DOSIMETRIC STUDY OF BETA-MINUS EMITTER PRASEODYMIUM-142 : APPLICATIONS IN MICROSPHERE BRACHYTHERAPY FOR HEPATOCELLULAR CARCINOMA AND BRACHYTHERAPY FOR OCULAR SQUAMOUS CELL CARCINOMA

Abstract

A dosimetric study of the beta-minus emitter Praseodymium-142 (¹⁴²Pr) was performed and two main innovative applications of this source in brachytherapy are proposed: microsphere brachytherapy and eye plaque brachytherapy. ¹⁴²Pr (96.3% beta and 3.7% gamma) has recently gained increased attention among beta sources for therapy. Its inherent physical and dosimetric characteristics may be suitable for several brachytherapy applications. This relatively short halflife (19.12 h) allows this source to deliver high dose rates to the target. Penetration of the beta component of ¹⁴²Pr radiation in tissue is limited to a few millimeters, therefore limiting dose deposition to the treated site. The large neutron activation cross-section of the parent isotope (11.40 barn including that of a meta-stable state) allows the activation of this nuclide in a low neutron fluence reactor, making its production easily available for both research and therapy purposes. From simulations and measurement of the doses, exposure rates due to the 3.7% gamma component showed to be clinically small for the patients' healthy organs and tissues as well as to medical staff and general public. Furthermore, it was studied whether its gamma component could be effective in performing pre-treatment quality assurance (QA) and dosimetry, as well as post-treatment biodistribution imaging and dose distributions of permanently implanted ¹⁴²Pr brachytherapy sources. Two main novel applications of ¹⁴²Pr beta emitters are studied in this work: (i) ¹⁴²Pr glass microspheres, as a possible choice of radionuclide for microsphere brachytherapy of nonresectable hepatocellular carcinoma (HCC) and metastasis of the liver, and (ii) ¹⁴²Pr glass rods for brachytherapy of ocular squamous cell carcinoma (OSCC), based on a novel design for eye brachytherapy -- also developed in this work. In order to simulate the dosimetric characteristics of ¹⁴²Pr, MCNPX2.6 Monte Carlo code and BRAIN-DOSE Dose Point Kernel code were used to determine the dose distributions of ¹⁴²Pr for different source distributions. Dosimetric properties of the currently used nuclide in microsphere brachytherapy, Yttrium-90 (⁹⁰Y), was also studied and compared to ¹⁴²Pr. A more realistic biodistribution model of microspheres within a single HCCs blood vessel was studied. In addition, the biological effective dose (BED) for different tumor doubling times (DT) for HCC was determined using the linear quadratic (LQ) model for both nuclides. Dose distributions for ¹⁴²Pr, ⁹⁰Y and Rhenium-188 were simulated within the eyeball and compared for application in eye brachytherapy. Measurements of dose distribution using GAFCHROMIC® EBT2 film were performed with ¹⁴²Pr glass microspheres and ¹⁴²Pr glass rods. Monte Carlo simulation validation with experimental measurements in phantom of the dose distribution due to ¹⁴²Pr glass microspheres and ¹⁴²Pr glass rods were performed. ¹⁴²Pr showed to be a possible choice of radionuclide for HCC microsphere brachytherapy and OSCC brachytherapy, delivering a high biological effective dose, and opening possibilities for post treatment imaging and biodistribution assessment.