Achievable accuracy in brain tumors by in vivo dosimetry with diode detectors

ABSTRACT Background: In vivo measurements of applied dose during radiotherapy treatment, is important to ensure accurate dose delivery to patients. Uncertainty in dose delivery should fall within ±5% of the prescribed dose as recommended by ICRU. Assessment of dose for radiotherapy applications performed with various types of detectors. In this study, semiconductor diodes were used which have some advantages for clinical dosimetry. Materials and Methods: The brain tumors have generally treated with two fields using SSD technique. Entrance and exit dose were measured for each patient with diodes during treatment. Entrance and exit dose measurements have converted to midline dose. Measured entrance and exit doses have compared with calculated ones and large deviations (more than 5%) have observed. A farmer ionization chamber (0.6 cm) was used as the reference dose detector and a Perspex water phantom (30 30cm2 area and thickness ranging from 5 cm to 30 cm) were used to determine calibration and correction factors. Results: Correction factors were determined and variations more than 1% have used to obtain correct doses. Large deviations between measured and calculated for entrance (5.3%), exit (42%) and midline (47%) were detected. The difference was not found to be significant when comparing the measured entrance dose with the calculated one (p=0.696) and the measured exit dose with calculated one (p=0.643) and measured midline dose with calculated one (p=0.104). Conclusion: In vivo dosimetry is very useful to check the dose delivered to the patient. A high precision obtained when the calibration and correction factors for each parameter of influence on the diode response are carefully determined and applied to convert the diode signal in the adsorbed dose. In this study, the target-absorbed doses were estimated from the measured entrance dose and the measured transmission.