Validation of a Simple Thyroid Cancer Dosimetry Model Based on the Fractional Whole-Body Retention at 48 Hours Post-Administration of (131)I

Advanced imaging and theranostics in thyroid cancer

PURPOSE OF REVIEW

Thyroid cancers are endocrine neoplasms with diverse gene expression and behavior, for which constantly evolving anatomic and functional imaging/theranostic agents have an essential role for diagnosis, staging, and treatment.

RECENT FINDINGS

To achieve definitive diagnosis, neck ultrasound and associated risk stratification systems, notably Thyroid Imaging Reporting and Data System (TI-RADS), allow improved thyroid nodule characterization and management guidance. Radioactive iodine-131 (RAI) has long played a role in management of differentiated thyroid cancer (DTC), with recent literature emphasizing its effectiveness for intermediate-high risk cancers, exploring use of dosimetry for personalized medicine, and potential for retreatment with RAI following tumor redifferentiation. Iodine-124 positron emission tomography/computed tomography (PET/CT) has promising application for DTC staging and dosimetry. F18-fluorodeoxyglucose (FDG) PET/CT is used for staging of high risk DTC and identification of noniodine-avid disease recurrences, with metabolic uptake consistently portending poor prognosis. Poorly differentiated and anaplastic thyroid cancers are best assessed with anatomic imaging and F18-FDG PET/ CT, though recent studies show a potential theranostic role for Ga68/Lu177-prostate-specific membrane antigen. Medullary thyroid cancers are evaluated with ultrasound, CT, magnetic resonance imaging, and various positron-emitting radiotracers for PET imaging (F18-DOPA, F18-FDG, and recently Ga68-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-octreotate (DOTATATE)); the latter may enable treatment with Lu177-DOTATATE.

SUMMARY

Multidisciplinary collaboration is essential to streamline appropriate management, given the wide array of available imaging and new therapies for metabolic and genetically complex cancers.

Reimbursement Approaches for Radiopharmaceutical Dosimetry: Current Status and Future Opportunities

Interest in performing dosimetry for clinical radiopharmaceutical therapy procedures has grown in recent years. Several approved therapies include dosimetry in the Food and Drug Administration-approved label instructions, and other therapies are best used under a patient-tailored paradigm. This paper, which is a product of the Society of Nuclear Medicine and Molecular Imaging Dosimetry Task Force, presents motivations and general workflows for radiopharmaceutical therapy dosimetry, as well as existing strategies for obtaining reimbursement for clinical activities related to dosimetry. Several specific patient examples are provided, including suggested codes for reimbursement. In addition to current reimbursement approaches, key dosimetry services that are not supported under the current coding structure are presented and suggested as areas of focus in the coming years.

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

The use of radiopharmaceutical therapies (RPTs) in the treatment of cancers is growing rapidly, with more agents becoming available for clinical use in last few years and many new RPTs being in development. Dosimetry assessment is critical for personalized RPT, insofar as administered activity should be assessed and optimized in order to maximize tumor-absorbed dose while keeping normal organs within defined safe dosages. However, many current clinical RPTs do not require patient-specific dosimetry based on current Food and Drug Administration-labeled approvals, and overall, dosimetry for RPT in clinical practice and trials is highly varied and underutilized. Several factors impede rigorous use of dosimetry, as compared with the more convenient and less resource-intensive practice of empiric dosing. We review various approaches to applying dosimetry for the assessment of activity in RPT and key clinical trials, the extent of dosimetry use, the relative pros and cons of dosimetry-based versus fixed activity, and practical limiting factors pertaining to current clinical practice.

Managing radioiodine refractory thyroid cancer: the role of dosimetry and redifferentiation on subsequent I-131 therapy

Poor responses to iodine-131 (I-131) therapy can relate to either low iodine uptake and retention in thyroid cancer cells or to increased radioresistance. Both mechanisms are currently termed radioactive iodine (RAI)-refractory (RAI-R) thyroid cancer but the first reflects unsuitability for I-131 therapy that can be evaluated in advance of treatment, whereas the other can only be identified post hoc. Management of both represents a considerable challenge in clinical practice as failure of I-131 therapy, the most effective treatment of metastatic thyroid cancer, is associated with a poor overall prognosis. The development of targeted therapies has shown substantial promise in the treatment of RAI-R thyroid cancer in progressive patients. Recent studies show that selective tyrosine kinase inhibitors (TKIs) targeting B-type rapidly accelerated fibrosarcoma kinase (BRAF) and mitogen-activated protein kinase (MEK) can be used as redifferentiation agents to re-induce RAI uptake, thereby (re)enabling I-131 therapy. The use of dosimetry prior- and post-TKI treatment can assist in quantifying RAI uptake and improve identification of patients that will benefit from I-131 therapy. It also potentially offers the prospect of calculating individualized therapeutic administered activities to enhance efficacy and limit toxicity. In this review, we present an overview of the regulation of RAI uptake and clinically investigated redifferentiation agents, both reimbursed and in experimental setting, that induce renewed RAI uptake. We describe the role of dosimetry in redifferentiation and subsequent I-131 therapy in RAI-R thyroid cancer, explain different dosimetry approaches and discuss limitations and considerations in the field.

123I Scan With Whole-Body Retention Measurement at 48 Hours for Simplified Dosimetry Before 131I Treatment of Metastatic Thyroid Cancer

ABSTRACT

A previously published model (Atkins) allows for calculation of 131I maximum tolerated activity on the basis of 48-hour whole-body retention of 131I on a pretherapy diagnostic scan. Our practice uses iodine 123I for diagnostic imaging of metastatic thyroid cancer for staging before 131I therapy, with images typically acquired 24 hours after administration of the radiopharmaceutical. We explored the feasibility of an additional 123I whole-body scan and retention measurement at 48 hours, with application of the model to estimate maximum tolerated activity of radioiodine before 131I treatment of metastatic thyroid cancer.

Conventional Radioiodine Therapy for Differentiated Thyroid Cancer

This article presents an overview of the use of radioactive iodine (131-I) in the treatment of patients with differentiated thyroid cancer. Topics reviewed include definitions; staging; the 2 principal methods for selection of 131-I dosage; the indications for ablation, adjuvant treatment, and treatment; the recommendations for the use of 131-I contained in the guidelines of the American Thyroid Association and the Society of Nuclear Medicine and Molecular Imaging; the dosage recommendations and selection of dosage approach for 131-I by these organizations; the use of recombinant human thyrotropin for radioiodine ablation, adjuvant therapy, or treatment; and the MedStar Washington Hospital Center approach.