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

The benefits and risks of I-131 therapy in patients with well-differentiated thyroid cancer

BACKGROUND

I-131 has been used in the therapy of well-differentiated thyroid cancer for over 50 years. Although the benefits and risks of I-131 remain issues of controversy and research, our understanding of them continues to improve. This review presents an overview of the benefits of I-131 therapy for ablation, adjuvant treatment, and treatment of locoregional and/or metastasis of well-differentiated thyroid cancer and considers the risks of complications of I-131 therapy.

SUMMARY

The benefits of I-131 remnant ablation include: [1] facilitating the interpretation of subsequent serum thyroglobulin levels, [2] increasing the sensitivity of detection of locoregional and/or metastatic disease on subsequent follow-up radioactive iodine whole-body scans, [3] maximizing the therapeutic effect of subsequent treatments, and [4] allowing a postablation scan to help identify additional sites of disease that were not identified on the preablation scan or when a preablation scan was not performed. The potential benefits of I-131 adjuvant treatment include decreasing recurrence and disease-specific mortality for unknown microscopic, locoregional, and/or distant metastatic disease. The potential benefits of I-131 treatment of known locoregional and/or distant metastases are [1] decreasing recurrence, and [2] decreasing disease-specific mortality and/or palliation. The more significant risks and side effects involve organ systems including eye/nasolacrimal, salivary, pulmonary, gastrointestinal, hematopoietic, and gonads as well as secondary primary malignancies.

CONCLUSIONS

Although there are never-ending controversies regarding I-131 therapy in well-differentiated thyroid cancer, the benefits and risks are becoming better understood. This in turn helps the treating physician and patient in making decisions regarding therapy.

Theoretical study of the influence of a heterogeneous activity distribution on intratumoral absorbed dose distribution

Radioimmunotherapy of hematopoeitic cancers and micrometastases has been shown to have significant therapeutic benefit. The treatment of solid tumors with radionuclide therapy has been less successful. Previous investigations of intratumoral activity distribution and studies on intratumoral drug delivery suggest that a probable reason for the disappointing results in solid tumor treatment is nonuniform intratumoral distribution coupled with restricted intratumoral drug penetrance, thus inhibiting antineoplastic agents from reaching the tumor's center. This paper describes a nonuniform intratumoral activity distribution identified by limited radiolabeled tracer diffusion from tumor surface to tumor center. This activity was simulated using techniques that allowed the absorbed dose distributions to be estimated using different intratumoral diffusion capabilities and calculated for tumors of varying diameters. The influences of these absorbed dose distributions on solid tumor radionuclide therapy are also discussed. The absorbed dose distribution was calculated using the dose point kernel method that provided for the application of a three-dimensional (3D) convolution between a dose rate kernel function and an activity distribution function. These functions were incorporated into 3D matrices with voxels measuring 0.10 x 0.10 x 0.10 mm3. At this point fast Fourier transform (FFT) and multiplication in frequency domain followed by inverse FFT (iFFT) were used to effect this phase of the dose calculation process. The absorbed dose distribution for tumors of 1, 3, 5, 10, and 15 mm in diameter were studied. Using the therapeutic radionuclides of 131I, 186Re, 188Re, and 90Y, the total average dose, center dose, and surface dose for each of the different tumor diameters were reported. The absorbed dose in the nearby normal tissue was also evaluated. When the tumor diameters exceed 15 mm, a much lower tumor center dose is delivered compared with tumors between 3 and 5 mm in diameter. Based on these findings, the use of higher beta-energy radionuclides, such as 188Re and 90Y is more effective in delivering a higher absorbed dose to the tumor center at tumor diameters around 10 mm.

Treatment of malignant pheochromocytomas with 131-I metaiodobenzylguanidine and chemotherapy

Malignant pheochromocytomas have exhibited partial responses to treatments with 131-I metaiodobenzylguanidine (MIBG) and with chemotherapy. The authors combined these two therapeutic methods to determine if beneficial effects from each would be additive. Patients with documented malignant pheochromocytomas were recruited with the intent of administering 131-I MIBG in three substantial amounts of radioactivity at 3-month intervals followed by a year of chemotherapy in which cyclophosphamide, dacarbazine, and vincristine were to be given in 21-day cycles. Six patients entered the protocol. After the 131-I MIBG treatments, three patients manifested declines in the presence of tumor (smaller tumor volume or abnormalities on bone and 131-I MIBG scans) and the function of tumor (decreased rate of normetanephrine excretion as the major index). Two patients completed at least 9 months of chemotherapy and showed further reductions in the presence and function of tumors and were classified as having partial responses. Progressive disease afflicted three of the other four subjects. Even though toxicity was minimal from 131-I MIBG, it was sufficient to force reduction in the dosages or duration of chemotherapy. A combination of 131-I MIBG treatments and chemotherapy produced additive effects in reducing malignant pheochromocytomas. Toxicity moderately curtailed the proposed chemotherapy protocol.