Otimizacao da dose terapeutica com 131I para carcinoma diferenciado da tireoidee

AUTOR(ES)
DATA DE PUBLICAÇÃO

2002

RESUMO

I-131 thyroid cancer therapy is based on the strategy of concentrating radioactive iodine in the thyroid tissue, to completely eliminate thyroid tissue and functioning thyroid cancer metastases remaining after thyroidectomy. In Brazil, fixed activities of 131 I generally are given, sometimes either delivering insufficient activities to ablate all of the remnants, or unnecessarily high activities, with patients remaining in the hospital for some period of time. This investigation proposes a protocol of individualized planning of ablative doses, based on individual patients metabolisms and measured thyroid remnant masses. Simulated thyroid remnants were fabricated in various forms, volumes and activities, and optimum image acquisition parameters were determined using Single- Photon Emission Computed Tomography (SPECT). Resultant images were evaluated, to determine the apparent volumes and the 131I concentrations. I-131 metabolism was studied in 9 patients who had undergone thyroidectomies. Their thyroid remnant masses were determined applying the same parameters used in SPECT simulation studies, and the optimum activity for their therapy was calculated and compared to the established fixed activity of 3.7 GBq (100 mCi), which would have normally been assigned. Background subtraction using the method of percent maximum counts, using a value of 67.5%, combined with scatter correction (triple energy window method), was shown to be optimum for SPECT quantification of volumes between 3-10 ml. Errors in the method were below 9% for sources with regular geometries and around 11% for sources with irregular geometries. In the patient studies, it was observed that 78% of patients could have received reduced activities of 131I (from 0.8-3.2 GBq (20-87 mCi)). In addition, 33% of these patients could have received low enough activities to have been discharged from the hospital, using an individualized administration scheme. This could have resulted in a cost savings for the institution, and perhaps an improvement in the psychological conditions for the patients, in being able to be at home during the course of therapy instead of in the hospital. This could also have resulted in a dose reduction for many organs, such as the marrow and gonads, of up to 78.4%. Possible benefits to the institution also include the use of less radioactive material and a reduction in radiation exposures to the staff during the manipulation and administration of the 131I. To facilitate the calculations of the optimum therapeutic activity of 131I for individual patients, a simple and fast dose planning program was created (PlanDose). The program has been set up to evaluate thyroid remnant ablation, but it can also be used for the calculation of the activity to be administered for treatment of hyperthyroidism. This protocol of calc ulated optimal patient-specific 131I activities allows a better determination of the necessary ablative dose for patients with differentiated carcinoma of the thyroid, and is an example of optimizing the practice of radiation protection

ASSUNTO(S)

tecido tiroideano engenharia nuclear iodo 131 - medicina nuclear

ACESSO AO ARTIGO

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