Measurements of the dose delivered during CT exams using AAPM Task Group Report No. 111.

The computed tomography dose index (CTDI) measured with a 10 cm long pencil ionization chamber placed in a 14 cm long PMMA phantom is typically used to evaluate the doses delivered during CT procedure. For the new generation of CT scanners, the efficiency of this methodology is low because it excludes the contribution of radiation scattered beyond the 100 mm range of integration along z. The AAPM TG111 Report proposes a new measurement modality using a small volume ionization chamber positioned in a phantom long enough to establish dose equilibrium at the location of the chamber. In this work, the AAPM report was implemented. The minimum scanning length needed to obtain cumulative dose equilibrium was evaluated. The equilibrium dose was determined and compared to CTDI values informed by the CT scanner, and the dose values were confirmed with TLD measurements. The difference between doses measured with TLD and with the ionization chamber (IC) was below 1% and the repeatability of the measurements' setup was 0.4%. The measurements showed that the scanning lengths needed to reach the cumulated dose equilibrium were 450 mm and 380 mm for the central and peripheral axes, respectively, which justifies the phantom length. For the studied clinical protocols, the doses measured were about 30% higher than those informed by the CT scanner. For the new generation of CT systems with wider longitudinal detector size or cone-beam technology, the current CTDI measurements may no longer be adequate, and the informed CTDI tends to undervalue the dose delivered. It is therefore important to evaluate CT radiation doses following the AAPM TG111 methodology.

Hospital discharge of patients with thyroid carcinoma treated with 131I.

UNLABELLED: A dose limit-based criterion was proposed to authorize hospital discharge of thyroid carcinoma patients treated with 131I. Evaluation of accumulated doses to determine the effective half-life, the expected accumulated dose at 1 m, and the hospitalization time was performed to ensure that the dose limit was satisfied for each patient. Situations involving different dose limits and occupancy factors were analyzed. This study dealt only with external exposure; the problem of internal contamination was not considered. METHODS: Fourteen patients treated postoperatively with 131I were studied. The range of activity was 1,110-8,175 MBq. Electronic dosimeters and thermoluminescent dosimeter chips were placed on the left pectoral muscle. Dose was measured for a mean of approximately 2.5 d. The accumulated doses were plotted as a function of time and then fitted using an exponential model to obtain the parameters of total accumulated dose and effective half-life. The doses to the public and relatives at 1 m were calculated with point source approximation and several occupancy factors. RESULTS: The fit function parameters of accumulated doses in the first 36 h predicted the behavior of the total accumulated dose within a 5% error in the parameters. Estimated values of the accumulated dose 1 m from the patient were generally <5 mSv, even for an occupancy factor of 100%. For more restrictive dose constraints, hospitalization times were calculated according to different occupancy factors, as suggested in the European Commission guide. From the fit of the measured data, values of effective half-life for each patient were obtained. CONCLUSION: To apply the dose limit-based criterion, one must calculate the patient-specific parameters, as can be done using the accumulated dose. Knowledge of patient-specific parameters ensures that the patient will not expose any individual to levels greater than the dose limit. The calculated hospitalization times were less than those recommended, especially for countries with more restrictive dose limits. The type of measurements performed in this study reveals more realistic doses for the treatment of thyroid carcinoma with 131I.