The purpose of this study was to develop a new ultraminiature dosimeter dedicated for in-vivo dosimetry in photon therapy. In particular, we applied this dosimeter for the dose measurement in the urethra of prostate cancer patients undergoing brachytherapy.
Materials/Methods
The present dosimeter consists of a small amount of plastic scintillator, a plastic optical fiber, a photo-multiplier tube, a charge pre-amplifier, a discriminator and a counter. A small amount of a partially polymerized plastic scintillator painted on the tip of the plastic optical fiber produces scintillation photons from the incident radiation which is then proportional to the absorbed dose. The scintillation photons are converted to electric pulse by the photo-multiplier and passed on to a counter. The source used for this study was Ir-192. The relationship of pulse counts and absorbed dose were analyzed and after which an equation to convert the pulse counts to water-equivalent dose was deduced. To confirm the accuracy of the dosimeter, we compared the doses measured by the present dosimeter and the calculated doses by the Monte Carlo n-Particle transport code (MCNP) for the dose rate distribution in a 20 × 20 × 20 cm3 water phantom. Additionally, the absorbed dose in the urethra of five prostate cancer patients were measured with the present dosimeter. These measured doses were compared with those generated by treatment planning software (PLATO).
Results
Figure 1 shows the dose rate distribution for the measurements done with the present dosimeter and calculated dose rates from MCNP. The measured values were in good agreement with the calculated results. The discrepancy between them was less than 5%. Moreover, it was found that the dosimeter has a wide dynamic range of linearity up to an order of magnitude of 102. In the in-vivo application of the dosimeter, the difference in between the measured and planned dose in the urethra was 7.42% in weighed average for five patients (range: 3.6–12.6%).
Conclusions
We developed a new ultraminiature dosimeter dedicated for in-vivo dosimetry. The accuracy of the dosimeter was within 5% as confirmed by the comparison of measured and calculated dose rate distributions. The usability for in-vivo dosimetry was also confirmed to be within reasonable difference when compared to calculated dose.
FULL TEXT
