Reference dose determination in 60Co and high-energy radiotherapy photon beams by using Farmer-type cylindrical ionization chambers - an experimental investigation

Development of a dose-rate dosimeter using a silicon photodiode for a medical linear accelerator in a 10 MV flattening filter-free mode

This study was aimed at developing a dose-rate dosimeter to measure the instantaneous dose rate of a commercially available medical linear accelerator. A dose-rate dosimeter composed of a silicon photodiode (Si-PD), a complementary metal-oxide semiconductor single operational amplifier, a resistor of 20 MΩ, a capacitor of 100 pF, and a mini-substrate measuring 16 × 16 mm2 was evaluated. Voltage outputs from the proposed dosimeter were measured using an analog-to-digital converter on a microcomputer. A custom-made x-ray tube generator at an energy of 120 kV with a tube current ranging from 0.1 to 2.0 mA was used for the dose-rate calibration. Dose-rate calibration was performed 83.3 mm from an x-ray source using a commercially available semiconductor dosimeter. The developed Si-PD dosimeter could measure up to 0.6 Gy/s at a distance of 19.3 mm from the x-ray source. Measurements were also performed using a medical linear accelerator in a 10 MV flattening filter-free mode at depths of 0, 25, 50, and 100 mm with an irradiation field of 100 × 100 mm2 at a constant distance of 1000 mm from the source to the dosimeter. A peak voltage variation corresponding to the instantaneous dose rate was observed using a sampling period of 1.0 ms, and the peak voltages decreased with the depth. The detected pulse numbers were 512, 484, 491, and 511 at depths of 0, 25, 50, and 100 mm, respectively.

Analysis of Uncertainties in Clinical High-Energy Photon Beam Calibrations Using Absorbed Dose Standards

We compared the results of absorbed dose measurements made using the TRS-398, TG-51, and DIN protocols and their associated uncertainties to reduce discrepancies in measurement results made using the three protocols. This experiment was carried out on two Varian Medical linear accelerators with 4, 6, 10, and 20 MV photon energies using FC65-G and CC15 (cylindrical) and NACP-02-type (plane-parallel) ion chambers in water phantoms. The radiation beam quality index (Q) was determined from the measurement of percentage depth dose. It was used to determine the photon beam quality factor required with the ionization chamber calibration factor to convert the ion chamber reading into the absorbed dose to water. For the same beam quality, the TRS-398/TG-51 varied from 0.01% to 1.8%, whereas the ratio for TRS-398/DIN 6800-2 varied from 0.1% to 0.88%. The chamber-to-chamber variation was 0.09% in TRS-398/TG-51, 0.03% in TRS-398/DIN, and 0.02% in TG-51/DIN 6800-2. The expanded uncertainties (k = 1) were 1.24 and 1.25 when using TRS-398 and DIN 6800-2, respectively. Using the aforementioned three protocols, the results showed little chamber-to-chamber variation and uncertainty in absorbed dose measurements. The estimated uncertainties when using cylindrical ion chambers were slightly lower than those measured using plane-parallel chambers. The results are important in facilitating comparisons of absorbed dose measurements when using the three protocols.

Evaluating the impact of ionization chamber-specific beam quality correction factor in dosimetry of filtered and unfiltered photon beams

Aim:

The response of ionization chamber changes when used at beam quality Q which is different from beam quality Q_o (usually ⁶⁰Co) that was used at the time of its calibration. Hence, one needs to apply beam quality correction factor (k_{Q, Qo}) during dosimetric measurements. However, k_{Q, Qo} data are unavailable for novel ion chambers in the literature. Moreover, most of such data do not differentiate between filtered (flat) and unfiltered (unflat) beams. In addition, literature-based data do not differentiate among different pieces of the ion chambers of the same make and model. Hence, the purpose of our study was to determine the ion chamber-specific experimental values of k_{Q, Qo} and to evaluate their impact in dosimetry.

Materials and Methods:

In this work, the value of k_{Q, Qo} were measured for six ionization chambers of three different types in 6, 10, and 15 MV filtered (with flattening filter [WFF]) as well as 6 and 10 MV unfiltered (flattening filter free [FFF]) photon beams. The measured values of k_{Q, Qo} were compared with Monte Carlo-calculated values available in the literature. The uncertainties in measurement of k_{Q, Qo} values were also evaluated.

Results:

For 6 MV FFF beam, the measured value of k_{Q, Qo} was found to be consistently lower than 6 MV WFF beam for all Sun Nuclear Corporation ion chambers, while it was higher as per the theoretical data. The inter-chamber variation in k_{Q, Qo} values was observed for the same model of the ion chambers. The maximum difference between absolute dose values on using the theoretical and experimental k_{Q, Qo} values was up to 3.23%.

Conclusion:

The measured absolute dose values by the ion chamber of a given make and model were found different due to the use of its theoretical and experimental k_{Q, Qo} values. Furthermore, the variation in response of different pieces of ion chambers of the same make and model cannot be accounted for theoretically, and hence, the use of theoretical k_{Q, Qo} data may introduce an inherent error in the estimation of absorbed dose to water. This necessitates the use of measured value of k_{Q, Qo} for each ionization chamber.