Measurements for dose distribution with a photo-stimulated luminescence dosimeter sheet

Dose distributions for photon beam were measured using a Photo-Stimulated Luminescence Dosimeter (PSLD) sheet, which has 18 × 24 cm2 dimension and 0.2 mm thickness. Its density and effective atomic number are 1.0 g/cm3 and 7.6, respectively. The read out was performed by blue LED lights for 20 seconds, which was much shorter than the readout time for TLD. The percent depth dose and dose profile were obtained as smooth curve after the calibration and it reproduced the measurements with ionization chamber, except for the tail region in the dose profile. We demonstrated the measurement of the prostate VMAT dose distribution also. The result reproduces the calculation by treatment planning system (TPS) qualitatively. It is shown that the PSLD sheet has the potential to measure the dose distribution.


Introduction
Accurate determination of the absorbed dose at the surface and in the buildup region requires careful considerations in the selection of detectors. It is known that the size of the detector along the beam direction should be as small as possible in order to minimize the effect of dosimeter volume. Recently, such a future is considered to be necessary for the QA of electron intensity modulated radiation therapy (IMRT) also since the surface dose should be measured with high accuracy. The report of AAPM TG-106 recommends the extrapolation chambers for the surface dose measurement [1], however, its availability is limited and the usage is time consuming. Instead, there have been measurements of the surface dose by various dosimeters, such as diode, film, TLD, gel, etc. [2][3][4][5][6]. The commercial TLD materials, such as rods, chips, powder, disks and so on, are usually used only for point measurements. Thus we have developed a TLD sheet for the verification of the spatial dose distribution [7][8][9][10][11][12][13] and showed that the sheet could be used for measurements of 3D dose distributions [14]. Although the TLD sheets have many attractive characters, it takes few minutes for heating the TLD crystal. On the other hand, we have investigated another type of dosimeter, a Photo-Stimulated Luminescence Dosimeter (PSLD) sheet [15]. A significant advantage of the PSLD sheet over the TLD sheet is that the PSLD sheet requires less time for light stimulation and it is easy for handling.
The purpose of this study is to demonstrate the PSLD sheet for dose distribution measurements of photon beam.

Method and Materials
Dose distributions of 10 x 10 cm 2 field size for 6 MV photon were measured using the PSLD sheet. In practice, the PSLD sheet can be formed in any size, and in this study we used 18 x 24 cm 2 dimension and 0.2 mm thickness as shown in Figure 1. The PSLD sheet is softer than film and it is tough flexible. The characters of PSLD sheet are summarized in Table 1 comparing with that of TLD sheets. We inserted these PSLD sheets in a tough water phantom (Kyoto Kagaku, Japan) in the same manner as film. The PSLD sheet does not have high uniformity across the sheet. Therefore, before the measurements of dose distributions, the irradiation of 200 cGy with 40 x 40 cm 2 field size was carried out to calibrate the response over 2D area of each PSLD sheet. Each sheet was characterized by an individual sensitivity value and we applied the calibration coefficients to each PSLD sheet. A light stimulation system includes computer controlled blue LED lights and CCD camera. The read out was performed by blue LED lights (100W) for 20 seconds and generated light was detected with a CCD camera which has 1024 x 1024 pixels (FLIML 1001E-2, Finger Lakes Instrument).
For the data comparison, a small thimble ionization chamber (PTW Semiflex ionization chamber) was also used.  Dose range Up to 1000 Gy 1 cGy to 10 Gy Readout Heat 180 o C for 240 sec. Blue LED light for 20 sec.

PDD and Profile
The present depth dose (PDD) and dose profile were obtained as smooth curve after the response calibration as shown in Figures 2 and 3. Obtained PDD well reproduced the results with ionization chamber measurement. For dose profile, the discrepancies were observed in the tail region and PSLD shows lower response comparing to the ionization chamber. In the tail region, the component of the lower energy and the low dose contribute. Thus it is necessary to study carefully the energy response and response to low dose of the PSLD sheet. The penumbra measured with PSLD is sharper than that with ionization chamber. It is because the PSLD has higher resolution compared to the ionization chamber. The measured filed sizes with the PSLD and ionization chamber showed agreement with 1%.

VMAT dose distribution
The comparison of dose distribution was carried out between the dose calculated by Pinnacle 3 treatment planning system (Philips Medical Systems, Madison, WI) and the dose measured by PSLD sheets in the phantom. The prostate plan of volumetric modulated arc therapy (VMAT) was used for the comparison as shown in Figures 4. Although the measurement of PSLD reproduces qualitatively the calculated result of Pinnacle 3 TPS, it was found that there were a couple of problems for the quantitative comparison. Our PSLD sheet has some non-uniformity over a 2D area since the PSLD powder were coated by hand for each piece, which was not performed by manufacture. The fluctuation of the PSLD coating influenced the result of the dose distribution comparison even though the response calibration over the 2D plane was applied before the measurement. We thus need to improve the uniformity of the PSLD sheet or the method of data processing for the application to the dose distribution measurement. It is expected that some manufacturer can improve the uniformity of the PSLD sheet.

Conclusion
We have developed the PSLD sheet for the measurement of dose distributions. The PSLD has significant advantage of the readout time over the TLD. Through the measurements, it has been shown that the PSLD sheet has the potential to measure the 3D dose distribution including low dose at surface. The measured results with PSLD qualitatively reproduced the calculated dose distribution. However, the uniformity variation of the PSLD sheet over 2D area influenced the PSLD readout and it caused some discrepancies between the measurements and TPS results. We thus need to improve the PSLD itself in order to use it for the measurement with high accuracy, such as VMAT and IMRT QA. After some improvements, it can be considered that the PSLD sheet are used for a surface dose measurement of cone-beam CT, dose distribution of electron IMRT, and the dose measurements where it is difficult for the film to access.