Dose Calculation for I131/ I124 Radioimmunoconjugates for Radioimmunotherapy

Research Article

Austin J Nucl Med Radiother. 2014;1(1): 3.

Dose Calculation for I131/ I124 Radioimmunoconjugates for Radioimmunotherapy

Bloch P and Plastaras JP*

Department Radiation Oncology, University of Pennsylvania, USA

*Corresponding author: Plastaras JP, Department of Radiation Oncology, Perleman Center for Advanced Medicine, University of Pennsylvania, 3400 Civic Center Boulevard, Philadelphia, PA 19104, USA

Received: August 20, 2014; Accepted: September 19, 2014; Published: September 24, 2014

Abstract

For clinical evaluation of Radioimmunotherapy (RIT) it would be valuable to measure the heterogeneity of the uptake and retention within the tumor bed of the radioimmunoconjugate agent. Presently, the patient-specific maximally tolerated therapeutic amount of radioactivity to administer is determined from measurements, prior to treatment, of the whole-body clearance of a trace amount of I-131 labeled radioimmunoconjugate agent. Labeling the therapeutic radioimmunoconjugate agent with the positron emitter I-124 would permit PET imaging to be employed to determine the regional uptake and retention of the therapeutic administered agent. An algorithm has been developed to calculate the 3D-dose distribution of I-124 and I-131 using Monte Carlo derived point dose kernels and measured regional Specific Uptake Values (SUVs) and clearance determined from PET/CT studies. The calculations indicate that for the same concentration of activity in a tumor volume (uCi/gm), I-124 would result in a higher dose rate (Gy/hr) and greater total dose than I-131.

Keywords: Cancer therapy; Radioimmuniotherapy; Dosimetry iodine-124/131

Introduction

To evaluate the dose delivered to the tumor and normal tissues for Radio Immunotherapy (RIT) requires measured data of both the regional distribution and the pharmacokinetics of the radio labeled agent. Therapeutic amounts of I-131 have been administered for RIT [1]. Although estimates of the dose deposition have been attempted using SPECT, detailed spatial information on the distribution of the I-131from nuclear medicine studies is limited by the physical properties of the isotope [2]. Attaching a positron emitter such as I-124 to the radioimmunoconjugate would permit using PET/CT imaging to determine the spatial distribution of the isotope in both the tumor and normal tissues.

In this paper we evaluate and compare the dosimetric differences of I-124 and I-131. For dosimetry proposes the decay products of isotopes are usually separated into two components; (1) a beta like component where the energy emitted is absorbed at the point of emission and (2) a gamma like component where the energy emitted is absorbed at a distance from them point of emission. ENSDF* data were used to evaluate these two components for I-124 and I-131. The equilibrium energy emission constant for the beta like Δβ and gamma components Δγ shown below demonstrates two important features: