Otimização na Avaliação de Doses em TC usando Dosímetros OSL em forma de Fita

Optimization in Dose Assessment in CT using Tape-shaped OSL Dosimeters

Abstract

Computed tomography (CT) images contribute to high-quality medical diagnosis, but radiation dose can be quite high requiring accurate assessment. The CT dose index (CTDI) was developed in the 80s for dosimetric purposes, but for scanners operated exclusively in axial mode. CTDI tends to underestimate patient dose in helical CT exams. AAPM report TG111 (2010) suggested a new metric in which the patient's radiation dose is obtained from dose profiles constructed from several measurements made with a small ionization chamber. It is also possible to obtain the dose distribution profiles using properly calibrated OSL (optically stimulated luminescence) tapes. In this work, a Radcal ionization (“pencil”) chamber and OSL Landauer (Luxel^TM) tapes 20 cm long and 0.3 cm wide were X-ray-irradiated, in air and in the holes of two cylindrical CT phantoms, using three peak voltage values (100, 120, 140 kV), both in lab and in a clinical CT scanner. Irradiated tapes were read using an OSL reader built in the GDRFM. The OSL profiles were calibrated against ion chamber, matching the CTDI₁₀₀ values obtained from both detectors. After this, CTDI_w and CTDI_vol values were determined from the calibrated profiles. CTDI values from OSL profiles differed approximately 3.9% related to those from the CT scanner. From the profiles, also the planar equilibrium dose D_eq,p (TG111) was evaluated in some CT protocols; this quantity exceeded the CTDI values provided by the CT scanner in all the cases. E.g.: The percentage difference found between D_eq,p and CTDI_vol for the head phantom ranged between 33-25%. Thus, in some cases, it could be advantageous to use calibrated OSL dosimeters instead of an ionization chamber to obtain the profiles, because the optimization in the measuring time (it is possible to obtain five OSL profiles from a single phantom irradiation).