Exposure levels associated with Na(131)I thyroid cancer patients: correlation with initial activity and clinical physical parameters

Initial radiation exposure levels X (0) at 1 m from the navel of thyroid cancer patients were measured for 165 individuals at the time of ingestion. Some 61 patients had previously signed informed consent so only those patients could be assayed with regard to body parameters. While the activity was in the stomach, resultant X (0) values were seen to be linearly correlated with the total (131)I activity (A) given orally. Yet large differences in X (0) were seen; e.g., at A = 7.4 GBq, variations of a factor of four were found between the largest and smallest exposure rates. Correlation analyses were performed between normalized rate X (0)A-1 and several patient physical parameters. These included age, sex, height, weight, and BMI (body mass index). Only weight and BMI had significant linear correlation (p < 0.05) with normalized exposure rate. In the former case, the correlation coefficient ρ (weight) was -0.296 (p = 0.02). Using BMI as the independent variable, ρ (BMI) was -0.386 (p = 0.0021). With further analysis of the BMI variation, 95% confidence intervals could be determined at various BMI levels. For example, at 28 kg m(-2), the normalized rate varied between 0.039 and 0.0446 μGy h(-1) MBq(-1)-approximately a ±6.5% variation on the mean value of 0.0419 μGy h(-1) MBq(-1) at this BMI. Given such clinical information, differences in normalized exposure rate can be reduced to values on the order of ±10% or less for BMI values over the clinically relevant interval 20 to 40 kg m(-2).

Using a single parameter to describe time-activity curves

Time-activity uptake curves [u(t) in % injected dose per gram of tissue] may be described by different--often complicated--functional forms. Because of the need to readily compare sequences of engineered radiopharmaceuticals, it is efficient to use mean residence time (MRT) as a one-parameter descriptor. In applying this computation to a sequence of five cognate anti-carcinoembryonic antigen (CEA) antibodies, it was found that the intact form had the longest MRT in the blood with the other four cognates having values less by approximately a factor of 10 or more. This difference probably follows from the lack of an intact Fc segment on the latter engineered molecules. MRT values for a sequence of six scFv-Fc engineered fragments demonstrated that the double mutant had the shortest blood residence time--30-fold less compared with the wild type. Whereas it is not possible to directly apply the MRT to nonbolus (tumor or organ) curves, a residence time (τ) may be assigned using the uptake function. Using τ, it was found that the intact (natural) form of the anti-CEA cognate set had the longest time at the tumor site in the human xenograft model in nude mice. The MRT and τ concept are proposed to also allow comparison of possible relative blood and tissue exposures, respectively, for cognate sets of unlabeled engineered antibodies used to treat malignancies although no data are yet available in the literature for this application.

The two types of correction of absorbed dose estimates for internal emitters

BACKGROUND

Two types of correction for absorbed dose (D) estimates are described for clinical applications of internal emitters. The first is appropriate for legal and scientific reasons involving phantom-based estimates; the second is patient-specific and primarily intended for radioimmunotherapy (RIT).

METHODS

The Medical Internal Radiation Dose (MIRD) relationship (D) = S A is used, where S is a geometric matrix factor and A is the integral of source organ activities. Internal consistency of the data and the size of organ systems in the humanoid phantom must be assured in both types of estimation.

RESULTS

The first dose estimate correction (I) is one whereby computations refer to one or another standard (e.g., MIRD-type) phantom. In this case the S value remains as given, but the measured patient A data must be standardized. The correction factor is the phantom's ratio of organ mass to whole-body mass divided by the same ratio for the volunteer or patient. The second dose estimate correction (II) is patient-specific. While the A value is unchanged for this application, a correction term is provided for the phantom-derived S matrix. The dominant (nonpenetrating radiation) component of this correction factor can be obtained via the ratio of the patient to phantom organ masses. In both corrections, we recommend that true organ sizes, necessary in each method of estimation, be determined in a separate sequence of anatomic images.

CONCLUSIONS

In both dose estimation corrections, true sizes of the patient's or volunteer's internal organs must be obtained. Correction due to organ mass size can be severalfold and is probably the dominant uncertainty in the internal emitter absorbed dose calculation process.

Truncation of blood curves to enhance imaging and therapy with monoclonal antibodies

Targeting of monoclonal antibody (Mab) to solid tumor sites is a function of the blood curve of activity versus time. It has been suggested that the blood curve be artificially reduced to approach zero so that the contrast between tumor and blood uptake is maximized. We analyzed tumor uptake as a function of the time tc of blood curve truncation. By using a convolution approach, we were able to find the optimal times for setting the blood curve to zero in either diagnostic or therapeutic animal examples. Two iodinated cT84.66 anti-CEA engineered fragments, diabody and minibody, were considered using previous data from nude mouse studies involving the LS174T colorectal tumor model. Figures of merit (FOMs) were used to compare ordinary and truncated blood curves and their associated tumor accumulations. Using a 1231 label, it was seen that the appropriate time for diagnostic truncation occurred when tumor uptake, as measured, was a maximum. The corresponding point for therapy (with 1311 as a label) was at infinite time. We also demonstrated that the use of traditional indices led to ambiguities in the choice of truncation times. The traditional therapy index, the ratio of the integral of the tumor uptake to the integral of the blood uptake, was found to be a numerical constant independent of tc. This ratio was proved to be the integral of the tumor impulse response function. Use of such convolution techniques to assess truncation of the perfused material is probably also applicable to multistep processes as well as to lesion targeting with other tumor-specific pharmaceuticals.

A radionuclide therapy treatment planning and dose estimation system

An object-oriented software system is described for estimating internal emitter absorbed doses using a set of computer modules operating within a personal computer environment. The system is called the Radionuclide Treatment Planning and Absorbed Dose Estimation System (RTDS). It is intended for radioimmunotherapy applications, although other forms of internal emitter therapy may also be considered.

METHODS

Four software modules interact through a database backend. Clinical, demographic and image data are directly entered into the database. Modules include those devoted to clinical imaging (nuclear, CT and MR), activity determination, organ compartmental modeling and absorbed dose estimation.

RESULTS

Both standard phantom (Medical Internal Radiation Dose [MIRD]) and patient-specific absorbed doses are estimated. All modules interact with the database backend so that changes in one process do not influence other operations. Results of the modular operations are written to the database as computations are completed. Dose-volume histograms are an intrinsic part of the output for patient-specific absorbed dose estimates. A sample dose estimate for a potential 90Y monoclonal antibody is described.

CONCLUSION

A four-module software system has been implemented to estimate MIRD phantom and patient-specific absorbed doses. Computations of the doses and their statistical distribution for a pure beta emitter such as 90Y take approximately 1 min on a 300 MHz personal computer.

Intrapatient consistency of imaging biodistributions and their application to predicting therapeutic doses in a phase I clinical study of 90Y-based radioimmunotherapy

Intrapatient variation in the biodistribution of the chimeric monoclonal antibody cT84.66 was assessed in 19 patients having a variety of carcinoembryonic antigen (CEA) positive tumors. The two studies, including whole-body imaging and blood and urine specimen collections, were conducted within 14 days of each other using (111)In-cT84.66 at a fixed total protein dose of 5 mg per patient per study. An initial pretherapy infusion of (111)In-cT84.66 was administered followed by a therapy coinfusion of (111)In-ct84.66 and 90Y-cT84.66 A closed five-compartment model was used to integrate source organ activity curves as residence time inputs into the MIRDOSE3 program. Normal organ absorbed doses were estimated for 90Y-cT84.66, the corresponding radiotherapeutic agent. For the two (111)In-cT84.66 biodistributions, all data were modeled with a R2 value of between 0.72 and 1.00 with the exception of the urine data taken during therapy. This was due to the need of diethylenetriaminepentaacetic acid during the therapy phase because of the possibility that yttrium might escape from the chelator attached to the antibody. With the assurance that the biodistributions were reproducible, we were able to estimate the 90Y-cT84.66 absorbed doses on a per-patient basis. Concordance coefficients showing the agreement between the imaging and therapy phase dose estimates were between the 0.60 and 0.99 levels for liver, spleen, red marrow, total body, and other organ systems. Median results were: 27, 17, and 2.7 rad/mCi of 90Y-cT84.66 for liver, spleen, and red marrow, respectively. Because of decreases in platelets and white cells as the amount of 90Y was increased, dose-limiting toxicity was found at 22 mCi/m2. We conclude that patient biodistributions were consistent over time to 14 days so as to allow absorbed dose estimation in a radioimmunotherapy trial involving the cT84.66 anti-CEA antibody.

Estimating residence times and their associated errors in patient absorbed-dose calculation

OBJECTIVE

An approach for estimating organ residence times (tau) and their errors in patient internal emitter radiation dosage calculations has been determined.

METHODS

Using a modeling algorithm and its associated parameters, chimeric anti-CEA monoclonal antibody (cT84.66) patient organ uptake data and residence times of source organ activity were calculated. Through the covariance matrix of the model's parameters and subsequent Monte Carlo simulations, errors in organ residence time (gamma tau) also were estimated

RESULTS

These relative tau errors were found to be model-dependent; increasing as the number of organs being simultaneously modeled in a set of two patients being considered for 90Y-cT84.66 radioimmunotherapy.

CONCLUSION

Use of modeling and Monte Carlo methods provide a general, direct procedure for calculating the degree of accuracy of activity integrals and other mathematical functions of kinetic variables.

Pharmacokinetic modeling and absorbed dose estimation for chimeric anti-CEA antibody in humans

The objective of this article was to model pharmacokinetic data from clinical diagnostic studies involving the 111In-labeled monoclonal antibody (MAb) chimeric T84.66, against carcinoembryonic antigen. Model-derived results based on the 111In-MAb blood, urine and digital imaging data were used to predict 90Y-MAb absorbed radiation doses and to guide treatment planning for future therapy trials. Fifteen patients with at least one carcinoembryonic antigen-positive lesion were evaluated. We report the kinetic parameter estimates and absorbed 111In-MAb dose and projected 90Y-MAb doses for each patient as well as describe our approach and rationale for modeling an extensive set of pharmacokinetic data.

METHODS

The ADAPT II software package was used to create three- and five-compartment models of uptake against time in the patient population. The "best-fit" model was identified using ordinary least squares. Areas under the curve were calculated using the modeled curves and input into MIRDOSE3 to estimate absorbed radiation doses for each patient.

RESULTS

A five-compartment model best described the liver, whole body, blood and urine data for a subcohort of nine patients with digital imaging data. A three-compartment model best described the blood and urine data for all 15 clinical patients accrued in the clinical trial. For the subcohort, the largest projected 90Y-MAb doses were delivered to the liver (mean, 24.78 rad/mCi; range, 15.02-37.07 rad/mCi), with red marrow estimates on the order of 3.32 rad/mCi (range, 1.24-5.55) of 90Y. Corresponding estimates for the 111In-MAb were 3.18 (range, 2.09-4.43) and 0.55 (range, 0.34-0.74), respectively.

CONCLUSION

The three- and five-compartment models presented here were successfully used to represent the blood, urine and imaging data. This was evidenced by the small standard errors for the kinetic parameter estimates and R2 values close to 1. As planned future therapeutic trials will involve stem cell support to alleviate hematological toxicities, the development of an approach for estimating doses to other major organs is crucial.