Radiation dose assessment in radioiodine therapy. Dose-response relationships in differentiated thyroid carcinoma using quantitative scanning and PET

Normal organ dosimetry for thyroid cancer patients treated with radioiodine as part of the multi-centre multi-national Horizon 2020 MEDIRAD project

PURPOSE

Dosimetry is rarely performed for the treatment of differentiated thyroid cancer patients with Na[131I]I (radioiodine), and information regarding absorbed doses delivered is limited. Collection of dosimetry data in a multi-centre setting requires standardised quantitative imaging and dosimetry. A multi-national, multi-centre clinical study was performed to assess absorbed doses delivered to normal organs for differentiated thyroid cancer patients treated with Na[131I]I.

METHODS

Patients were enrolled in four centres and administered fixed activities of 1.1 or 3.7 GBq of Na[131I]I using rhTSH stimulation or under thyroid hormone withdrawal according to local protocols. Patients were imaged using SPECT(/CT) at variable imaging time-points following standardised acquisition and reconstruction protocols. Whole-body retention data were collected. Dosimetry for normal organs was performed at two dosimetry centres and results collated.

RESULTS

One hundred and five patients were recruited. Median absorbed doses per unit administered activity of 0.44, 0.14, 0.05 and 0.16 mGy/MBq were determined for the salivary glands of patients treated at centre 1, 2, 3 and 4, respectively. Median whole-body absorbed doses for 1.1 and 3.7 GBq were 0.05 Gy and 0.16 Gy, respectively. Median whole-body absorbed doses per unit administered activity of 0.04, 0.05, 0.04 and 0.04 mGy/MBq were calculated for centre 1, 2, 3 and 4, respectively.

CONCLUSIONS

A wide range of normal organ doses were observed for differentiated thyroid cancer patients treated with Na[131I]I, highlighting the necessity for individualised dosimetry. The results show that data may be collated from multiple centres if minimum standards for the acquisition and dosimetry protocols can be achieved.

Initial results of the INSPIRE clinical trial-investigating radiation dosimetry for differentiated thyroid cancer patients

Introduction

The optimal strategy for differentiated thyroid cancer (DTC) patients treated with radioiodine (RAI) following thyroidectomy remains controversial. Multi-centre clinical studies are essential to identify strategies to improve patient outcomes while minimising treatment-induced toxicity.

Materials and Methods

The INSPIRE clinical trial (ClinicalTrials.gov Identifier: NCT04391244) aims to investigate patient-specific dosimetry for DTC patients and to determine the range of absorbed doses delivered to target and non-target tissues and their relationship with treatment outcome and toxicity.

Results

We report here initial results of the first 30 patients enrolled onto the INSPIRE trial. A large range of absorbed doses are observed for both thyroid remnants and salivary glands, with median values of 4.8 Gy (Range 0.2 - 242 Gy) and 0.3 Gy (Range 0.1 to 1.7 Gy), respectively.

Discussion

The preliminary study results are encouraging and could help to improve our understanding of absorbed doses to thyroid remnants and normal organs following RAI therapy. Such knowledge could potentially enable patient-specific treatment planning with improved clinical outcomes and quality-of-life of patients.

Iodine-131 and Iodine-131-Meta-iodobenzylguanidine Dosimetry in Cancer Therapy

Radioactive iodine was first used for the treatment of benign thyroid disease and thyroid cancer 80 years ago. I-131 mIBG was later developed for the treatment of adult and pediatric neuroendocrine tumors. Physicists were closely involved from the outset to measure retention, to quantify uptake and to calculate radiation dosimetry. As the treatment became widespread, contrasting treatment regimes were followed, either given with empirically derived fixed levels of activity or guided according to the radiation doses delivered. As for external beam radiotherapy, individualized treatments for both thyroid cancer and neuroendocrine tumors were developed based on the aim of maximizing the radiation doses delivered to target volumes while restricting the radiation doses delivered to organs-at-risk, particularly the bone marrow. The challenge of marrow dosimetry has been met by using surrogate measures, often the blood dose for thyroid treatments and the whole-body dose in the case of treatment of neuroblastoma with I-131 mIBG. A number of studies have sought to establish threshold absorbed doses to ensure therapeutic efficacy. Although different values have been postulated, it has nevertheless been conclusively demonstrated that a fixed activity approach leads to a wide range of absorbed doses delivered to target volumes and to normal organs. Personalized treatment planning is now technically feasible with ongoing multicenter clinical trials and investigations into image quantification, biokinetic modelling and radiobiology.

Dependency of the remnant 131I-NaI biokinetics on the administered activity in patients with differentiated thyroid cancer

PURPOSE

To study the dependency of the effective half-life on the administered activity and the correlation between the time-integrated activity and the remnant uptake at 2d and 7d in patients treated for DTC with 1.11 GBq, 3.7 GBq or 5.55 GBq of ¹³¹I-NaI.

METHODS

Ninety-two patients undergoing total thyroidectomy and lymph node removal were included. If cancer had not spread to lymph nodes, patients received 1.11 GBq of ¹³¹I-NaI when the lesion maximal diameter was smaller than 4 cm, and 3.7 GBq for bigger sizes. If cancer had spread to lymph nodes patients received 5.55 GBq. There were 30, 49 and 13 patients respectively treated with 1.11 GBq(Group 1), 3.7 GBq(Group 2) and 5.55 GBq(Group 3). Two SPECT/CT scans were performed at 2d and 7d after radioiodine administration for each patient to determine the thyroid remnant activities and effective half-lives of the radioiodine.

RESULTS

Statistical analysis showed significant differences (p < 0.05) in the effective half-life among patients treated with 1.11 GBq, 3.7 GBq and 5.55 GBq. A high positive correlation (ρ > 0.95) was found between the time-integrated activity and the remnant activity at 2d for the three groups of patients.

CONCLUSIONS

There were significant differences in the effective half-life of the radioiodine in remnants of patients treated with activities of 1.11 GBq, 3.7 GBq or 5.55 GBq. The high positive linear correlation found between the time-integrated activity and the remnant activity at 2d for the three groups of patients indicate that the time-integrated activity could be estimated from one time-point.

Setting up a quantitative SPECT imaging network for a European multi-centre dosimetry study of radioiodine treatment for thyroid cancer as part of the MEDIRAD project

Background

Differentiated thyroid cancer has been treated with radioiodine for almost 80 years, although controversial questions regarding radiation-related risks and the optimisation of treatment regimens remain unresolved. Multi-centre clinical studies are required to ensure recruitment of sufficient patients to achieve the statistical significance required to address these issues. Optimisation and standardisation of data acquisition and processing are necessary to ensure quantitative imaging and patient-specific dosimetry.

Material and methods

A European network of centres able to perform standardised quantitative imaging of radioiodine therapy of thyroid cancer patients was set-up within the EU consortium MEDIRAD. This network will support a concurrent series of clinical studies to determine accurately absorbed doses for thyroid cancer patients treated with radioiodine. Five SPECT(/CT) systems at four European centres were characterised with respect to their system volume sensitivity, recovery coefficients and dead time.

Results

System volume sensitivities of the Siemens Intevo systems (crystal thickness 3/8″) ranged from 62.1 to 73.5 cps/MBq. For a GE Discovery 670 (crystal thickness 5/8″) a system volume sensitivity of 92.2 cps/MBq was measured. Recovery coefficients measured on three Siemens Intevo systems show good agreement. For volumes larger than 10 ml, the maximum observed difference between recovery coefficients was found to be ± 0.02. Furthermore, dead-time coefficients measured on two Siemens Intevo systems agreed well with previously published dead-time values.

Conclusions

Results presented here provide additional support for the proposal to use global calibration parameters for cameras of the same make and model. This could potentially facilitate the extension of the imaging network for further dosimetry-based studies.

Radionuclide therapy: current status and prospects for internal dosimetry in individualized therapeutic planning

The efficacy and toxicity of radionuclide therapy are believed to be directly related to the radiation doses received by target tissues; however, nuclear medicine therapy continues to be based primarily on the administration of empirical activities to patients and less frequently on the use of internal dosimetry for individual therapeutic planning. This review aimed to critically describe the techniques and clinical evidence of dosimetry as a tool for therapeutic planning and the main limitations to its implementation in clinical practice. The present article is a nonsystematic review of voxel-based dosimetry. Clinical evidence pointing to a correlation between the radiation dose and therapeutic response in various diseases, such as thyroid carcinoma, neuroendocrine tumors and prostate cancer, is reviewed. Its limitations include technical aspects related to image acquisition and processing and the lack of randomized clinical trials demonstrating the impact of dosimetry on patient therapy. A more widespread use of dosimetry in therapeutic planning involves the development of user-friendly dosimetric protocols and confirmation that dose estimation implies good efficacy and low treatment-related toxicity.

Impact of PET and MRI threshold-based tumor volume segmentation on patient-specific targeted radionuclide therapy dosimetry using CLR1404

Variations in tumor volume segmentation methods in targeted radionuclide therapy (TRT) may lead to dosimetric uncertainties. This work investigates the impact of PET and MRI threshold-based tumor segmentation on TRT dosimetry in patients with primary and metastatic brain tumors. In this study, PET/CT images of five brain cancer patients were acquired at 6, 24, and 48 h post-injection of ¹²⁴I-CLR1404. The tumor volume was segmented using two standardized uptake value (SUV) threshold levels, two tumor-to-background ratio (TBR) threshold levels, and a T1 Gadolinium-enhanced MRI threshold. The dice similarity coefficient (DSC), jaccard similarity coefficient (JSC), and overlap volume (OV) metrics were calculated to compare differences in the MRI and PET contours. The therapeutic ¹³¹I-CLR1404 voxel-level dose distribution was calculated from the ¹²⁴I-CLR1404 activity distribution using RAPID, a Geant4 Monte Carlo internal dosimetry platform. The TBR, SUV, and MRI tumor volumes ranged from 2.3-63.9 cc, 0.1-34.7 cc, and 0.4-11.8 cc, respectively. The average  ±  standard deviation (range) was 0.19  ±  0.13 (0.01-0.51), 0.30  ±  0.17 (0.03-0.67), and 0.75  ±  0.29 (0.05-1.00) for the JSC, DSC, and OV, respectively. The DSC and JSC values were small and the OV values were large for both the MRI-SUV and MRI-TBR combinations because the regions of PET uptake were generally larger than the MRI enhancement. Notable differences in the tumor dose volume histograms were observed for each patient. The mean (standard deviation) ¹³¹I-CLR1404 tumor doses ranged from 0.28-1.75 Gy GBq^-1 (0.07-0.37 Gy GBq^-1). The ratio of maximum-to-minimum mean doses for each patient ranged from 1.4-2.0. The tumor volume and the interpretation of the tumor dose is highly sensitive to the imaging modality, PET enhancement metric, and threshold level used for tumor volume segmentation. The large variations in tumor doses clearly demonstrate the need for standard protocols for multimodality tumor segmentation in TRT dosimetry.

Radioiodine for High Risk and Radioiodine Refractory Thyroid Cancer: Current Concepts in Management

The management of early stage differentiated thyroid cancer (DTC) with low risk of recurrence has been the subject of much interest and investigation in the recent years. Locally advanced DTC and patients with a high risk of recurrent disease however needs further investigation. This short review will look at what constitutes high risk thyroid cancer, the definition of radioiodine refractory disease, the current management and areas of debate within this clinical setting.

Ten Year Experience of Radioiodine Dosimetry: is it Useful in the Management of Metastatic Differentiated Thyroid Cancer?

AIMS

When a fixed activity of radioiodine is given for differentiated thyroid cancer (DTC), absorbed doses of radioiodine can vary widely and are not usually measured. Leeds Cancer Centre has routinely used a form of lesion-specific dosimetry for radioiodine patients. This study investigated if the results of dosimetry influenced treatment decisions for patients with advanced DTC.

MATERIALS AND METHODS

Since 2005, patients with regionally advanced/metastatic DTC, who underwent radioiodine treatment together with dosimetry, were included in this study. Patients were excluded if their radioiodine post-treatment scan showed no abnormal uptake. Dosimetry was calculated using images taken 2, 3 and 7 days post-radioiodine. Regions of interest were drawn around lesions that required dosimetry and a time-dose activity curve was created. The total cumulative activity was equal to the area under the curve. Each patient's results were prospectively assessed by their oncologist regarding the usefulness of dosimetry in making management decisions.

RESULTS

Thirty patients were studied and underwent 102 admissions of radioiodine between them. Dosimetry was carried out during 83 of 102 admissions. An absorbed dose of >20 Gy was taken as significant from dosimetry calculations, following which further radioiodine was considered. In 80% of patients, dosimetry was found to be useful when making treatment decisions. Only on 1/19 admissions did dosimetry calculate a minimum dose above 20 Gy in patients who had a total of four or more admissions for radioiodine. Ten per cent (3/30) had a complete response to radioiodine, both biochemically and radiologically, with a median follow-up of 6.7 months. Thirty-three per cent had a partial response/stable disease to radioiodine. The remainder had progressive disease. The decision to discontinue radioiodine therapy was often based on dosimetry and thyroglobulin results. Dosimetry was very useful for patients with thyroglobulin antibodies.

CONCLUSION

Only 10% had a complete response. Therefore, a significant number of patients became refractory to radioiodine during a course of repeat admissions for treatment. Dosimetry (often together with thyroglobulin and anatomical scans) helped to identify these patients to avoid further futile radioiodine therapy.

Whole-remnant and maximum-voxel SPECT/CT dosimetry in 131 I-NaI treatments of differentiated thyroid cancer

PURPOSE

To investigate the possible differences between SPECT/CT based whole-remnant and maximum-voxel dosimetry in patients receiving radio-iodine ablation treatment of differentiated thyroid cancer (DTC).

METHODS

Eighteen DTC patients were administered 1.11 GBq of 131 I-NaI after near-total thyroidectomy and rhTSH stimulation. Two patients had two remnants, so in total dosimetry was performed for 20 sites. Three SPECT/CT scans were performed for each patient at 1, 2, and 3-7 days after administration. The activity, the remnant mass, and the maximum-voxel activity were determined from these images and from a recovery-coefficient curve derived from experimental phantom measurements. The cumulated activity was estimated using trapezoidal-exponential integration. Finally, the absorbed dose was calculated using S-values for unit-density spheres in whole-remnant dosimetry and S-values for voxels in maximum-voxel dosimetry.

RESULTS

The mean absorbed dose obtained from whole-remnant dosimetry was 40 Gy (range 2-176 Gy) and from maximum-voxel dosimetry 34 Gy (range 2-145 Gy). For any given patient, the activity concentrations for each of the three time-points were approximately the same for the two methods. The effective half-lives varied (R = 0.865), mainly due to discrepancies in estimation of the longer effective half-lives. On average, absorbed doses obtained from whole-remnant dosimetry were 1.2 ± 0.2 (1 SD) higher than for maximum-voxel dosimetry, mainly due to differences in theS-values. The method-related differences were however small in comparison to the wide range of absorbed doses obtained in patients.

CONCLUSIONS

Simple and consistent procedures for SPECT/CT based whole-volume and maximum-voxel dosimetry have been described, both based on experimentally determined recovery coefficients. Generally the results from the two approaches are consistent, although there is a small, systematic difference in the absorbed dose due to differences in the S-values, and some variability due to differences in the estimated effective half-lives, especially when the effective half-life is long. Irrespective of the method used, the patient absorbed doses obtained span over two orders of magnitude.

Thyroid cancer: radiation safety precautions in 131I therapy based on actual biokinetic measurements

PURPOSE

To formulate radiation precautions for patients with thyroid cancer who are undergoing thyroid hormone withdrawal-induced hypothyroidism and iodine 131 ((131)I) therapy through actual biokinetic measurements.

MATERIALS AND METHODS

Informed consent and institutional review board approval were obtained. From October 2008 to December 2011, consecutive patients with differentiated thyroid cancer who had been prepared for (131)I ablation treatment or (131)I treatment for metastatic disease during follow-up were prospectively recruited. Calculations based on deduced whole-body retention and measured iodine biokinetics in thyroidal tissue were derived to determine the thyroidal and extrathyroidal compartment uptake fractions and effective half-lives. Precaution times necessary to avoid close contact with family members and the general public were derived from these parameters and regulatory dose limits.

RESULTS

Seventy-seven patients (36 with ablation treatments, 41 with follow-up treatments) were eligible for the analysis. Actual dose rates from patients after therapeutic (131)I administration were greatly lower than those described in the American Thyroid Association (ATA) and Nuclear Regulatory Commission (NRC) models: The mean initial dose rate at 0.3 m for patients with ablation treatment was only 28% (0.183/0.655 µSv/h/MBq) ± 2.9 (standard deviation) (range, 12.1%-38.3%) and 36% (0.183/0.511 µSv/h/MBq) ± 3.7 (range, 15.5%-49.1%) of that described in the NRC and ATA models, respectively; the equivalent values for patients with follow-up treatment were only 30% (0.195/0.655 µSv/h/MBq) ± 3.5 (range, 12.5%-45.3%) and 38% (0.195/0.511 µSv/h/MBq) ± 4.5 (range, 16.0%-58.1%), respectively. The actual mean effective (131)I half-life in the thyroid remnant tissue was greatly lower than that described in the ATA and NRC models: 47.6 versus 175.2 hours.

CONCLUSION

On the basis of the current dose limits, typically administered activities of 3.7 GBq to a patient with ablation treatment or 7.4 GBq to a patient with follow-up treatment required 3 days of sleeping apart for keeping the doses to pregnant women and children below 1 mSv. No precautions were required for non-cosleeping nonpregnant adult family members.

Radioiodine: the classic theranostic agent

Radioiodine has the distinction of being the first theranostic agent in our armamentarium. Millennia were required to discover that the agent in orally administered seaweed and its extracts, which had been shown to cure neck swelling due to thyromegaly, was iodine, first demonstrated to be a new element in 1813. Treatment of goiter with iodine began at once, but its prophylactic value to prevent a common form of goiter took another century. After Enrico Fermi produced the first radioiodine, (128)I, in 1934, active experimentation in the United States and France delineated the crucial role of iodine in thyroid metabolism and disease. (130)I and (131)I were first employed to treat thyrotoxicosis by 1941, and thyroid cancer in 1943. After World War II, (131)I became widely available at a reasonable price for diagnostic testing and therapy. The rectilinear scanner of Cassen and Curtis (Science 1949;110:94-95), and a dedicated gamma camera invented by Anger (Nature 1952;170:200-201), finally permitted the diagnostic imaging of thyroid disease, with (131)I again the radioisotope of choice, although there were short-lived attempts to employ (125)I and (132)I for this purpose. (123)I was first produced in 1949 but did not become widely available until about 1982, 10 years after a production technique eliminated high-energy (124)I contamination. I continues to be the radioiodine of choice for the diagnosis of benign thyroid disease, whereas (123)I and (131)I are employed in the staging and detection of functioning thyroid cancer. (124)I, a positron emitter, can produce excellent anatomically correlated images employing positron emission tomography/computed tomography equipment and has the potential to enhance heretofore imperfect dosimetric studies in determining the appropriate administered activity to ablate/treat thyroid cancer. Issues of acceptable measuring error in thyroid cancer dosimetry and the role in (131)I therapy of tumor heterogeneity, tumor hypoxia, and kinetics must be overcome, and long-term outcome studies following (131)I given based on this new dosimetry must be completed before the nuclear medicine community will be able to predictably cure our thyroid cancer patients with this technology.

The role of postoperative external-beam radiotherapy in the management of patients with papillary thyroid cancer invading the trachea

PURPOSE

To determine the role of adjuvant external-beam radiotherapy (EBRT) in the management of patients with well-differentiated thyroid cancer invading the trachea.

METHODS AND MATERIALS

Of 1,098 thyroid cancer patients, 68 (6%) were found to have tracheal invasion, and they all received "shave" excision of the tracheal cartilage. Among them, 12 patients had no postoperative residuum, 43 patients had microscopic residuum, and 13 patients had macroscopic residuum. All patients were divided into two groups according to treatment modality with or without EBRT; (1) the control group (n = 43) and (2) the EBRT group (n = 25).

RESULTS

The locoregional recurrence rate for EBRT patients was much lower than that of control patients (51% for the control group vs. 8% for the EBRT group) (p < 0.01). The 10-year local progression-free survival rate for the EBRT group was significantly better than that of the control group (89% in the EBRT group vs. 38% in the control group) (log-rank, p < 0.01). The use of adjuvant EBRT after conservative surgery was an independent prognostic factor in univariate and multivariate analyses.

CONCLUSIONS

External-beam radiotherapy was found to be effective, particularly in patients with thyroid cancer invading the trachea with microscopic or gross residuum after conservative surgery.

Treatment of advanced differentiated thyroid carcinoma with high activity radioiodine therapy

OBJECTIVES

This was a retrospective study to assess the efficacy and morbidity of high activity I therapy in patients with advanced differentiated thyroid carcinoma.

METHODS

From 1975 to 2003, 38 patients with locally advanced or metastatic differentiated thyroid cancer (16 follicular, 20 papillary, one Hurthle cell, one insular) were treated with high activity radioiodine therapy (9 GBq) as the cancers had previously not responded to standard activities (5.5 GBq).

RESULTS

Cumulative total activities received ranged from 11.8 to 84.5 GBq (mean 29.4 GBq per patient). Staging at presentation showed pT4 and/or M1 disease in 27/38 of patients (71.1%). Moderate (grade 2) and poorly differentiated (grade 3) tumours were present in a total of 9/38 patients (23.7%). Outcomes were evaluated according to the results of I whole-body scans, serum thyroglobulin, radiological assessments and physical examination. Neither [18F]flurodeoxyglucose positron emission tomography (F-FDG PET) nor 99mTc sestamibi were available during this study. The mean duration of follow-up was 83 months. A complete response was observed in 7/38 patients (18.4%), progressive disease in 27/38 (71.1%) and stable disease in 4/38 (10.5%). The mean survival from initiation of high activity treatment was 36.6 months. For patients with lung disease the mean survival was 45 months, neck disease 38.9 months, bone disease 35 months and multiple sites was 30.9 months. Twelve patients died during follow-up (10 due to thyroid carcinoma). After high activity treatment, 9.7% of patients suffered grade 3 and 3.2% suffered grade 4 WHO haematological toxicity. Significant salivary gland morbidity was observed (30% dry mouth, 27% salivary swelling).

CONCLUSIONS

Repeated treatment with high activity (9 GBq) I in patients with advanced differentiated thyroid carcinoma appears to be of no apparent benefit and may lead to late morbidity.

Management of thyroid cancer

Thyroid cancers are a diverse group of malignant disorders ranging from indolent micropapillary carcinoma, which has no effect on life expectancy, to anaplastic tumours, which are invariably fatal even with aggressive treatment. Although the estimated incidence has increased by 14.6% over the past 40 years, the estimated death rate has fallen by 21%, probably as a result of earlier diagnosis. The natural history of thyroid tumours is no longer a mystery, and the prognostic factors identified can predict outcome fairly accurately. Improvements in management have mostly depended on information from large retrospective series, though there are still many areas open to debate. There has been, however, a general acceptance that thyroid cancer should be managed by multidisciplinary teams in specialised units following evidence-based guidelines.

Improvements in target coverage and reduced spinal cord irradiation using intensity-modulated radiotherapy (IMRT) in patients with carcinoma of the thyroid gland

BACKGROUND AND PURPOSE

External beam radiotherapy for thyroid carcinoma poses a significant technical challenge as the target volume lies close to or surrounds the spinal cord. The potential of intensity-modulated radiotherapy (IMRT) to improve the dose distributions was investigated.

MATERIALS AND METHODS

A planning study was performed on patients with thyroid carcinoma. Plans were generated to irradiate the thyroid bed alone or to treat the thyroid bed and the loco-regional lymph nodes in two phases. Conventional plans with minimal beam shaping were compared to three-dimensional conformal radiotherapy (3DCRT) and inverse-planned IMRT plans to assess target coverage and normal tissue sparing. IMRT techniques were optimized to find the minimum number of equispaced beams required to achieve the clinical benefit and a concomitant boost technique was explored.

RESULTS

For the thyroid bed alone and the thyroid bed plus loco-regional lymph nodes, conventional and conformal techniques produced low minimum doses to the planning target volume (PTV) if spinal cord tolerance was respected. 3DCRT reduced the irradiated volume of normal tissue (P=0.01). IMRT plans achieved the goal dose to the PTV (P<0.01) and also reduced the spinal cord maximum dose (P<0.01). IMRT, using a concomitant boost technique, produced better target coverage than a two-phase technique. For both the two-phase and concomitant boost techniques, IMRT plans with seven and five equispaced fields produced similar dose distributions to nine fields, but three fields were significantly worse.

CONCLUSIONS

3DCRT reduced normal tissue irradiation compared to conventional techniques, but did not improve PTV or spinal cord doses. IMRT improved the PTV coverage and reduced the spinal cord dose. A simultaneous integrated boost technique with five equispaced fields produced the best dose distribution. IMRT should reduce the risk of myelopathy or may allow dose escalation in patients with thyroid cancer.

Glucose metabolism of the thyroid in Graves' disease measured by F-18-fluoro-deoxyglucose positron emission tomography

The radiolabeled glucose surrogate F-18-fluoro-deoxyglucose (F-18-FDG) and positron emission tomography (PET) were used to measure glucose metabolism of the thyroid in vivo. We evaluated patients with Graves' disease before therapy with radioiodine in comparison to patients with normal thyroids. Thirty-six patients with Graves' disease underwent scanning the day before radioiodine therapy. Twenty patients with head tumors and normal thyroids were the controls. Overall F-18-FDG uptake was determined for all thyroids and modeling of glucose metabolism was performed in order to differentiate between glucose concentration in the fractional blood volume, glucose transport, and glucose utilization. F-18-FDG uptake was significantly higher in Graves' disease patients compared with controls. In these patients F-18-FDG uptake increased with increasing antithyroid antibodies and shorter radioiodine half-life. Modeling of glucose metabolism revealed substantial differences in thyroid F-18-FDG utilization constants (k3 values) corresponding to enhanced local metabolic rates in Graves' disease. No significant differences in the remaining rate constants and the fractional blood volume were detected. These results indicate that glucose metabolism is enhanced in the thyroid of Graves' disease patients not only due to enhanced fractional blood volume but to enhanced utilization. Whether a lymphocytic infiltration or thyroid epithelial cells utilize this surplus of glucose cannot be determined using in vivo PET measurements in humans. Still, the correlation of radioiodine half-life and glucose hypermetabolism suggests direct or nondirect connections of glucose metabolism and hormone synthesis in thyroid cells.

The effects of radioactive iodine in thyroid remnant ablation and treatment of well differentiated thyroid carcinoma

Although the use of radioactive iodine (131I) in the treatment of thyroid cancer is well established, treatment dose is not well standardized. In order to deduce the appropriate dose for thyroid remnant ablation and the effect of 131I in the treatment of distant metastases, data for 544 patients with papillary or follicular thyroid cancer were retrospectively reviewed. All patients received surgical treatment followed by post-operative 131I. If remnants were present in the 0.2 GBq 131I diagnostic scan, 1.1-3.7 GBq 131I were administered for ablation. For the treatment of distant metastases 3.7-5.6 GBq were used. Of 318 patients receiving 131I for thyroid remnant ablation, 290 were successfully ablated. After one dose of 1.1 GBq 131I, 82% (159/194) of thyroid remnants were ablated. During the follow-up period, two of 14 Stage IV patients with lung or mediastinal metastases at the time of operation achieved complete clinical remission. Factors identified as influencing response to 131I therapy included age, clinical stage, survival, recurrence, extent of surgery and the 1 month post-operative serum thyroglobulin (Tg) level. In conclusion 1.1 GBq 131I was adequate for thyroid remnant ablation unless distant metastases were present. Radioactive 131I has a role in the treatment of well differentiated thyroid carcinoma with pulmonary metastases but seems to be less effective for treatment of bone metastases.

Treatment of micronodular lung metastases of papillary thyroid cancer: are the tumors too small for effective irradiation from radioiodine?

Our purpose was to determine if micronodular lung metastases from papillary thyroid cancer had diameters that were less than 1 mm and therefore of a size for which irradiation by radioiodine (131I) is inefficient. In five patients, lung metastases seen on computed tomography (CT) were enumerated and sized in the entire right lung and right upper lung giving volumes of measurable, ie, more than 1 mm diameter, tumors. Concentrations of diagnostic 131I were quantified scintigraphically in the same regions. Fractions of administered 131I per milliliter of tumor and the absorbed radiation from the subsequent treatments were calculated to see if the 131I levels in lungs were greater than expected for the visible tumor volumes. Two other patients manifesting similar findings had lung biopsies that were reviewed for size of metastases. The calculated fractions of administered activity per milliliter of tumor and the absorbed radiations from the treatments with 131I were exceptionally high. Biopsies revealed numerous tumors below the resolution of CT. We conclude that the fractions of administered activity and absorbed radiations of 131I in tumors were high because the measured tumor volumes underestimated the total tumor volumes. Many lung metastases were less than 1 mm in diameter.

The changing role of radioiodine in the management of differentiated thyroid cancer

This article discusses several aspects of the evaluation and management of differentiated thyroid carcinoma that are changing or may change in the near future. Although conventional treatment of this disease is highly effective, some modification may improve the welfare of patients and the overall results. Because the symptoms of hypothyroidism are vexing, there has been great interest in using recombinant human thyroid-stimulating hormone (rhTSH) to prepare patients for iodine 131 imaging. rhTSH has been about as effective as thyroid hormone withdrawal for diagnostic imaging so that approval for this use is expected. Another topic of interest is the administration of 131I therapy to patients whose serum thyroglobulin levels are abnormal but whose diagnostic 131I scans are negative. Because the 131I scans after therapy are often abnormal in these patients and a reduction of serum thyroglobulin can occur, this approach seems effective. The long-term impact of this therapy on recurrence and survival, however, is unknown. A third issue that is currently under review is the amount of 131I that should be used for diagnostic scanning. Although past opinion favored larger doses, "stunning" of thyroid remnant and tumor can occur with diagnostic 131I imaging. Substituting iodine 123 is an alternative for postthyroidectomy scanning, but when administered as 300 uCi it is less accurate than 131I for recurrent disease or distant metastases. Related to these issues, two other topics are reviewed: the use of other radiopharmaceuticals for imaging patients with thyroid cancer, and 131I dosimetry.

131-I treatment of micronodular pulmonary metastases from papillary thyroid carcinoma

BACKGROUND

Pulmonary metastases from papillary thyroid carcinoma shorten the survival of the hosts. Treatments with 131-I have been reported to induce disappearance of these tumors in a large proportion of afflicted patients. In this study, consecutive patients with diffuse micronodular lung metastases from papillary thyroid carcinoma were examined to determine if disappearance of tumor occurred, and how much disappeared, after substantial amounts of 131-I were administered.

METHODS

Of 232 patients treated with 131-I for thyroid carcinoma between 1985 and 1994, 12 patients between the ages of 5 and 45 years exhibited evidence of micronodular metastases to the lungs that concentrated 131-I. Each patient had undergone total or nearly total thyroidectomy and cervical lymph node dissection. All neoplasms were well differentiated papillary carcinoma, but one also had focal, poorly differentiated insular components. Follicle formation by tumors varied from less than 10% to 100% of the histologic sections. Effects of treatment were measured by three indices: chest X-ray and/or CT images, scintigraphic images, and serum thyroglobulin levels. Individual activities of 131-I ranged from 2.2 gigabequerel (GBq) (initial activity in the 5-year-old patient) to 13 GBq, and were greater than 7.4 GBq in 6 patients. Only one treatment was given to three patients, two were given to seven, and more than two were given to two. The duration of follow-up was at least one year.

RESULTS

In two patients, the only evidence of lung metastases was on scintigraphic images made a few days after treatment. Another patient had a normal X-ray but showed diffuse uptake of 131-I in the lungs on a diagnostic scintiscan. Of the nine patients with abnormal X-ray and CT images, seven showed improvement, but tumors disappeared in only two. In the ten patients with abnormalities on the diagnostic scintiscans, five eventually manifested no abnormality. At the outset, thyroglobulin levels exceeded 10 ng/mL in each patient; 3 individuals exhibited a decline in level by 25% or more, and a value of less than 6 ng/mL, uncomplicated by thyroglobulin antibodies, was seen in two patients. Only two patients attained normality in all three indices. Hematologic toxicity was modest and reversible.

CONCLUSIONS

Despite a number of previous reports that pulmonary metastases from thyroid carcinoma disappear in approximately half of patients treated with 131-I, evidence of tumor reduction was found in most, but a complete remission occurred in only 2 of 12 patients. Nevertheless, 131-I therapy may be useful to decrease the tumor burden in many such patients.

Imaging technologies for radionuclide dosimetry

Targeted radionuclide therapy is becoming an increasingly popular treatment modality as an alternative or as an adjunct to external beam radiotherapy and chemotherapy. The present method of dosimetry based on the MIRD system requires measurements of the concentration of the radionuclide in the target and risk tissues and the effective half-life of the radionuclide in these tissues. Radionuclide imaging techniques including planar scintigraphy, rectilinear scanning, single-photon emission computed tomography and positron emission tomography have all been used to provide data from which this information can be obtained. Additionally anatomical imaging has been used to aid these estimates. This paper reviews the application of imaging technology and methodology to radionuclide dosimetry.

Imaging in thyroid cancer

No ideal imaging method is available for the diagnosis of MTC. Once the diagnosis is suspected, serum calcitonin or CEA may help to confirm it. Often, multiple studies are done, especially a combination of MR imaging or ultrasound and an isotopic method. Tc(V)DMSA is an excellent agent but not readily available in the United States. Indium-labeled pentreotide is probably the imaging agent of choice in the United States.

Brain metastases in patients with differentiated thyroid carcinoma

We present a retrospective review of six patients who developed brain metastases in our series of 649 with differentiated thyroid carcinoma seen at the Royal Marsden Hospital between 1936 and 1991. Prognosis was poor, with survival 1-19 months from the diagnosis of brain metastases in five patients. One patient remains alive at 18 months. A dosimetric approach may help to individualize and optimize treatment.

Double-tracer dosimetry of organs in assessment of bone marrow involvement by two monoclonal antibodies

Five patients with ductal breast cancer were studied using simultaneous administration of 99Tcm-labelled BW250/183 and 131I-labelled B72.3 monoclonal antibodies (MAbs). The distribution and dosimetry of these tracers were evaluated using the information from simultaneous anterior and posterior whole body scintigrams, together with 99Tcm and 131I standard activity sources, recorded on an average of 1, 4, 24, 90 and 224 h after injection. A method to eliminate 131I Scatter on 99Tcm-channel was developed. The geometric means of conjugate views and region-of-interest analysis were used to determine organ uptakes, mean residence times and absorbed radiation dose estimates of organs induced by the tracers. Organ uptakes (% of injected activity/ml) varied from 6.2 x 10(-3 /red marrow to 3.1 X 10(-2)/liver for 99Tcm-MAb and from 3.1 x 10(-2)/red marrow to 1.8 x 10(-1)/liver for 131I-MAb, one hour after injection. Calculated average residence times of organs for 99Tcm-labelled BW250/183 were in the range of physical mean-life of 99Tcm and from 71 to 95 h for 131I-B72.3 respectively. The average total absorbed dose from 99Tcm-MAb to the bone marrow was 0.01 and to the spleen 0.14 mGy/MBq and from 131I-MAb the corresponding values were 0.48 and 10.76 mGy/MBq. This double-tracer technique provides information from two antibodies having different kinetic behaviour and may facilitate in distinguishing various antigens in targeting and control MAb applications.

Results of external beam radiotherapy in differentiated thyroid carcinoma: a retrospective study from the Royal Marsden Hospital

Between 1969 and 1991, 113 patients with differentiated thyroid carcinoma (follicular and papillary) received radical dose megavoltage external beam radiotherapy. There were 70 females and 43 males, mean age 53 years (range 11-84). Radiotherapy was delivered to both sides of the neck and superior mediastinum, using either megavoltage photons via anterior and posterior portals, delivering a 60 Gy mid-plane dose in 30 fractions and treating daily over 6 weeks (with spinal cord shielding from the posterior field after 40 Gy), or matched 20 MeV and 35 MeV electron beams (to the neck and superior mediastinum, respectively) delivering a 75 Gy applied dose in 30 daily fractions. All patients received suppressive thyroid hormone and 74 radioiodine. Local recurrence, mostly within field, occurred in 19% of 53 patients with probable and definite residual microscopic disease (both follicular and papillary histologies). For gross residual disease (both follicular and papillary) in 49 patients, complete regression was obtained in 37.5%, partial regression in 25% and no regression in 37.5%. Median follow-up from diagnosis was 49 months (range 3-335). Overall 5-year survival rates were 85% for residual microscopic disease but only 27% for gross disease. 61 patients have died. Nineteen deaths were due to unrelated causes, 15 to distant metastases, 15 to uncontrolled local disease and 12 died with both local and distant tumours.