Patient-specific Dosimetry Research Articles

WE-F-105-04: Evaluation of the Eclipse Electron Monte Carlo Algorithm Based On Patient Specific Dosimetry Measurement

Purpose: To evaluate the Eclipse electron Monte Carlo (eMC) dose calculation algorithm using patient specific dosimetry measurements, and to investigate possible patient specific QA guideline. Methods: Electron treatments were planned for 58 patients on Eclipse (Version 10.0) using the eMC algorithm for two Varian TrueBeam linear accelerators (Linac‐A and Linac‐B). The dose calculations were based on patient anatomy (patient plan), and a solid‐water phantom (QA plan). The calculation results were compared with physics measurement using a solid‐water phantom and a pinpoint ion chamber. Results: The monitor units as calculated on the QA plans were (2.1±2.0)% higher than the measurement on both accelerators, excluding outliers with small fields (3 cm or less) and extended SSD (above 105 cm). The difference of QA plan and measurement was consistent with the calculation uncertainty of the eMC algorithm (2%). The monitor units as calculated on the patient plans were (8.0±5.0)% (Linac‐A) and (5.5±5.0)% (Linac‐B) higher than the corresponding measurement. However, we observed that the eMC algorithm under‐estimated the dose for one patient treated on nose, by as much as 28.5% for the patient plan and 12.9% for the QA plan. For this patient, a calculation based on an output look‐up table was within 2% of the measurement. Conclusion: The measurements indicate that the eMC algorithm predicts dose reasonably well for QA plans using the same homogeneous phantom. However, for CT‐based patient plans, the eMC algorithm generates a larger number of monitor units versus the QA plan. It may reflect the impact of patient surface curvature and tissue inhomogeneity to the eMC algorithm and further investigation is needed. Caution should be taken for eMC patient plan with small field size and extended SSD. A lookup table of output factors based on measurement can be generated as a quick reference for electron dose calculation.

SU-E-T-472: Personalized 3D Dose Prediction for Intrabeam Treatments Based On Patient CT Imaging with Monte Carlo GATE Simulations

Purpose: The aim of partial breast irradiation is to reduce the risk of local recurrence into the tumor bed. After surgical resection of the tumor, the Intrabeam™ system delivers a single dose fraction of 20 Gy at the surface of the applicator, irradiating the tumor bed through a 50keV X‐ray beam. However, there is currently no latitude for personalized dose prescription which is in contrast to official recommendations for individual dose optimization in radiotherapy. In this context, the study aim was to perform personalized 3D dose prediction for Intrabeam™ treatments based on patient CT imaging using Monte Carlo (MC) simulations. Methods: A model of the Intrabeam™ system was developed with GATE taking into account the different parts of the device. A preoperative CT acquisition of the patient breast is performed and included in the simulation allowing accurate dose calculation. During intra‐operative radiotherapy, thermo‐luminescent dosimeters are placed on the skin at 1 and 3 cm around the spherical applicator. Comparison between simulation results and TLD measurements are performed to confirm the dose prediction at the TLD locations. This study was performed on 10 patients, including 3 who had an intra‐operative CT acquisition in order to validate the overall dosimetry evaluation protocol. Results: Patient results show good agreement between clinical experiments and simulations. Indeed the relative mean deviation between TLD dose measurements and GATE dose scoring is 0.09% ± 0.11% with a maximum of 0.334% and less than 1% of dose estimation uncertainty (min 0.41% max 0.95%). Conclusion: We propose the use of an accurate model of the Intrabeam system on the GATE platform based on Geant4 MC toolkit. The dosimetric evaluation of the proposed platform with patient datasets supports its use for patient specific dosimetry planning.

SU‐E‐T‐239: Implementation of QA Procedures and Their Effect On the Radiation Treatment Delivery Error Rate Over a 12 Year Period

Purpose: Treatment deviations have been tracked from 2000 to the present covering over 23,000 treatment courses and 420,000 fractions. During this period multiple error reduction actions were taken including implementation of an electronic record and verify system (ERVS), a departmental weekly QA meeting, a time out procedure, a dosimetry boarding pass and ACR accreditation preparation. This study will examine error rates before and after these actions to estimate the effect on the overall and categorical error rates. Methods: A treatment deviation was defined as any error in radiation treatment delivery that reached the patient. Deviations were catalogued by type of error, detection method, number of fractions delivered prior to detection, and percentage of treatment course affected by the deviation. Results: The use of ERVS was incrementally applied to linacs from 1999 until 2006 by which time all linac treatments were electronically verified. In 2006 a weekly QA meeting and a requirement for a patient specific dosimetry boarding pass were started. A treatment timeout procedure was initiated in 2007. Standards were modified to meet ACR standards during 2007–08 resulting in accreditation. From 2000– 2002 the overall error rate averaged 5.4 per 1000 treatments (0.0054) and decreased to 0.0024 for the 2005–2007 period with 2004 being an outlier due to a single upstream error affecting multiple patients. The rate continued to decline to 0.0019 for 2008– 09 and to 0.0010 for 2010–12. Conclusion: Simultaneous technology and procedural changes make it difficult to correlate the effect of individual changes directly to the error rate. Nevertheless, the data clearly shows a progressive error rate reduction as QA procedures were implemented. Reduction in some classes of errors can more easily be tied to specific changes. We believe the combined implementation strongly contributed to an overall culture of safety resulting in a reduced error rate.

SU-E-T-520: Deriving Energy Spectrum From Depth Dose Measurements for X-Rays of CT Scanner

Purpose: The goal of this study is to derive x-ray energy spectrum from the measured depth dose data for any CT scanner without a priori knowledge of x-ray tube configurations. The obtained energy spectrum, together with other beam characteristics, can be used as an effective way to estimate the patient-specific organ doses from CT scans clinically. Methods: EGSnrc Monte Carlo code was used to generate a series of percent depth dose (PDD) curves in water for mono-energetic x-rays with energy ranging from 11 keV to 150 keV. Specifically, a fan beam was simulated with a fan angle of 55 degrees and 2 cm thickness along superior-inferior direction. Based on BEAMnrc Monte Carlo characterization of x-rays from CT scanners, an empirical function with three variables was proposed to represent the energy spectrum of those beams. An iterative multivariate search algorithm has been developed to derive the energy spectrum based on the correlation coefficient between the measured PDD and the fitted PDD. The developed algorithm has been tested with a BEAMnrc simulated x-ray spectrum serving as the ‘original’ spectrum and the subsequent dose simulation serving as the ‘measured’ PDD. Results: Except for the characteristic energy peaks, the derived energy spectrum reproduced the ‘original’ spectrum very well with a smooth energy dependency, peaking around 39 keV. The maximum error between the measured PDD and the fitted PDD was less than 1%, indicating the validity of our proposed method for spectrum derivation. Conclusion: According to this study, it is possible to derive the energy spectrum from the measured PDD data for the x-rays of a CT scanner, without a priori knowledge of x-ray tube configurations. Future work will seek to demonstrate its clinical efficacy by establishing a patient-specific dosimetry system for any CT scanners, particularly for organ dose estimation on real patients.

Dosimetry and Therapeutic Ratios for Rhenium-186 HEDP

Rhenium-186 (Re-186) is a β-emitting radionuclide. Emitted β-particles have ranges up to 4.5 mm in tissue, capable of delivering high doses to skeletal regions of high Re-186 concentrations while sparing adjacent radiosensitive regions and thus making the irradiation well tolerated for the patient. Along with the β-emissions, γ-rays are emitted having an adequate energy for imaging during therapy and biodistribution assessment for patient-specific dosimetry calculations. The relatively short physical half-life combined with the β-emissions allows the delivery of relatively high activity rate for a short period of time in areas of concentration. This study is a short review concerning the palliative treatment of skeletal metastases using 186Re-HEDP. After presenting the dominant ways of 186Re production, special emphasis is given to dosimetry issues while the effect of palliation therapy can be evaluated through the comparison of the absorbed dose in metastatic lesion relatively to the normal bone region. Accurate dose estimation is required taking into account the anatomic individual difference of each patient. For this purpose a patient specific dosimetric model considering metastatic lesions as spherical nodules is introduced. In order to quantify in a representative way the results of palliation treatment, the concept of therapeutic ratios is analyzed.

Radioiodine Therapy for Graves Disease

Based on the committed thyroid absorbed dose, the aim was to compare the efficiency of I therapy against Graves disease (GD) within 1 year after treatment and, by exploring the dose-response relationship, indicate an absorbed dose to be targeted into patient therapeutic planning. Thyroid-absorbed doses were calculated to 196 patients with GD by applying Medical Internal Radiation Dose formalism and taking into account administered I activity, thyroid radioiodine uptake, effective half-life, and gland tissue mass. Statistical analysis was applied to assess the relationship between absorbed doses and the patient's clinical response. Overall, successful therapy was achieved in 167 patients, whereas in 29 the disease persisted, even though 64.8% and 89.3% of all the treated patients had received, respectively, thyroid absorbed dose and activity superior to 300 Gy and 11.1 MBq/g (300 μCi/g) of thyroid tissue. Among those in whom the disease persisted, 24 (83%) had a 6- to 24-hour I uptake ratio equal or superior to 0.9, whereas only 5 (17%) presented a lower ratio. According to statistical analysis, there was no difference in cure rate between the groups that received 300 Gy or less and that which received more (84.1% vs 85.8%, P = 0.8336). A thyroid absorbed dose of 300 Gy is plausible as a targeted therapeutic dose in GD therapy planning, because statistical analysis has proven there to be no advantage in treating patients with doses above this level. On the other hand, numerous efforts should be made to develop an optimized and easily applicable protocol of patient-specific dosimetry and to provide data that show its clinical impact on patient management.

Patterns of patient specific dosimetry in total body irradiation

Total body irradiation (TBI) has been used for bone marrow transplant for hematologic and immune deficiency conditions. The goal of TBI is to deliver a homogeneous dose to the entire body, with a generally accepted range of dose uniformity being within ± 10% of the prescribed dose. The moving table technique for TBI could make dose uniform in whole body by adjusting couch speed. However, it is difficult to accurately estimate the actual dose by calculation and hence in vivo dosimetry (IVD) is routinely performed. Here, the authors present patterns of patient-specific IVD in 161 TBI patients treated at our institution. Cobalt-60 teletherapy unit (Model C9 Cobalt-60 teletherapy unit, Picker X-ray Corporation) with customized moving bed (SITI Industrial Products, Inc., Fishers, IN) were used for TBI treatment. During treatment, OneDose(TM) (Sicel Technology, NC) Metal Oxide-silicon Semiconductor Field Effect Transistor detectors were placed at patient body surface; both entrance and exit side of the beam at patient head, neck, mediastinum, umbilicus, and knee to estimate midplane dose. When large differences (>10%) between the prescribed and measured dose were observed, dose delivery was corrected for subsequent fractions by the adjustment of couch speed and/or bolus placement. Under IRB exempt status, the authors retrospectively analyzed the treatment records of 161 patients who received TBI treatment between 2006 and 2011. Across the entire cohort, the median ± SD (range) percent variance between calculated and measured dose for head, neck, mediastinum, umbilicus, and knee was -2.3 ± 10.2% (-66.2 to +35.3), 1.1 ± 11.5% (-62.2 to +40.3), -1.9 ± 9.5% (-66.4 to +46.6), -1.1 ± 7.2% (-35.2 to +42.9), and 3.4 ± 12.2% (-47.9 to +108.5), respectively. More than half of treatments were within ± 10% of the prescribed dose for all anatomical regions. For 80% of treatments (10%-90%), dose at the umbilicus was within ± 10%. However, some large differences greater than 35% were also found at several points. For one case, the knee received double the prescribed dose. When the dose differences for multiple fractions were averaged, compliance (± 1 0%) between the prescription and measured dose was improved compared to the dose difference of the first single fraction, for example, as at umbilicus, which improved from 83.9% to 98.5%. Actual dose measurement analysis of TBI patients revealed a potentially wide variance from the calculated dose. Based from their IVD method for TBI using Cobalt-60 irradiator and moving table, ± 10% over entire body is hard to achieve. However, it can be significantly improved with immediate feedback after the first fraction prior to subsequent treatments.

Emission of β+ Particles Via Internal Pair Production in the 0+ – 0+ Transition of 90Zr: Historical Background and Current Applications in Nuclear Medicine Imaging

90Y is traditionally considered as a pure β– emitter. However, the decay of this radionuclide has a minor branch to the 0+ first excited state of 90Zr at 1.76 MeV, that is followed by a β+/β– emission. This internal pair production has been largely studied in the past because it is generated by a rare electric monopole transition (E0) between the states 0+/0+ of 90Zr. The positronic emission has been recently exploited for nuclear medicine applications, i.e. positron emission tomography (PET) acquisitions of 90Y-labelled radiopharmaceuticals, widely used as therapeutic agents in internal radiation therapy. To date, this topic is gaining increasing interest in the radiation dosimetry community, as the possibility of detecting β+ emissions from 90Y by PET scanners may pave the way for an accurate patient-specific dosimetry. This could lead to an explosion in scientific production in this field. In the present paper the historical background behind the study of the internal pair production of the 0+/0+ transition of 90Zr is presented along with most up to date measured branch ratio values. An overview of most recent studies that exploit β+ particles emitted from 90Y for PET acquisitions is also provided.

EP-1137: Rapid Arc Patient-specific dosimetry with a novel ISORAD diode

EP-1137: Rapid Arc Patient-specific dosimetry with a novel ISORAD diode

Investigation on the effect of sharp phantom edges on point dose measurement during patient-specific dosimetry with Rapid Arc

The objective of this work was to investigate and quantify the effect of sharp edges of the phantom on the point dose measurement during patient-specific dosimetry with Rapid Arc (RA). Ten patients with carcinoma of prostate were randomly selected for this dosimetric study. Rapid Arc plans were generated with 6 MV X-rays in the Eclipse (v 8.6.14) with single arc (clockwise). Dosimetry verification plans were generated for two phantoms (cylindrical and rectangular). The cylindrical phantom was solid water (diameter 34 cm) and the rectangular phantom was a water phantom (25 cm × 25 cm × 10 cm). These phantoms were pre-scanned in computed tomography (CT) machine with cylindrical ionization chamber (FC65) in place. The plans were delivered with Novalis Tx linear accelerator with 6 MV X-rays for both the phantoms separately. The measured dose was compared with the planned dose for both the phantoms. Mean percentage deviation between measured and planned doses was found to be 4.19 (SD 0.82) and 3.63 (SD 0.89) for cylindrical and rectangular phantoms, respectively. No significant dosimetric variation was found due to the geometry (sharp edges) of the phantom. The sharp edges of the phantom do not perturb the patient specific Rapid Arc dosimetry significantly.

Welch Informationist Collaboration With The Johns Hopkins Medicine Department of Radiology

With the announcement of the NLM Administrative Supplement Grant, the informationists at the Welch Medical Library found an interested and enthusiastic Principal Investigator (PI) in the Department of Radiology at The Johns Hopkins School of Medicine. Although the PI had not used the services of a Welch informationist before, he was eager to collaborate with an informationist on the grant “Dose-Response in Radionuclide Therapy” (Parent Grant Number: 5R01CA116477-07), and had self-identified the need for “a literature review by experts” to fulfill his funded work. Targeted radionuclide therapy is an emerging modality for cancer therapy that involves the delivery of radioactive atoms using carriers that preferentially bind to tumor cells. Such treatment is best implemented with patient-specific dosimetry calculations. This mutually beneficial partner ship brings the specialized skill set, knowledge base and expertise of an informationist to the biomedical research team to assist in accomplishing the broader work of improving RPT delivery in metastatic cancer patients. The informationist will carry out multiple tasks to produce an effective and efficient workflow that facilitates the dissemination of the information to a globally dispersed research team. This collaboration will be evaluated both quantitatively and qualitatively, and the important role of the informationist in the success of this project will demonstrate the value of including an informationist in the research team. This demonstrated value will assist Welch Library in continuing to promote the embedded informationist program to other interested researchers throughout the institution. When the NLM Administrative Supplement grant opportunity was announced, the informationists at the Welch Medical Library partnered with a keenly interested and enthusiastic Principal Investigator in Dr. George Sgouros, Professor of Radiology, Oncology and Radiation Oncology, at the Johns Hopkins School of Medicine. Although he had not used the services of a Welch informationist before, Dr. Sgouros was eager to collaborate with an informationist on his existing grant, and had self-identified the need for “a literature review by experts” to fulfill his funded work. Performing literature searches and gathering data for scientific review from relevant studies are central to the current work of the grant. This work is in the wheelhouse of the informationist.

Feasibility of using PRESAGE® for relative 3D dosimetry of small proton fields

Small field dosimetry is challenging due to the finite size of the conventional detectors that underestimate the dose distribution. With the fast development of the dynamic proton beam delivery system, it is essential to find a dosimeter which can be used for 3D dosimetry of small proton fields. We investigated the feasibility of using a proton formula PRESAGE® for 3D dosimetry of small fields in a uniform scanning proton beam delivery system with dose layer stacking technology. The relationship between optical density and the absorbed dose was found to be linear through small volume cuvette studies for both photon and proton irradiation. Two circular fields and three patient-specific fields were used for proton treatment planning calculation and beam delivery. The measured results were compared with the calculated results in the form of lateral dose profiles, depth dose, isodose plots and gamma index analysis. For the circular field study, lateral dose profile comparison showed that the relative PRESAGE® profile falls within ± 5% from the calculated profile for most of the spatial range. For unmodulated depth dose comparison, the agreement between the measured and calculated results was within 3% in the beam entrance region before the Bragg peak. However, at the Bragg peak, there was about 20% underestimation of the absorbed dose from PRESAGE®. For patient-specific field 3D dosimetry, most of the data points within the target volume passed gamma analysis for 3% relative dose difference and 3 mm distance to agreement criteria. Our results suggest that this proton formula PRESAGE® dosimeter has the potential for 3D dosimetry of small fields in proton therapy, but further investigation is needed to improve the dose under-response of the PRESAGE® in the Bragg peak region.

Dosimetry of 223Ra-chloride: dose to normal organs and tissues

(223)Ra-Chloride (also called Alpharadin®) targets bone metastases with short range alpha particles. In recent years several clinical trials have been carried out showing, in particular, the safety and efficacy of palliation of painful bone metastases in patients with castration-resistant prostate cancer using (223)Ra-chloride. The purpose of this work was to provide a comprehensive dosimetric calculation of organ doses after intravenous administration of (223)Ra-chloride according to the present International Commission on Radiological Protection (ICRP) model for radium. Absorbed doses were calculated for 25 organs or tissues. Bone endosteum and red bone marrow show the highest dose coefficients followed by liver, colon and intestines. After a treatment schedule of six intravenous injections with 0.05 MBq/kg of (223)Ra-chloride each, corresponding to 21 MBq for a 70 kg patient, the absorbed alpha dose to the bone endosteal cells is about 16 Gy and the corresponding absorbed dose to the red bone marrow is approximately 1.5 Gy. The comprehensive list of dose coefficients presented in this work will assist in comparing and evaluating organ doses from various therapy modalities used in nuclear medicine and will provide a base for further development of patient-specific dosimetry.

A dose point kernel database using GATE Monte Carlo simulation toolkit for nuclear medicine applications: Comparison with other Monte Carlo codes

GATE is a Monte Carlo simulation toolkit based on the Geant4 package, widely used for many medical physics applications, including SPECT and PET image simulation and more recently CT image simulation and patient dosimetry. The purpose of the current study was to calculate dose point kernels (DPKs) using GATE, compare them against reference data, and finally produce a complete dataset of the total DPKs for the most commonly used radionuclides in nuclear medicine. Patient-specific absorbed dose calculations can be carried out using Monte Carlo simulations. The latest version of GATE extends its applications to Radiotherapy and Dosimetry. Comparison of the proposed method for the generation of DPKs was performed for (a) monoenergetic electron sources, with energies ranging from 10 keV to 10 MeV, (b) beta emitting isotopes, e.g., (177)Lu, (90)Y, and (32)P, and (c) gamma emitting isotopes, e.g., (111)In, (131)I, (125)I, and (99m)Tc. Point isotropic sources were simulated at the center of a sphere phantom, and the absorbed dose was stored in concentric spherical shells around the source. Evaluation was performed with already published studies for different Monte Carlo codes namely MCNP, EGS, FLUKA, ETRAN, GEPTS, and PENELOPE. A complete dataset of total DPKs was generated for water (equivalent to soft tissue), bone, and lung. This dataset takes into account all the major components of radiation interactions for the selected isotopes, including the absorbed dose from emitted electrons, photons, and all secondary particles generated from the electromagnetic interactions. GATE comparison provided reliable results in all cases (monoenergetic electrons, beta emitting isotopes, and photon emitting isotopes). The observed differences between GATE and other codes are less than 10% and comparable to the discrepancies observed among other packages. The produced DPKs are in very good agreement with the already published data, which allowed us to produce a unique DPKs dataset using GATE. The dataset contains the total DPKs for (67)Ga, (68)Ga, (90)Y, (99m)Tc, (111)In, (123)I, (124)I, (125)I, (131)I, (153)Sm, (177)Lu (186)Re, and (188)Re generated in water, bone, and lung. In this study, the authors have checked GATE's reliability for absorbed dose calculation when transporting different kind of particles, which indicates its robustness for dosimetry applications. A novel dataset of DPKs is provided, which can be applied in patient-specific dosimetry using analytical point kernel convolution algorithms.

MIRD Pamphlet No. 23: Quantitative SPECT for Patient-Specific 3-Dimensional Dosimetry in Internal Radionuclide Therapy

In internal radionuclide therapy, a growing interest in voxel-level estimates of tissue-absorbed dose has been driven by the desire to report radiobiologic quantities that account for the biologic consequences of both spatial and temporal nonuniformities in these dose estimates. This report presents an overview of 3-dimensional SPECT methods and requirements for internal dosimetry at both regional and voxel levels. Combined SPECT/CT image-based methods are emphasized, because the CT-derived anatomic information allows one to address multiple technical factors that affect SPECT quantification while facilitating the patient-specific voxel-level dosimetry calculation itself. SPECT imaging and reconstruction techniques for quantification in radionuclide therapy are not necessarily the same as those designed to optimize diagnostic imaging quality. The current overview is intended as an introduction to an upcoming series of MIRD pamphlets with detailed radionuclide-specific recommendations intended to provide best-practice SPECT quantification-based guidance for radionuclide dosimetry.

90Y PET-based dosimetry after selective internal radiotherapy treatments

The decay of 90Y has a minor branch to the O+ first excited state of 89Zr, the de-excitation of which to the fundamental state is followed by a β+–β- emission that has been used recently for biodistribution assessment after selective internal radiotherapy (SIRT) treatments. The purpose of the present study is to demonstrate the feasibility of 90Y PET imaging for dose assessment after radioembolization with 90Y microspheres. Activity quantification was validated through preliminary phantom studies using a cylindrical body phantom composed of six inserts of different volumes filled with a calibrated amount of 90Y microspheres. A GE Discovery ST PET/CT scanner provided with bismuth germinate (BGO) crystals was used for image acquisition. Images were reconstructed with an ordered subset expectation–maximization method. The effect of object size and the effect of the number of iterations on dose evaluation and volume recovery were investigated. Microsphere dose distribution was then evaluated on one patient (one lesion) who underwent liver SIRT treatment. Dose calculations were made with a MATLAB-based code developed in our department. Dedicated Monte Carlo calculations were executed to evaluate dose S-values for the 90Y source. The activity distribution derived from 90Y PET acquisitions was convolved with the voxel S-values to obtain a three-dimensional absorbed dose distribution and dose–volume histograms. Dosimetry studies carried out on the body phantom with ordered subset expectation–maximization algorithm, three iterations, provided an accuracy of 7.62% in determining the absorbed dose in the largest insert. The dose difference increases as the insert size reduces. Preliminary results on a patient provided a high-resolution absorbed dose distribution map. An average dose of 139.3 Gy was evaluated for the tumor area, with a maximum dose as high as 237.9 Gy. The absorbed dose to the healthy liver was below the tolerance dose of 35 Gy (33.8 Gy). A clear correlation between absorbed dose and tumor response was observed at 18F-fluorodeoxyglucose PET acquired 6 months after treatment. According to our experience, 90Y PET is a promising and reliable technique for microsphere dose assessment and might pave the way for a patient-specific PET-based dosimetry after liver SIRT treatments.

SU-E-J-182: Patient Specific Assessment of External Radiation Exposure to Bystanders Interacting with Patients Following 131I Therapy.

Conventional calculation methods of patient release criteria for compliance with NRC regulations are based on the assumption that both patient and bystander are each a single point in space. This study was intended to assess the patient-specific external radiation exposure to a bystander interacting with the patient following radionuclide therapy with 131I. 131I-sodium iodide treatment for hyperthyroidism and thyroid cancer and 131I-tositumomab treatment of non-Hodgkin's lymphoma were considered. 131I distribution provided by the patient SPECT image was rendered on the SPECT-fused CT images. The CT images were then imported to a Monte Carlo based simulation code, MCNPX 2.7, as a source phantom. For a target phantom, we employed the adult male hybrid phantom developed at the University of Florida and National Cancer Institute. A single orientation - patient and a bystander facing one another at 1.0 m - was considered. S factors (dose per unit cumulative activity (A)) for each organ in a bystander was obtained from the MC calculations and effective dose (EDE) per A was calculated based on tissue-weighted individual organ doses. The results were compared with the calculations using UF/NCI adult hybrid source/target phantoms and the revised adult ORNL stylized source/target phantoms. EDE per A of the stylized phantom was 1.5% higher than that of the hybrid phantom for uniform source localization in the thyroid. However, EDE per A of the hybrid phantom was 20% less than that of stylized phantoms for a torso source. The difference is attributed to the realistic shape of the frontal body comparing to the simple ellipsoidal trunk of the stylized phantom. Based on the realistic hybrid phantoms and accurate MC radiation transport calculation tools, patient specific dosimetry for a bystander is feasible. S factors will be calculated using the patient CT image with 131I bio-distributions and hybrid phantoms.

SU-E-T-287: Patterns of Patient Specific Dosimetry in Total Body Irradiation.

Total body irradiation (TBI) is commonly used for conditioning prior to transplant in hematologic and immunologic diseases. Due to variability in body thickness, achieving dose uniformity across body within ±10% of the prescribed dose is challenging. The dose uniformity is further complicated by, techniques and beam energy used, lung shielding and selection of detector. The translational table technique for TBI could compensate for estimated delivered dose to whole body by adjusting couch speed during treatment. However, it is difficult to accurately estimate the dose by calculation and hence in vivo dosimetry (IVD) is routinely performed for TBI. The patterns of patient specific dosimetry, IVD are presented in this study. Under IRB exempt status, 161 patients who received TBI treatment between 2006 and 2011 were retrospectively analyzed using the treatment records from Cobalt-60 teletherapy unit and translational treatment couch. During treatment, IVD detectors (TLD, diode, or MOSFET) were placed on patient surface; both entrance and exit dose were recorded at the patient's head, neck, mediastinum, umbilicus, and knee. When large differences between prescribed and measured dose were observed, the dose delivery was corrected for subsequent fractions by adjustment in couch speed and/or bolus placement. Across the entire cohort, the mean (range) percent variance between calculated and measured dose were -2.3% (-66.2 - 35.3), 1.1% (-62.2 - 40.3), -1.9% (-66.4 - 46.6), -1.1% (-35.2 - 42.9), and 3.4% (-47.9 - 108.5) for head, neck, mediastinum, umbilicus, and knee, respectively. When the dose differences for multiple fractions were averaged, the compliance (±10%) between prescription and measured dose was improved as at umbilicus from 83.9% to 98.5%. Actual dose measurement analysis of TBI patients reveals a potentially wide variance from calculated dose. Dose uniformity can be significantly improved with immediate feedback after the first fraction prior to subsequent treatments. This work was supported by the JSPS Core-to-Core Program No. 23003.

Peptide Receptor Radionuclide Therapy with Somatostatin Analogues in Neuroendocrine Tumors

Neuroendocrine tumors (NETs) are rare tumors with variable malignant behavior. The majority of NETs express increased levels of somatostatin (SST) receptors, particularly SST2 receptors. Radiolabeled peptides specific for the SST2 receptors may be used for diagnosis of NETs and for peptide receptor radionuclide therapy (PRRT). [(111)In-DTPA(0)]-octreotide has been the first peptide used for PRRT. This radiolabeled peptide, emitting Auger electrons, often induced symptomatic relief, but objective morphological responses were rarely documented. After the introduction of the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) other peptides, primarily [DOTA(0),Tyr(3)]octreotate (DOTATATE) and [DOTA(0),Tyr(3)]octreotide (DOTATOC) were labeled with (90)Y or (177)Lu and used for therapy applications. The rate of objective response obtained with these radiolabeled peptides ranges between 6% and 46%, owing to differences in inclusion criteria adopted in different studies, length and type of therapy, and criteria of evaluation of the response. The present data in the literature do not allow defining the most suitable peptide and radionuclide for the treatment of NETs. Instead emerging evidence indicates that a combination of nuclides with different physical characteristics might be more effective than the use of a single nuclide. Kidney and bone marrow toxicity are the limiting factors for PRRT. Mild toxicity is often encountered while severe toxicity is rarer. Toxicity could be reduced and therapeutic efficacy enhanced by patient-specific dosimetry. Future directions include different issues of PRRT, such as defining the most suitable treatment scheme, evaluation of new peptides with different affinity profiles to other SST receptor subtypes, and reduction of toxicity.

Feasibility of bremsstrahlung dosimetry for direct dose estimation in patients undergoing treatment with 90Y-ibritumomab tiuxetan

Radioimmunotherapy with (90)Y-ibritumomab tiuxetan has been used successfully used in the treatment of CD20-positive non-Hodgkin's lymphoma (NHL). Pretherapy imaging with (111)In-ibritumomab tiuxetan has been used in provisional dosimetry studies. Posttherapy imaging of (90)Y-ibritumomab tiuxetan for clinical use is appealing as it would simplify the data acquisition process and allow measurements of actual doses absorbed during treatment. The study included 29 patients with non-Hodgkin's lymphoma, of whom 16 (group I) received a pretherapy (111)In-ibritumomab tiuxetan diagnostic study and (90)Y-ibritumomab tiuxetan treatment 1 week later, and 13 (group II) received only (90)Y-ibritumomab tiuxetan treatment. Planar imaging and blood sampling were performed in all patients. The doses absorbed by organs at risk were calculated using a whole-body average attenuation correction factor (relative dosimetry approach) and, in the case of the (111)In-ibritumomab tiuxetan image sets, also using organ-specific attenuation correction factors (absolute dosimetry method). Red marrow absorbed doses were based on gamma counting of blood samples. The estimated red marrow absorbed doses from (111)In and (90)Y data were equivalent. In all cases, the doses absorbed by organs at risk were found to be within prescribed limits. The relative dosimetry approach applied to both the (90)Y and (111)In data significantly underestimated the doses relative to those obtained with the (111)In absolute dosimetry method which is generally accepted as the reference method (MIRD 16). In the case of (111)In, the relative dosimetry approach values were highly correlated (R(2) = 0.61) with the reference method values. Relative dosimetry estimates may be adjusted multiplying by a correction factor of 2.8. The (90)Y-ibritumomab tiuxetan relative dosimetry data correlated poorly with the reference method values (R (2) = 0.02). Based on patient-specific dosimetry, the administered activity may be increased by an average factor of 2.4, indicating that most patients could be undertreated. The relative dosimetry approach based on planar imaging largely underestimates doses relative to reference values. Dosimetry based on planar bremsstrahlung imaging is not a dependable alternative to (111)In dosimetry.