Direct measurement system:
There is a necessity to measure the ESD and its mapping,
but accurate dose measurement method has not been established.
We devised a accurate measurement system for ESD and maximum patient skin dose (MSD),
using many RPLDs in an array.
In this study,
we examined the fundamental characteristics of an RPLD.
Materials:
The x-ray device used in this study was a digital cine single-plane x-ray system (Infinix Celeve-i,
Toshiba,
Japan) with a flat-panel detector (FPD).
This equipment includes a fixed additional filter (0.06 mm Tantalum).
The total filtration of this equipment is 1.1 mm Al + 0.06 mm Ta.
RPLDs made of silver-activated phosphate glass used in this study were 12 mm in length and 1.5 mm in diameter with a plastic capsule (GD-302M,
Chiyoda Technol Corporation,
Japan),
and measurement/readout system of RPLD was Dose Ace FGD-1000 (Chiyoda Technol Corporation).
The equipment uses a pulsed UV laser measurement technique.
RPLDs:
- RPLDs can provide highly accurate measurement from low levels of natural radiation to high levels of radiotherapy (10μGy to 10Gy).
- RPLDs have no fading effect,
compared to TLD.
- RPLDs are usually equipped with energy compensation filters made of tin.
- Radiopaque materials will be viewed in the display monitor and impeded the PCI procedure.
- In this study,
we chose the GD-302M with no energy compensation filter as the RPLDs.
Reference dosimeter:
Reference measurements were made using an ionization chamber,
model 9015 (Fig.2),
manufactured by Radcal Corp.
(California,
USA),
and a calibrated (reference: traceable from the national standard exposure) thimble-type 6 mL ion chamber with ±5% energy dependence in the 40 to 80 keV range.
GD-302M measurements were compared to measurements obtained with a reference ionization chamber.
For each measurement,
five GD-302M dosimeters were irradiated in free air (Fig.2).
The average ± SD value based on measurements of the elements of the GD-302M dosimeters were determined.
Methods:
- Effective energy dependence:
The tube voltage range for PCI in our hospital is 70 to 120 kVp.
We measured the effective energy for a tube voltage of 70 to 120kVp (70,
80,
90,
100,
110,
and 120 kVp) using the aluminum half-value layer (HVL) measurement method.
We also evaluated the calibration factor for the GD-302M in the 70 to 120 kVp range.
The factor was calculated as: Calibration factor = reference ion chamber measurement /GD-302M measurement.
- Dose dependence and the dose-rate dependence:
The dose dependence was measured at five different doses (0.1,
0.5,
1.0,
2.0,
3.0Gy) using a tube voltage of 80 kVp.
The dose-rate dependence,
at a fixed voltage (80kVp) and time (100ms),
was measured by varying the tube current from 1 to 1000 mA.
The angular dependence of the GD-302M dosimeter in the short and long axes was measured at a tube voltage of 90 kVp.
We assumed at seven points (0°,
±30°,
±60°,
and ±90°) for each of the axes (Fig.3).
The angular dependence was evaluated with the 0° measurement used as the reference value.