

Treatment planning for molecular targeted radionuclide therapy

Molecular targeted radionuclide therapy promises to expand the usefulness of radiation to successfully treat widespread cancer. The unique properties of radioactive tags make it possible to plan treatments by predicting the radiation absorbed dose to both tumors and normal organs, using a pre-treatment test dose of radiopharmaceutical. This requires a combination of quantitative, high-resolution, radiation-detection hardware and computerized dose-estimation software, and would ideally include biological dose-response data in order to translate radiation absorbed dose into biological effects. Data derived from conventional (external beam) radiation therapy suggests that accurate assessment of the radiation absorbed dose in dose-limiting normal organs could substantially improve the observed clinical response for current agents used in a myeloablative regimen, enabling higher levels of tumor control at lower tumor-to-normal tissue therapeutic indices. Treatment planning based on current radiation detection and simulations technology is sufficient to impact on clinical response. The incorporation of new imaging methods, combined with patient-specific radiation transport simulations, promises to provide unprecedented levels of resolution and quantitative accuracy, which are likely to increase the impact of treatment planning in targeted radionuclide therapy.

Absorbed fractions in a voxel-based phantom calculated with the MCNP-4B code

A new approach for calculating internal dose estimates was developed through the use of a more realistic computational model of the human body. The present technique shows the capability to build a patient-specific phantom with tomography data (a voxel-based phantom) for the simulation of radiation transport and energy deposition using Monte Carlo methods such as in the MCNP-4B code. MCNP-4B absorbed fractions for photons in the mathematical phantom of Snyder et al. agreed well with reference values. Results obtained through radiation transport simulation in the voxel-based phantom, in general, agreed well with reference values. Considerable discrepancies, however, were found in some cases due to two major causes: differences in the organ masses between the phantoms and the occurrence of organ overlap in the voxel-based phantom, which is not considered in the mathematical phantom.

Absorbed dose estimates for 131I-labelled monoclonal antibody therapy in patients with intraperitoneal pseudomyxoma

Seven patients with intraperitoneal pseudomyxoma originating from the appendix (4 cases) and from the ovary (3 cases) were treated with radioimmunotherapy. During the therapy, nine infusions of 3.0-4.2 GBq of 131I-labelled B72.3 monoclonal antibody were administered. We developed three-dimensional dose calculation software that can utilize activity maps based on SPET images to calculate the absorbed dose distribution using point source kernels. The dose calculation program was employed to calculate absorbed doses to various organs. The calculated dose distributions enable us to evaluate the variation in dose within the organs, which is normally not available using approaches based on geometric models. The patient-specific absorbed dose calculations were compared with doses based on a model that uses photon S-factors derived from a standard phantom. The compared doses agreed well on average, but in some organs showed large discrepancies.

The use of TL detectors in dosimetry of systemic radiation therapy

A method for determining absorbed doses to organs in systemic radiation therapy (SRT) is evaluated. The method, based on thermoluminescent (TL) dosimeters placed on the patient's skin, was validated and justified through a phantom study showing that the difference between measured (TL dosimeters in the phantom) and derived (TL method) values is within 10%. Six radioimmunotherapy (RIT) patients with widespread intraperitoneal pseudomyxoma were also studied. In dose evaluations, special emphasis was on kidneys. In addition to the TL method, the absorbed doses to kidneys were calculated using MIRD formalism and a point dose kernel technique. We conclude that in SRT the described TL method can be used to estimate the absorbed doses to those critical organs near the body surface within 50% (1 SD).

Radiation dosimetry in nuclear medicine

Radionuclides are used in nuclear medicine in a variety of diagnostic and therapeutic procedures. A knowledge of the radiation dose received by different organs in the body is essential to an evaluation of the risks and benefits of any procedure. In this paper, current methods for internal dosimetry are reviewed, as they are applied in nuclear medicine. Particularly, the Medical Internal Radiation Dose (MIRD) system for dosimetry is explained, and many of its published resources discussed. Available models representing individuals of different age and gender, including those representing the pregnant woman are described; current trends in establishing models for individual patients are also evaluated. The proper design of kinetic studies for establishing radiation doses for radiopharmaceuticals is discussed. An overview of how to use information obtained in a dosimetry study, including that of the effective dose equivalent (ICRP 30) and effective dose (ICRP 60), is given. Current trends and issues in internal dosimetry, including the calculation of patient-specific doses and in the use of small scale and microdosimetry techniques, are also reviewed.

Use of the fast Hartley transform for three-dimensional dose calculation in radionuclide therapy

Effective radioimmunotherapy may depend on a priori knowledge of the radiation absorbed dose distribution obtained by trace imaging activities administered to a patient before treatment. A new, fast, and effective treatment planning approach is developed to deal with a heterogeneous activity distribution. Calculation of the three-dimensional absorbed dose distribution requires convolution of a cumulated activity distribution matrix with a point-source kernel; both are represented by large matrices (64 x 64 x 64). To reduce the computation time required for these calculations, an implementation of convolution using three-dimensional (3-D) fast Hartley transform (FHT) is realized. Using the 3-D FHT convolution, absorbed dose calculation time was reduced over 1000 times. With this system, fast and accurate absorbed dose calculations are possible in radioimmunotherapy. This approach was validated in simple geometries and then was used to calculate the absorbed dose distribution for a patient's tumor and a bone marrow sample.

Calculating internal dose by convolution from SPECT/MR fusion images

A new computer program was developed to calculate the absorbed dose. The program is based on the use of the convolution method and abdominal SPECT/MR fusion images. The applicability of the method was demonstrated by using data from (111)In-labeled thrombocyte and 99mTc-labeled colloid studies of three healthy volunteers. Dose distributions in the volunteers and the average absorbed doses in liver and spleen were calculated. The average doses for 99mTc-labeled colloid study were 0.07 +/- 0.02 (liver) and 0.046 +/- 0.005 mGy/MBq (spleen). The results are in good agreement with a Monte Carlo (MC) based method (0.074 for liver and 0.077 mGy/MBq for spleen) used by the International Commission on Radiological Protection (ICRP). For (111)In-labeled thrombocyte study the doses were 0.33 +/- 0.05 (liver) and 8.9 +/- 1.2 mGy/MBq (spleen) versus 0.730 and 7.50, respectively. The differences in dose estimates in the (111)In-labeled thrombocyte study are mainly due to the approximation used in activity quantitation. Convolution of the activity distribution with a point dose kernel is an effective method for calculating absorbed dose distribution in a homogeneous media. Activity distribution must be aligned to anatomical data in order to utilize the calculated dose distribution. The program developed is applicable to and practical for clinical use provided that the input data needed are available.