RESUMO
BACKGROUND: An increase in the incidence of second primary cancers is the late effect of greatest concern that could occur in differentiated thyroid carcinoma (DTC) patients treated with radioactive iodine (RAI). The decision to treat a patient with RAI should therefore incorporate a careful risk-benefit analysis. The objective of this work was to adapt the risk-estimation models developed by the Biological Effects of Ionizing Radiation Committee to local epidemiological characteristics in order to assess the carcinogenesis risk from radiation in a population of Brazilian DTC patients treated with RAI. Absorbed radiation doses in critical organs were also estimated to determine whether they exceeded the thresholds for deterministic effects. METHODS: A total of 416 DTC patients treated with RAI were retrospectively studied. Four organs were selected for absorbed dose estimation and subsequent calculation of carcinogenic risk: the kidney, stomach, salivary glands, and bone marrow. Absorbed doses were calculated by dose factors (absorbed dose per unit activity administered) previously established and based on standard human models. The lifetime attributable risk (LAR) of incidence of cancer as a function of age, sex, and organ-specific dose was estimated, relating it to the activity of RAI administered in the initial treatment. RESULTS: The salivary glands received the greatest absorbed doses of radiation, followed by the stomach, kidney, and bone marrow. None of these, however, surpassed the threshold for deterministic effects for a single administration of RAI. Younger patients received the same level of absorbed dose in the critical organs as older patients did. The lifetime attributable risk for stomach cancer incidence was by far the highest, followed in descending order by salivary-gland cancer, leukemia, and kidney cancer. CONCLUSION: RAI in a single administration is safe in terms of deterministic effects because even high-administered activities do not result in absorbed doses that exceed the thresholds for significant tissue reactions. The Biological Effects of Ionizing Radiation Committee mathematical models are a practical method of quantifying the risks of a second primary cancer, demonstrating a marked decrease in risk for younger patients with the administration of lower RAI activities and suggesting that only the smallest activities necessary to promote an effective ablation should be administered in low-risk DTC patients.
Assuntos
Adenocarcinoma Folicular/radioterapia , Adenoma Oxífilo/radioterapia , Carcinoma Papilar/radioterapia , Radioisótopos do Iodo/uso terapêutico , Neoplasias Induzidas por Radiação/epidemiologia , Segunda Neoplasia Primária/epidemiologia , Neoplasias da Glândula Tireoide/radioterapia , Adulto , Neoplasias da Medula Óssea/epidemiologia , Brasil/epidemiologia , Feminino , Humanos , Incidência , Neoplasias Renais/epidemiologia , Leucemia/epidemiologia , Masculino , Pessoa de Meia-Idade , Doses de Radiação , Radiometria , Estudos Retrospectivos , Medição de Risco , Neoplasias das Glândulas Salivares/epidemiologia , Neoplasias Gástricas/epidemiologia , Câncer Papilífero da TireoideRESUMO
A 46 year-old male presented with persistently high level of serum parathyroid hormone (PTH), despite successful resection of an oxyphilic cell parathyroid adenoma of the left lower gland. Renal function and serum calcium were normal, leading to vitamin D deficiency being considered. Tc99m-sestamibi parathyroid scintigraphy showed no capitation, but a cervical ultrasound demonstrated an increase in the lower parathyroids. Surgery confirmed that the right gland was normal but the left corresponded to parathyroid carcinoma. The patient developed severe hypocalcemia, with PTH values being consistent with hypoparathyroidism for a few months. However, a progressive increase in calcium and PTH serum levels indicated recurrence of disease. Tc99m-sestamibi scintigraphy demonstrated hyperfixation in topography of the left inferior parathyroid and the patient was subjected to a third and more extensive surgery, with removal of lymph nodes and adjacent thyroid tissue. Serum calcium and PTH remained elevated, requiring loop diuretics and intravenous bisphosphonates to control hypercalcemia. Cervical radiotherapy was implemented as adjuvant therapy. After two months the patient complained of dyspnea, and a CT scan of the chest demonstrated areas of parenchymal condensation, suggestive of actinic pneumonitis. At the 2-year follow-up no major issues were evident.