engleski

In-vivo dosimetry with silicon diodes for Co-60 beams

Calibration of semiconductor detectors for entrance dose measurements was performed in order to implement in-vivo dosimetry as a part of a quality assurance programme for external beam radiotherapy in our department. The DPD-3 Scanditronix basic unit and three p-type silicon diodes EDE-5, designed for Co-60 beams, and calibrated Farmer type ionisation chamber connected to a PTW Unidos electrometer were used. Prior to calibration, diodes were checked for reproducibility and stability of signal. Calibration is performed once per month for each diode under the reference conditions (10x10 cm2 field size at isocenter, SSD=80 cm, gantry angle 0o) using the polystyrene phantom (RW3) with irradiation time calculated by the treatment planning system to give 100 cGy at dmax. Calibration factors are then determined as a ratio of dose measured by ionisation chamber placed at reference depth (5 cm) multiplied with plastic to water correction factor and PDD to give dose at dmax in phantom, and signal from the diode placed at the phantom surface. A number of correction factors were determined to compensate for non-reference conditions during the actual patient treatment. Diodes were individually characterized for non-linearity in dose response, different angle of beam incidence, SSD, field size, wedges, block and tray. Corrections less than 1% were neglected in further measurements. In addition, diode perturbation of radiation field was measured to be about 3% at 10 cm depth on the central field axis. The diode dose rate dependence was investigated as a comparison of calibration factors at reference and extended SSD. Set of entrance dose measurements was done on Alderson phantom for pelvic and head and neck localization. Alderson phantom CT scans were used for planning purpose. Three-field arrangement with and without wedges for pelvis and two parallel-opposed fields with and without thermoplastic mask for head and neck treatments were investigated. Differences between expected dose, either manually or TPS calculated, and measured dose were below 3%, which was satisfactory for commencing with patient in-vivo dosimetry. Patient measurements are performed for all localizations treated in the department, preferably in first three fractions of the treatment. In three months of routine patient dosimetry, total number of 179 fields was inspected. For instance, the mean percentage deviations of the measured from expected dose were 0.8% with 1.8% SD for 68 pelvis fields, and 1.3% with 2.1% SD for 35 head and neck fields. Diode measurements detected 3 cases of mean deviation grater than 5% due to the missing wedge, block not included in time calculation and wrong SSD. Furthermore, we are expecting to perform the calibration for exit dose measurements and use it for patient dosimetry as well.

in-vivo dosimetry; diode; radiotherapy

nije evidentirano