Red marrow and blood dosimetry in¹³¹I treatment of metastatic thyroid carcinoma: pre-treatment versus in-therapy results

Abstract

Treatment with radioiodine is a standard procedure for patients with well-differentiated thyroid cancer, but the main approach to the therapy is still empiric, consisting of the administration of fixed activities. A predictive individualized dosimetric study may represent an important tool for physicians to determine the best activity to prescribe. The aim of this work is to compare red marrow and blood absorbed dose values obtained in the pre-treatment (PT) dosimetry phase with those obtained in the in-treatment (IT) dosimetry phase in order to estimate the predictive power of PT trial doses and to determine if they can be used as a decision-making tool to safely administer higher (131)I activity to potentially increase the efficacy of treatment. The PT and IT dosimetry for 50 patients has been evaluated using three different dosimetric approaches. In all three approaches blood and red marrow doses, are calculated as the sum of two components, the dose from (131)I activity in the blood and the dose from (131)I activity located in the remainder of the body (i.e. the blood and whole-body contributions to the total dose). PT and IT dose values to blood and red marrow appear to be well correlated irrespective of the dosimetric approach used. Linear regression analyses of PT and IT total doses, for blood and red marrow, and the whole-body contribution to these doses, showed consistent best fit slope and correlation coefficient values of approximately 0.9 and 0.6, respectively: analyses of the blood dose contribution to the total doses also yielded similar values for the best fit slope but with correlation coefficient values of approximately 0.4 reflecting the greater variance in these dose estimates. These findings suggest that pre-treatment red marrow dose assessments may represent an important tool to personalize metastatic thyroid cancer treatment, removing the constraints of a fixed activity approach and permitting potentially more effective higher (131)I activities to be safely used in-treatment.

Absorbed Dose Evaluation in Radioiodine Therapy with Different Approaches

“…Even if clinical data have not yet demonstrated the therapeutic effectiveness of higher activities, some authors have suggested that high-activity single therapy may result in a higher efficacy than multiple low-activity treatments, because of the potential for smaller fractionated doses to increase the radio-resistance of metastatic tumors [2]. Moreover, Giostra et al [21] found that the administered therapeutic activity could be safely increased compared to the current prescribed activities (from 7.4 GBq to 11.1 GBq). Dorn et al [22] combined red marrow and lesion dosimetry showing that high-activity administrations with less than 3 Gy to the red marrow are a safe and more effective method with respect to fixed activity administrations.…”

Personalized Dosimetry in the Context of Radioiodine Therapy for Differentiated Thyroid Cancer

“…Implicit in all such approaches is the biokinetics assessed pre-treatment with a tracer activity and that of the in-treatment therapeutic administration are the same. To verify this assumption, a recent study [ 151 ] compared red marrow and blood absorbed dose values obtained pre-treatment with that obtained in-treatment for a group of 50 patients treated for metastatic thyroid cancer, using different dosimetric approaches: pre-treatment and in-treatment absorbed dose values to blood and red marrow appeared to be well correlated irrespective of the dosimetric approach used. In a retrospective analysis of 124 metastatic DTC patients who underwent dosimetric evaluation over a period of 15 y, the MTA approach was adopted using a risk-adapted approach [ 122 ]: the maximum administered activity was limited with radiation doses of 3 Gy to the red marrow or 30 Gy to the lungs, both limiting values corresponding to the absorbed dose of the external radiation therapy which is associated with a 5% risk of severe damage within 5 y. Eighty-three patients received 104 treatments, of which 13 were given for postsurgical ablation of thyroid remnants, 41 were given with curative intent, and 50 were for palliation (for the others, treatment resulted not indicated).…”

Personalized dosimetry for a deeper understanding of metastatic response to high activity 131I-mIBG therapy in high risk relapsed refractory neuroblastoma

“…Whereas, the D RM is the sum of two contributions: (I) direct irradiation due to blood activity (self blood contribution) and (II) indirect irradiation due to rest of body activity. It has been calculated using the formula reported in (15,16):…”

“…where I Ant and I Post represent counts inside lesion's ROI in anterior and posterior views respectively, corrected for background; f j is the self attenuation correction; C and μ(I-131) are the sensitivity of the gamma camera and the attenuation coefficient for 131 I obtained using an abosolute quantification described in (15,21) and are equal to 12.83 cps/MBq and 0.106 cm −1 , respectively. Finally the absorbed dose to lesion (D Lesion ) was calculated using the formula:…”