Sacral scintigraphy for bone marrow dosimetry in radioimmunotherapy

Dosimetry for Radiopharmaceutical Therapy: Practical Implementation, From the AJR Special Series on Quantitative Imaging

Radiopharmaceutical therapy (RPT) is advancing rapidly and achieving wider clinical application. However, RPT is not yet optimized in practice, as tumor and normal-organ dose estimates and, in turn, dose-response relationships remain poorly defined. Internal dosimetry is evolving to address such issues, transitioning from the estimation of population-average organ-level or tumor-level doses to individualized patient-specific sub-organ or sub-tumor doses. Derivation of patient-specific doses allows the further development of reliable dose-response relationships for diseased tissues and dose-toxicity relationships for normal tissues. Resources such as commercially available or publicly downloadable software are being increasingly developed to facilitate the use of these emerging methods. This review addresses the determination of patient-specific radiation doses for target tissue and at-risk normal tissues in the setting of RPT. Topics covered include: quantities, units, and radionuclides relevant to RPT; dose prescription algorithms; the steps in the dosimetry workflow; and bioeffects modeling. Implementation of patient-specific dosimetry will be essential for this therapeutic modality's optimization and further clinical expansion.

Dosimetry in Radiopharmaceutical Therapy

The application of radiopharmaceutical therapy for the treatment of certain diseases is well established, and the field is expanding. New therapeutic radiopharmaceuticals have been developed in recent years, and more are in the research pipeline. Concurrently, there is growing interest in the use of internal dosimetry as a means of personalizing, and potentially optimizing, such therapy for patients. Internal dosimetry is multifaceted, and the current state of the art is discussed in this continuing education article. Topics include the context of dosimetry, internal dosimetry methods, the advantages and disadvantages of incorporating dosimetry calculations in radiopharmaceutical therapy, a description of the workflow for implementing patient-specific dosimetry, and future prospects in the field.

Dosimetric Approaches for Radioimmunotherapy of Non-Hodgkin Lymphoma in Myeloablative Setting

Radioimmunotherapy (RIT) is a safe and active treatment available for non-Hodgkin lymphomas (NHLs). In particular, two monoclonal antibodies raised against CD20, that is Zevalin (⁹⁰Y-ibritumomab-tiuxetan) and Bexxar (¹³¹I-tositumomab) received FDA approval for the treatment of relapsing/refractory indolent or transformed NHLs. RIT is likely the most effective and least toxic anticancer agent in NHLs. However, its use in the clinical setting is still debated and, in case of relapse after optimized rituximab-containing regimens, the efficacy of RIT at standard dosage is suboptimal. Thus, clinical trials were based on the hypothesis that the inclusion of RIT in myeloablative conditioning would allow to obtain improved efficacy and toxicity profiles when compared to myeloablative total-body irradiation and/or high-dose chemotherapy regimens. Standard-activity RIT has a safe toxicity profile, and the utility of pretherapeutic dosimetry in this setting can be disputed. In contrast, dose-escalation clinical protocols require the assessment of radiopharmaceutical biodistribution and dosimetry before the therapeutic injection, as dose constrains for critical organs may be exceeded when RIT is administered at high activities. The aim of the present study was to review and discuss the internal dosimetry protocols that were adopted for non-standard RIT administration in the myeloablative setting before hematopoietic stem cell transplantation in patients with NHLs.

Dosimetry for Radiopharmaceutical Therapy: Current Practices and Commercial Resources

With the ongoing dramatic growth of radiopharmaceutical therapy, research and development in internal radiation dosimetry continue to advance both at academic medical centers and in industry. The basic paradigm for patient-specific dosimetry includes administration of a pretreatment tracer activity of the therapeutic radiopharmaceutical; measurement of its time-dependent biodistribution; definition of the pertinent anatomy; integration of the measured time-activity data to derive source-region time-integrated activities; calculation of the tumor, organ-at-risk, and/or whole-body absorbed doses; and prescription of the therapeutic administered activity. This paper provides an overview of the state of the art of patient-specific dosimetry for radiopharmaceutical therapy, including current methods and commercially available software and other resources.

Prospective evaluation of organ-specific dose and lesional doses following therapeutic [177Lu]Lu-EDTMP administration in patients with multiple skeletal metastases and its correlation with clinical hematological toxicity

AIM

In patients with multiple skeletal metastases, accurate estimation of absorbed doses to radiosensitive bone marrow in bone-directed systemic radionuclide therapies (RNT) is critically important from clinical dose determination standpoint. The primary aim of the present study was to estimate the radiation absorbed doses of therapeutic [177Lu]Lu-EDTMP to bone marrow by two methods viz. Medical Internal Radiation Dose (MIRD) schema and using OLINDA software and correlate with hematological toxicity.

METHODS

A total of 15 patients diagnosed to have multiple painful skeletal metastases being treated with [177Lu]Lu-EDTMP for palliation of pain, were enrolled for this prospective study. For all patients, urine was collected immediately after infusion of [177Lu]Lu-EDTMP up to 24 h post-administration and cumulative activity excreted from body via urine was calculated. For dosimetry, patients underwent post-administration whole-body scintigraphy at five-time points: 0.5 (pre-void), 2, 24, 48 and 120 h (post-void). From the time-activity curves generated by drawing regions of interest (ROIs) on the images, number of disintegrations was determined. Absorbed doses for organs and bone lesions were calculated using OLINDA 2.2.0 software. For bone marrow dose estimates, in addition to OLINDA 2.2.0 software, MIRD schema was also adopted. Hematological profile was monitored in all patients during the treatment and post-treatment follow-up (estimating complete blood counts, every 15 d for 3 months after therapy).

RESULTS

The mean ± standard deviation activity of [177Lu]Lu-EDTMP administered per patient per cycle was 2.08 ± 0.45 GBq. The results demonstrated higher uptake of [177Lu]Lu-EDTMP in bone metastases compared to normal bones. Within 2 and 24 h of administration of [177Lu]Lu-EDTMP, [177Lu]Lu activity excreted from the body was 24 ± 9% and 39 ± 14%, respectively. The mean absorbed organ doses (mean ± SD) in Gy/GBq were as follows: osteogenic cells 3.15 ± 1.85, bone marrow 0.57 ± 0.31, kidneys 0.08 ± 0.05, urinary bladder 0.32 ± 0.04, and bone lesions 2.91 ± 1.88. Strong correlation was found between (a) MIRD schema and OLINDA 2.2.0 software method for estimation of bone marrow doses (r = 0.96; P = <0.0001) and (b) Bone marrow absorbed dose and hematological toxicity (r = 0.81, P = 0.0027).

CONCLUSION

Radiation absorbed doses to the bone marrow and skeletal metastatic lesions, following therapeutic [177Lu]Lu-EDTMP were estimated using a convenient and non-invasive quantitative imaging method. The estimated bone marrow absorbed dose, either by MIRD schema or the OLINDA 2.2.0 software method, demonstrated strong correlation. Strong correlation was also observed between bone marrow absorbed dose and hematological toxicity.

Description of the methodology for dosimetric quantification in treatments with 177Lu-DOTATATE

Implementation of dosimetry calculations in the daily practice of Nuclear Medicine Departments is, at this time, a controversial issue, partly due to the lack of a standardized methodology that is accepted by all interested parties (patients, nuclear medicine physicians and medical physicists). However, since the publication of RD 601/2019 there is a legal obligation to implement it, despite the fact that it is a complex and high resource consumption procedure. The aim of this article is to review the theoretical bases of in vivo dosimetry in treatments with 177Lu-DOTATATE. The exposed methodology is the one proposed by the MIRD Committee (Medical Internal Radiation Dose) of the SNMMI (Society of Nuclear Medicine & Molecular Imaging). According to this method, the absorbed dose is obtained as the product of 2factors: the time-integrated activity of the radiopharmaceutical present in a source region and a geometrical factor S. This approach, which a priori seems simple, in practice requires several SPECT/CT acquisitions, several measurements of the whole body activity and taking several blood samples, as well as hours of image processing and computation. The systematic implementation of these calculations, in all the patients we treat, will allow us to obtain homogeneous data to correlate the absorbed doses in the lesions with the biological effect of the treatment. The final purpose of the dosimetry calculations is to be able to maximize the therapeutic effect in the lesions, controlling the radiotoxicity in the organs at risk.

A novel planar image-based method for bone marrow dosimetry in (177)Lu-DOTATATE treatment correlates with haematological toxicity

Background

^177Lu-DOTATATE is a valuable treatment option for patients with advanced neuroendocrine tumours overexpressing somatostatin receptors. Though well tolerated in general, bone marrow toxicity can, besides renal exposure, become dose limiting and affect the ability to sustain future therapies. The aim of this study was to develop a novel planar image-based method for bone marrow dosimetry and evaluate its correlation with haematological toxicity during ¹⁷⁷Lu-DOTATATE treatment. In this study, 46 patients with advanced neuroendocrine tumours were treated with 7.2 GBq (3.5–8.3 GBq) of ¹⁷⁷Lu-DOTATATE on two to five occasions. Planar gamma camera images were acquired at 2, 24, 48 and 168 h post-injection. Whole-body regions of interest were created in the images, and a threshold-based segmentation algorithm was applied to separate the uptake of ¹⁷⁷Lu-DOTATATE into high and low uptake compartments. The conjugate view method was used to quantify the activity, the accumulated activity was calculated and the absorbed dose to the bone marrow was estimated according to the MIRD scheme. Patients were monitored for haematological toxicity based on haemoglobin (Hb), white blood cell (WBC) and platelet (PLT) counts every other week during the treatment period.

Results

The mean absorbed dose to the bone marrow was estimated to 0.20 Gy (0.11–0.37 Gy) per 7.4 GBq of ¹⁷⁷Lu-DOTATATE, and the mean dose per fraction correlated with a decrease in Hb (p = 0.01), WBC (p < 0.01) and PLT (p < 0.01) counts. The total mean absorbed dose to the bone marrow was 0.64 Gy (0.30–1.5 Gy) per 24 GBq (8.2–37 GBq) of ¹⁷⁷Lu-DOTATATE and also correlated with a decrease in Hb (p < 0.01), WBC (p = 0.01) and PLT (p < 0.01) counts.

Conclusions

The planar image-based method developed in this study resulted in similar absorbed doses to the bone marrow as reported in earlier studies with blood-based bone marrow dosimetry. The results correlated with haematological toxicity, making it a promising method for estimating bone marrow doses in ¹⁷⁷Lu-DOTATATE treatment without the need for blood and urine sampling.

90Y-ibritumomab tiuxetan therapy in allogeneic transplantation in B-cell lymphoma with extensive marrow involvement and chronic lymphocytic leukemia: utility of pretransplantation biodistribution

BACKGROUND

Biodistribution data to date using In-ibritumomab tiuxetan have been initially obtained in patients with less than 25% lymphomatous bone marrow involvement and adequate hematopoietic synthetic function. In this article we present the results of an analysis of the biodistribution data obtained from a cohort of patients with extensive bone marrow involvement, baseline cytopenias, and chronic lymphocytic leukemia (CLL).

MATERIALS AND METHODS

Thirty-nine patients with a diagnosis of B-cell lymphoma or CLL expressing the CD20 antigen, who had failed at least one prior regimen, and had evidence of persistent disease were included in this analysis; however, only 38 of them completed the treatment. Semiquantitative analysis of the biodistribution was performed using regions of interest over the liver, lungs, kidneys, spleen, and sacrum. The observed interpatient variability including higher liver uptake in four patients is discussed.

RESULTS

No severe solid organ toxicity was observed at the maximum administered activity of 1184 MBq (32 mCi) Y-ibritumomab tiuxetan. After accounting for differences in marrow involvement, patients with CLL exhibit comparable biodistributions to those with B-NHL. We found that the estimated sacral marrow uptake on 48 h images in patients with bone marrow involvement may be an indicator of bone marrow involvement. There was no correlation between tumor visualization and response to treatment.

CONCLUSION

These data suggest that the imaging step is not critical when the administered activity is below 1184 MBq (32 mCi). However, our analysis confirms that the semiquantitative imaging data can be used to identify patients at risk for liver toxicity when higher doses of Y-ibritumomab tiuxetan are used. Patients with CLL can have excellent targeting of disease by In-ibritumomab tiuxetan, indicating potential efficacy in this patient population.

Three methods assessing red marrow dosimetry in lymphoma patients treated with radioimmunotherapy

BACKGROUND

Maximum injected activity in radioimmunotherapy (RIT) is limited by bone marrow toxicity. Many dosimetric approaches have been proposed, leading to high variability in the results and elusive absorbed dose-effect relations. This study presents the results of red marrow (RM) absorbed dose estimates performed with 3 methods.

METHODS

Five patients received 2 co-infusions of (90)Y-labeled (370 MBq/m2) and (111)In- labeled (120 MBq) epratuzumab (1.5 mg/kg) 1 week apart. RM-absorbed dose was estimated by 3 methodologies. The first approach (M1) used L(2)-L(4) lumbar vertebrae imaging. M2 and M3 methods used different red marrow to blood ratios (RMBLR) to assess RM-absorbed dose. RMBLR was set to a fixed value of 0.36 in M2 or assessed according to each patient's hematocrit in M3.

RESULTS

Median RM-absorbed doses were 4.1 (2.9-8.4), 2.3 (2.0-2.7), and 2.3 (1.6-2.5) mGy/MBq for M1, M2, and M3, respectively. No trend could be found between total RM-absorbed dose and toxicity for M2 and M3. Conversely, M1 seemed to provide the best absorbed dose-effect relation. The 4 patients with the highest RM-absorbed doses exhibited grade 4 toxicity. The fifth patient, with the lowest RB absorbed dose, exhibited only a mild (grade 2) toxicity.

CONCLUSIONS

Image-based methodology (M1) seems to better predict hematological toxicity as compared with blood-based methods. Only this method provides for bone marrow involvement.

State of the art in nuclear medicine dose assessment

Basic calculational methods and models used in dose assessment for internal emitters in nuclear medicine are discussed in this overview. Methods for quantification of activity in clinical and preclinical studies also are discussed, and we show how to implement them in currently available dose calculational models. Current practice of the use of internal emitters in therapy also is briefly presented here. Some of the future challenges for dose assessment in nuclear medicine are discussed, including application of patient-specific dose calculational methods and the need for significant advances in radiation biology.

A method to correct for radioactivity in large vessels that overlap the spine in imaging-based marrow dosimetry of lumbar vertebrae

Accurate marrow dosimetry for radionuclide therapy based on imaging methods has been challenging because of a variety of factors. One of the uncertainties in image quantification of lumbar vertebrae is correction for radioactivity in large blood vessels anterior to the vertebrae. We developed a method to correct for background radioactivity contributed from blood in large vessels and tested it in a pilot study.

METHODS

CT images of 26 patients receiving (111)In- or (131)I-labeled conjugates were used to measure the inside diameters of the aorta and inferior vena cava (IVC) at the top of L2 and the bottom of L4 and to measure the length of this vessel segment. The volume was calculated for this vessel segment, and then the radioactivity in that volume at each imaging time was determined using a time-variant blood radioactivity concentration as established by serial blood samples. This vessel segment typically overlapped with lumbar vertebrae in anterior and posterior whole-body images. The contribution of this background radioactivity to the cumulated activity of the lumbar spine region of interest (ROI) from serial gamma-camera images was determined, taking into account differences in attenuation between vessel segments and lumbar vertebrae.

RESULTS

The total blood volumes varied from 25 to 94 mL, with a mean of 51 mL. This mean is 76% of the mean marrow volume of 3 lumbar vertebrae measured in some of these patients. Thirteen of the 14 patients evaluated for aortic position had the aortic segment completely within the L2-L4 ROI. For the IVC, a mean of 72% was in the L2-L4 ROI. Adjusting for radioactivity in major blood vessels that were in the ROI led to lower marrow dose estimates.

CONCLUSION

To improve the accuracy of lumbar spine imaging-based marrow dosimetry, one can adjust radioactivity in the large vessels by methods that measure the volume, position, and depth of vessels in the ROI.

Establishing a clinically meaningful predictive model of hematologic toxicity in nonmyeloablative targeted radiotherapy: practical aspects and limitations of red marrow dosimetry

In either heavily pretreated or previously untreated patient populations, dosimetry holds the promise of playing an integral role in the physician's ability to adjust therapeutic activity prescriptions to limit excessive hematologic toxicity in individual patients. However, red marrow absorbed doses have not been highly predictive of hematopoietic toxicity. Although the accuracy of red marrow dose estimates is expected to improve as more patient-specific models are implemented, these model-calculated absorbed doses more than likely will have to be adjusted by parameters that adequately characterize bone marrow tolerance in the heavily pretreated patients most likely to receive nonmyeloablative radiolabeled antibody therapy. Models need to be established that consider not only absorbed dose but also parameters that are indicative of pretherapy bone marrow reserve and radiosensitivity so that a clinically meaningful predictive model of hematologic toxicity can be established.

A model-based method for the prediction of whole-body absorbed dose and bone marrow toxicity for 186Re-HEDP treatment of skeletal metastases from prostate cancer

In high-activity rhenium-186 hydroxyethylidene diphosphonate ((186)Re-HEDP) treatment of bone metastatic disease from prostate cancer the dose-limiting factor is haematological toxicity. In this study, we examined the correlation of the injected activity and the whole-body absorbed dose with treatment toxicity and response. Since the best response is likely to be related to the maximum possible injected activity limited by the whole-body absorbed dose, the relationship between pre-therapy biochemical and physiological parameters and the whole-body absorbed dose was studied to derive an algorithm to predict the whole-body absorbed dose prior to injection of the radionuclide. The whole-body retention of radioactivity was measured at several time points after injection in a cohort of patients receiving activities ranging between 2,468 MBq and 5,497 MBq. The whole-body absorbed dose was calculated by fitting a sequential series of exponential phases to the whole-body time-activity data and by integrating this fit over time to obtain the whole-body cumulated activity. This was then converted to absorbed dose using the Medical Internal Radiation Dose (MIRD) committee methodology. Treatment toxicity was estimated by the relative decrease in white cell (WC) and platelet (Plt) counts after the injection of the radionuclide, and by their absolute nadir values. The criterion for a treatment response was a 50% or greater decrease in prostate-specific antigen (PSA) value lasting for 4 weeks. Alkaline phosphatase (AlkPh), chromium-51 ethylene diamine tetra-acetate ((51)Cr-EDTA) clearance rate and weight were measured before injection of the radionuclide. The whole-body absorbed dose showed a significant correlation with WC and Plt toxicity ( P=0.005 and 0.003 for the relative decrease and P=0.006 and 0.003 for the nadir values of WC and Plt counts respectively) in a multivariate analysis which included injected activity, whole-body absorbed dose, pre-treatment WC and Plt baseline counts, PSA and AlkPh values, and the pre-treatment Soloway score. The injected activity did not show any correlation with WC or Plt toxicity, but it did correlate with PSA response ( P=0.005). These results suggest that the administration of higher activities would be likely to generate a better response, but that the quantity of activity that can be administered is limited by the whole-body absorbed dose. We have derived and evaluated a model that estimates the whole-body absorbed dose on an individual patient basis prior to injection. This model uses the level of injected activity and pre-injection measurements of AlkPh, weight and (51)Cr-EDTA clearance. It gave good estimates of the whole-body absorbed dose, with an average difference between predicted and measured values of 15%. Furthermore, the whole-body absorbed dose predicted using this algorithm correlated with treatment toxicity. It could therefore be used to administer levels of activity on a patient-specific basis, which would help in the optimisation of targeted radionuclide therapy. We believe that algorithms of this kind, which use pre-injection biochemical and physiological measurements, could assist in the design of escalation trials based on a toxicity-limiting whole-body absorbed dose, rather than using the more conventional activity escalation approach.

Additional radiation absorbed dose estimates for Zevalin radioimmunotherapy

Zevalin (ibritumomab tiuxetan) radioimmunotherapy is a novel treatment for non-Hodgkin's lymphoma (NHL). The Zevalin regimen includes 5 mCi (111)In-labeled Zevalin on Day 1, followed by serial anterior and posterior planar gamma images for imaging or dosimetry. On Day 8, patients receive 0.4 mCi/kg (90)Y Zevalin for radioimmunotherapy. Both Zevalin doses are preceded by 250 mg/m(2) rituximab to clear peripheral B cells and improve biodistribution of the radiolabeled antibody. In a 143-patient, Phase III, randomized study, the Zevalin regimen produced a significantly higher overall response rate than rituximab for relapsed or refractory, low-grade, follicular, or transformed NHL (80% versus 56%, p = 0.02). Fifteen patients from the Zevalin arm of this study were randomly selected for additional radiation dosimetry. (90)Y residence times were calculated from (111)In image analysis data. MIRDOSE3.1 radiation absorbed dose estimates to normal tissues were highest for spleen, testes, and liver, with considerably lower doses reaching heart, lung, intestines, red marrow, and kidneys. Radiation absorbed doses to organs and marrow were within a safe range following administration of 0.4 mCi/kg (90)Y Zevalin.

Comparison of methods for predicting myelotoxicity for non-marrow targeting I-131-antibody therapy

Although marrow suppression is usually the dose-limiting toxicity in non-marrow ablative radionuclide therapy, calculated marrow dose has rarely been used for prescribing the radioactivity to be administered. This study assesses the correlation of myelotoxicity with mCi/m(2), patient-specific lean body dose, marrow dose from blood and body of reference man, or from blood and body using the patient-specific mass. Fourteen prostate cancer patients were treated with (131)I-CC49. Radioactivity in blood and body was determined and used to calculate their contributions to the marrow dose. Platelet nadir expressed as percentage (%) of the initial baseline was used as an indicator for myelotoxicity. Correlation between platelet nadir (%) and myelotoxicity predictors was evaluated. Platelet nadirs (%) varied substantially (5-33%) for a small range of injected radioactivity/m(2) (68-78 mCi/m(2), 2.5-2.9 GBq/m(2)). Patient-specific total body dose based on lean body mass exhibited a weak correlation (r = 0.48) with platelet nadir. Marrow dose from blood and body of reference man had a better correlation (r = 0.73). Patient-specific marrow dose from blood and body (or lean body) had a similar correlation (r = 0.74 or 0.73). Radioactivity in the remainder of the body contributed only 28% of the total dose, and thus changes to this dose component had small impact on total marrow dose. Marrow dose was a better predictor for myelotoxicity than mCi/m(2) or lean total body dose in this non-marrow targeting (131)I-antibody therapy with high blood contributions to total dose.

Sensitivity of model-based calculations of red marrow dosimetry to changes in patient-specific parameters

We have investigated several of the key model parameters and assumptions involved in the calculation of red marrow absorbed dose in order to better understand the sensitivity of the predicted results to changes in these model features and the subsequent effect on correlations of the red marrow absorbed dose values with observed hematologic toxicity. Red marrow dose calculations based on measured blood activity concentrations (to determine red marrow cumulated activity) and measured total body cumulated activity have a mass-independent and mass-dependent term. Adjustments for patient mass should be made in these calculations when patients' lean body masses are more than 10% different from that in the assumed standard models. The blood-based red marrow dose methodology has the potential to provide a reasonable estimate of red marrow dose as long as there is no specific uptake in red marrow or bone due to the presence of free radionuclide, disease, or retention of activity due to metabolism by the reticuloendothelial system. If these additional sources of red marrow dose are present, the blood-based methodology will significantly underestimate red marrow dose. For radiometals, such as in (90)Y-labeled antibodies, bone or red marrow uptake of free yttrium or catabolized (90)Y products may have a significant impact on the calculated dose, assuming fairly low amounts of free (90)Y or marrow activity uptake (5-10%), even in the absence of disease in red marrow and/or bone. This is also true for (131)I-labeled antibodies, although to a lesser extent due to typically reduced activity retention in the bone marrow in the absence of disease and lack of bone uptake of free radionuclide. Radiation dose calculations for the red marrow must be made as carefully as possible, taking into account all possible sources of radiation dose, and considering all sources of uncertainties, in order to give the best possible correlations of radiation dose with observed toxicity.

Hematologic toxicity in radioimmunotherapy: dose-response relationships for I-131 labeled antibody therapy

Bone marrow toxicity is generally dose-limiting for radioimmunotherapy (RIT) with beta-emitting radionuclides. Treatment may be prescribed on the basis of administered activity or absorbed dose. An optimal definition of maximum tolerated dose will enable the clinical benefits of RIT to be maximized.

METHODS

We examined data from six clinical studies of RIT with various 131-I labeled antibodies and antibody fragments that treated a total of 114 patients. We also examined a sub-set of 36 patients with minimal prior chemotherapy who were treated with 131I-labeled intact murine IgG at a single institution. For both these groups the ability of absorbed dose-based methods to predict bone marrow tolerance was compared with that of activity-based methods.

RESULTS

Marrow toxicity was more accurately predicted by absorbed dose than by activity in the general case where a variety of different antibodies and antibody fragments were used. For the more homogeneous smaller group, well defined "dose-response" relationships were observed for both absorbed dose and administered activity. However, absorbed dose-based definitions of maximally tolerated dose yielded a better stratification of patients than activity-based definitions (including per meter squared) such that fewer patients had major toxicity when treated below "tolerance", and fewer patients had minor toxicity when treated above "tolerance".

CONCLUSIONS

Absorbed dose-based definitions of maximum tolerated dose and escalation variables are optimal for 131I-labeled antibody therapy. The ability of pre-therapy dosimetry studies to predict the behavior of therapeutic administrations must be validated for prospective clinical applications.

Correlation of red marrow radiation dosimetry with myelotoxicity: empirical factors influencing the radiation-induced myelotoxicity of radiolabeled antibodies, fragments and peptides in pre-clinical and clinical settings

Usually, the red marrow (RM) is the first dose-limiting organ in systemic radionuclide therapy, e.g., radioimmuno-or radiopeptide therapy. However, several studies have obtained rather poor correlations between the marrow doses and the resulting toxicities. Red marrow doses are mostly not determined directly, but are derived from blood or whole-body doses. The aim of our recent work was to analyze, in a nude mouse model in more detail, additional factors than just total dose, such as dose rate or relative biological effectiveness (RBE) factors, that may influence the resulting myelotoxicity. Furthermore, we wanted to analyze, whether correlations between the red marrow doses and the resulting myelotoxicities can be found in clinical metabolic endo-radiotherapy. The maximum tolerated activities (MTAs) and doses (MTDs) of several murine, chimeric and humanized immunoconjugates as complete IgG or fragments (F(ab)(2), Fab), as well as peptides, labeled with beta(-)- (such as (131)I or (90)Y), Auger electron- (such as (125)I or (111)In), or alpha-emitters (such as (213)Bi) were determined in nude mice. Blood counts were monitored at weekly intervals; bone marrow transplantation (BMT) was performed in order to support the assumption of the RM as dose-limiting. The radiation dosimetry was derived from biodistribution data of the various conjugates, accounting for cross-organ radiation; the activities in the blood, bone, bone marrow, and major organs were determined over time. Dosimetry and myelotoxicity data of three clinical radioimmunotherapy trials, involving a total of 82 colorectal cancer patients, treated with (131)I-labeled anti-CEA IgG, and twelve non-Hodgkin's lymphoma patients, treated with (131)I-labeled anti-CD20 IgG, were analyzed. In the preclinical model, at the respective MTAs, the RM doses differed significantly between the three conjugates: e.g., with (131)I-labeled conjugates, the maximum tolerated activities were#10; 260 microCi for IgG, 1200 microCi for F(ab)(2), and 3 mCi for Fab, corresponding to blood doses of 17 Gy, 9 Gy, and 4 Gy, respectively. However, initial dose rates were 10 times higher with Fab as compared to IgG, and still 3 times higher as compared to F(ab)(2); interestingly, all 3 deliver approximately 4 Gy within the first 24 h. The MTDs of all three conjugates were increased by BMT by approximately 30%. Similar observations were made for the (90)Y-labeled conjugates. Higher blood-based RM doses were tolerated with Auger-emitters than with conventional beta(-)-emitters, whereas the MTDs were similar between alpha- and beta(-)-emitters. In accordance to dose rates never exceeding those occurring at the single injection MTA, re-injections of (131)I-, (90)Y-, or (213)Bi-labeled Fab' were tolerated without increased lethality, if administered 24-48 h apart, whereas reinjection of bivalent conjugates was not possible. Clinically, a sigmoidally shaped dose-effect correlation was found in colorectal cancer patients treated with (131)I-anti-CEA IgG. Previous mitomycin chemotherapy was identified as additional myelosensitizing factor leading to enhanced myelotoxicity. At comparable doses, non-Hodgkin's lymphoma patients developed higher degrees of myelotoxicity with a less clearly pronounced predictability from red marrow doses. In summary, results in the murine model suggest a strong influence of the dose rate (or better: dose per unit time), not only total dose on the resulting myelotoxicity, whereas the influence of high- (alpha, Auger/conversion electrons) versus low-LET (beta,gamma) type radiation seems to be much lower than expected from previous in vitro data. The lower myelotoxicity of Auger e(-) emitters is probably due to the short path length of their low-energy electrons, which cannot reach the nuclear DNA if the antibody is not internalized into the stem cells of the red marrow. Clinically, additional factors than just marrow dose (e.g., previous myelotoxic therapy, bone marrow involvement by metastatic malignancy) seem to a, bone marrow involvement by metastatic malignancy) seem to affect the resulting myelotoxicity.

Radiation dosimetry for radionuclide therapy in a nonmyeloablative strategy

Radionuclide therapy extends the usefulness of radiation from localized disease of multifocal disease by combining radionuclides with disease-seeking drugs, such as antibodies or custom-designed synthetic agents. Like conventional radiotherapy, the effectiveness of targeted radionuclides is ultimately limited by the amount of undesired radiation given to a critical, dose-limiting normal tissue, most often the bone marrow. Because radionuclide therapy relies on biological delivery of radiation, its optimization and characterization are necessarily different than for conventional radiation therapy. However, the principals of radiobiology and of absorbed radiation dose remain important for predicting radiation effects. Fortunately, most radionuclides emit gamma rays that allow the measurement of isotope concentrations in both tumor and normal tissues in the body. By administering a small "test dose" of the intended therapeutic drug, the clinician can predict the radiation dose distribution in the patient. This can serve as a basis to predict therapy effectiveness, optimize drug selection, and select the appropriate drug dose, in order to provide the safest, most effective treatment for each patient. Although treatment planning for individual patients based upon tracer radiation dosimetry is an attractive concept and opportunity, practical considerations may dictate simpler solutions under some circumstances. There is agreement that radiation dosimetry (radiation absorbed dose distribution, cGy) should be utilized to establish the safety of a specific radionuclide drug during drug development, but it is less generally accepted that absorbed radiation dose should be used to determine the dose of radionuclide (radioactivity, GBq) to be administered to a specific patient (i.e., radiation dose-based therapy). However, radiation dosimetry can always be utilized as a tool for developing drugs, assessing clinical results, and establishing the safety of a specific radionuclide drug. Bone marrow dosimetry continues to be a "work in progress." Blood-derived and/or body-derived marrow dosimetry may be acceptable under specific conditions but clearly do not account for marrow and skeletal targeting of radionuclide. Marrow dosimetry can be expected to improve significantly but no method for marrow dosimetry seems likely to account for decreased bone marrow reserve.

Role of radiation dosimetry in radioimmunotherapy planning and treatment dosing

Cancer-seeking antibodies (Abs) carrying radionuclides can be powerful drugs for delivering radiotherapy to cancer. As with all radiotherapy, undesired radiation dose to critical organs is the limiting factor. It has been proposed that optimization of radioimmunotherapy (RIT), that is, maximization of therapeutic efficacy and minimization of normal tissue toxicity, depends on a foreknowledge of the radiation dose distributions to be expected. The necessary data can be acquired by established tracer techniques, in individual patients, using quantitative radionuclide imaging. Object-oriented software systems for estimating internal emitter radiation doses to the tissues of individual patients (patient-specific radiation dosimetry), using computer modules, are available for RIT, as well as for other radionuclide therapies. There is general agreement that radiation dosimetry (radiation absorbed dose distribution, cGy) should be utilized to establish the safety of RIT with a specific radiolabeled Ab in the early stages (i.e. phase I or II) of drug evaluation. However, it is less well established that radiation dose should be used to determine the radionuclide dose (amount of radioactivity, GBq) to be administered to a specific patient (i.e. radiation dose-based therapy). Although treatment planning for individual patients based upon tracer radiation dosimetry is an attractive concept and opportunity, particularly for multimodality RIT with intent to cure, practical considerations may dictate simpler solutions under some circumstances.

Kinetics of 76Br-labeled anti-CEA antibodies in pigs; aspects of dosimetry and PET imaging properties

A monoclonal antibody labeled with the positron-emitting radionuclide 76Br (T(1/2) 16.2 h) has previously been shown useful for positron emission tomography (PET) imaging of experimental tumors. Our aim in the present study was to investigate the effects of the complex decay scheme of this radionuclide on normal organ dosimetry and PET image quality. Three mini-pigs were injected intravenously with 46-75 MBq of the 76Br-labeled anti-CEA antibody 38S1, and the whole-body kinetics followed by PET imaging for 19 h. From PET data, absorbed doses in human organs were estimated using the MIRDOSE 3.0 software. The highest 76Br concentrations were found in lungs, after a correction for the air volume in this organ. The lungs received the highest absorbed dose (mGy/MBq, mean+/-maximum error), 0.84+/-0.16, followed by liver, 0.74+/-0.28, and small intestine, 0.55+/-0.05, while the effective dose equivalent was 0.41+/-0.03 mSv/MBq. The PET imaging properties of 76Br in a two-dimensional 2D PET camera, including central area resolution and scattering effects, were investigated in phantoms and compared to those of 18F. In a 0.97 g/cm3 material, approximating soft tissue density, the FMHW ("full width at half-maximum") value of the point spread function was 7.7+/-0.2 mm for 76Br and 6.0+/-0.1 mm for 18F. In conclusion, radioimmuno PET using 76Br-labeled antibodies resulted in a fairly even distribution of the radiation dose, where the highest absorbed organ doses were only about two to three times higher than the mean absorbed body dose. The high energy beta+ spectrum in the 76Br decay had only minor effects on the resolution, but may decrease the quantification accuracy, especially in organs with a lower density such as a lung.

Low-dose, fractionated radioimmunotherapy for B-cell malignancies using 131I-Lym-1 antibody

PURPOSE

This trial was conducted to assess the toxicity and efficacy of 131I-Lym-1 in patients with either malignant B-cell non-Hodgkin's lymphoma (NHL) or chronic lymphocytic leukemia (CLL) using low-dose, fractionated radioimmunotherapy (RIT).

MATERIALS AND METHODS

Thirty adult patients who had advanced B-cell malignancies (25 NHL and 5 CLL) had progressed despite standard therapy; 12 patients entered the trial with Karnofsky performance status (KPS) of equal to or greater than 60. Patients were treated with a series of intravenous doses of 131I-Lym-1 with a goal of reaching a cumulative dose in each patient of at least 300 mCi. All patients were Lym-1 reactive. Clinical responses and immediate toxicity were evaluable in all 30 patients and delayed toxicity in 26.

RESULTS

Toxicity to Lym-1 antibody occurred with 28% of the 176 doses and was transient. Human antimouse antibodies (HAMA) were generated in 30% after a mean of 4 doses, but interrupted therapy in only 10% of the patients. Thrombocytopenia was dose-limiting; there were no deaths due to toxicity. Tumor regression occurred in 25 (83%) of the patients and was great enough, and durable enough, in 17 (57%) to qualify them as responders; 13 NHL patients and 4 CLL patients. Advanced disease often interrupted therapy prematurely. However, 18 patients received at least 180 mCi of 131I-Lym-1; 17 (94%) of these responded to the therapy.

CONCLUSION

Although advanced disease often interrupted therapy prematurely, the results from 131I-Lym-1 therapy are clearly promising and warrant additional trials.

Overview of radiation myelotoxicity secondary to radioimmunotherapy using 131I-Lym-1 as a model

The radiation dose-limiting toxicity from radioimmunotherapy has been myelotoxicity in the absence of bone marrow reconstitution (transplantation). Myelotoxicity can be assessed directly by biopsy examination of the bone marrow and indirectly by peripheral blood counts. In patients with B-cell malignancies, thrombocytopenia has been the initial and most severe manifestation of 131I-Lym-1 radiation toxicity from treatment. Manifestations of myelotoxicity varied greatly among the patients and from one treatment dose to another in the same patient, suggesting that additional factors were present. There was an increased likelihood of Grade 3-4 hematopoietic toxicity after 131I-Lym-1 treatment if the patient had peripheral blood cell abnormalities before undergoing 131I-Lym-1 treatment. Fractionation of the total 131I-Lym-1 dose was associated with less toxicity. In many patients, myelotoxicity could not be explained by marrow radiation dose (0.36 +/- 0.13 rads per administered mCi) from 131I-Lym-1 in the blood and body alone. Bone marrow examination and 131I-Lym-1 imaging usually provided evidence for additional marrow radiation from 131I-Lym-1-targeting of marrow malignancy and also for residual toxic effects from prior treatment in these patients. Immunohistologic and imaging examination of the bone marrow performed with the intended treatment antibody allowed assessment of extent of marrow malignancy and prediction of degree of myelotoxicity from subsequent treatment. Treatment programs (and protocols) for radioimmunotherapy should incorporate these methods into the decision process. Larger amounts of 131I-Lym-1 can be used in patients selected to have relatively normal peripheral blood cell counts and normocellular bone marrows uninvolved by the malignancy. These observations appear to be relevant to the maximum tolerated dose in radioimmunotherapy for other malignancies as well.