ROC7080: RADIATION THERAPY PHYSICS LABORATORY LAB TWO : THERMO LUMINESCENT DOSIMETRY

Transcription

1 ROC7080: RADIATION THERAPY PHYSICS LABORATORY LAB TWO : THERMO LUMINESCENT DOSIMETRY GROUP I SPRING 2014 KEVIN JORDAN GRADUATE STUDENT, RADIOLOGICAL PHYSICS KARMANOS CANCER CENTER WAYNE STATE UNIVERSITY

2 Table of Contents I. Introduction & Theory... 3 II. Laboratory Procedures & Methods... 4 II.a TLD Reader Identification and Setup Parameters... 4 II.b TLD Linearity... 4 II.c Patient Measurement... 4 III. Questions & Discussion... 4 III. Appendix A: Experimental Data... 8 IV. Appendix B: Sample Calculations V. Appendix C: References JORDAN,KEVIN 2

3 I. Introduction & Theory In this laboratory, the uses and mechanisms of Thermo luminescent Dosimetry have been explored. The proceeding sections can be found to include Laboratory Procedures & Methods, Questions & Discussion along with the appendices, which include the experimental data, sample calculations and references. Recall, that TLD phenomenon occurs through solid state and band gap theory. TL materials are non-conductors, which contain activators and holes within the material. These activators create traps or holes for electrons. The depth of the trap is proportional to the energy and for TL the E trap = 1eV, which is much larger than E kt =(1/40)eV. Thus, fluorescence, phosphorescence and thermoluminescence reflect increasing trap depth. JORDAN,KEVIN 3

4 II. Laboratory Procedures & Methods II.a TLD Reader Identification and Setup Parameters Upon entering the dosimetry room, the Harshaw 3500 TLD reader can be found to reveal all necessary information in terms of identification and setup parameters, of which can be found in Table 1. II.b TLD Linearity Using the LINAC, the group placed the dosimeters in a solid water phantom at a depth of d max for 6 MV photons, 2.8 cm. Setting a 100 cm SSD and a 10x10 cm 2 field size. In this geometry, 1 MU equals 1 cgy. The group then proceeded to irradiate groups of three dosimeters to the following dose levels: 5, 10, 20, 50, 100, 200, 500, 1000, 2000 cgy. The resulting readings, average, standard deviation, standard deviation of the mean and respective background (un-irradiated TLD) can be found in Table 2. II.c Patient Measurement Again using the LINAC, the group placed place at least 5cm of solid water phantom on the table and then set 95cm SSD and a 15x15 cm 2 field size. Finally, the grouped placed a group of three dosimeters just outside the border of the field under 1.5 cm of bolus and delivered 100 MU. The readings, average, and background (un-irradiated TLD) can be found in Table 3. III. Questions & Discussion Q1: What percentage of the l00 cgy reading was the background reading with no TLD on the planchet? What was the percent background with the unirradiated TLD on the planchet? Is it necessary to correct for the background in the subsequent analysis? The background reading with no TLD in the planchet can be found to be 0.02% of the 100cGy reading, while the unirradiated TLD on the planchet could be found to be 0.18% of the 100cGy reading. Since both these are of such small quantities, much below 1%, it is not necessary to correct for the 100cGy reading and these values would only get smaller with increasing dose and thus charge collected. JORDAN,KEVIN 4

5 Q2: How many peaks are evident in the glow curves? At what temperature is the main glow curve peak? How does this compare with your expectation for this TLD material? Between two and three peaks are evident in the glow curves and the temperature of the main glow curve peak came to approximately 255 C. The expectation would be for three peaks on the glow curve and 240 C, so these results are satisfactory with that expectation. One can look to the ROC7040 course pack, pp. 147, to see and confirm these expectations, a screenshot has been provided below. Q3: What component in the reader requires high voltage? What effect would increasing the high voltage setting have on the readings? Why do TLD readers incorporate an internal light source? The photo multiplier tube (PMT) is what requires the high voltage in the TLD reader. The high voltage provides the correct accelerating potential for the photoelectrons. Recall the PMT converts light into an electrical signal and thus the reader requires the proper high voltage to be provided in order to provide the proper gain for a proper electrical signal. The TLD reader incorporates an internal light source in order to calibrate the output of the reader, thus providing more accurate data output for the temperatures and intensities along the glow curve. JORDAN,KEVIN 5

6 This light source thus provides a PMT self check and gain adjustment as necessary based on the results of the internal light source s iterative algorithm. Q4: Over what range is the TLD response linear? Calculate the regression line relating reading to dose for this range. Determine also the uncertainty of the fit parameters. Use this relation to determine the relative over or under response of the system at the 2000 cgy level. Plot the data and the regression line on a linear and a log scale. Are there advantages/disadvantages to either representation? Based on the ROC7040 course pack, pp. 151, one would expect the TLD response to be linear between the range of 5 300cGy. The log-log representation provides a nice visual representation of the data, especially for the lower half of the data set. One can easy see in Figure 3, compared to Figure 1, that the data becomes nicely spread out across the page, instead of just crunched in to the left hand side of the graph, thus better showing the relatively strong linearity, despite the inherent over response for the cGy data, the R 2 = Additionally, the uncertainty of the fit parameters for cGy and 5-200cGy can be found in Table 4 and Table 5 respectively. One can easily take some values and calculate the obvious over response above 300 cgy. For example, using the linear equation for 5-200cGy, one can plug 2000cGy to this equation (y= mx+b) to find the expected charge reading to be 126,279nC, yet a value of 205,867nC is recorded, thus showing an over response or supralinearity for this particular dose of 79,588nC. Recall, that the over response is thought to be from increased availability of luminescent centers when charged particle tracks come closer together and also from radiation induced trap formation. JORDAN,KEVIN 6

7 Q5: Perform dose calculations for the Patient measurement. Which reading from section 2 do you use as the calibration and why? One can see in Table 3 that one reading is particularly high, so if one were to compare the other two and take their mean, the average charge reading would be 526nC for the 100MU dose application. With these results, one can assume and test that this should be well below 200cGy and thus use the linear equation from Figure 2 to back calculate for the dose since the R 2 of shows the trend line providing strong correlation. Thus, the dose can be found to be 6.42cGy to the TLD s placed just outside the light field. JORDAN,KEVIN 7

8 III. Appendix A: Experimental Data Table 1. TLD Reader Identification & Setup Parameters Table&1&((&TLD&Reader&Identif&&&Setup&Parameters Item Value Units TLD&Material: LiF n/a High&Voltage: Volts Preheat&Temp: degc Heat&Rate: degC/s Bkg&Reading&(no&TLD): 1.49 nc Un(irrad&TLD&reading: nc Light&Source&Reading nc Light&Source&Reading nc Light&Source&Reading nc Light&Source&Rdg&Mean: nc TLD&Reader&Model&#: Harshaw&3500 n/a Acquisition&Time: seconds Preheat&time: 0.00 seconds Maximum&Temperature: degc Table 2. TLD Linearity Table&2&((&TLD&Linearity Dose&(cGy) Readings&(nC) Mean Std.&Dev Std.&Dev.&Of&Mean Mean&Bkg JORDAN,KEVIN 8

9 Table 3. Patient Measurement Table&3&((&Patient&Measurement Item Value Reading&(nC)& Reading&(nC)& Reading&(nC)& Readings&Mean Mean(Bkg Figure 1. Charge Reading (nc) vs. Dose (cgy) - 5cGy cGy JORDAN,KEVIN 9

10 Figure 2. Charge Reading (nc) vs. Dose (cgy) -- 5cGy - 200cGy Figure 3. Charge Reading (nc) vs. Dose (cgy) (5-2000cGy) [log-log] JORDAN,KEVIN 10

11 Figure 4. Charge Reading (nc) vs. Dose (cgy) (5-200cGy) [log-log] Table 4. LINEST Linear Regression Analysis (5-2000cGy) slope intercept SE(slope) SE(intercept) r^ SE(y) F df ss?reg ss?resid Table 5. LINEST Linear Regression Analysis (5-200cGy) slope intercept SE(slope) SE(intercept) r^ SE(y) F df ss?reg ss?resid JORDAN,KEVIN 11

12 Figure 5. Glow Curve 100MU (100cGy) Figure 6. Glow Curve 500MU (500cGy) JORDAN,KEVIN 12

13 Figure 7. Glow Curve Background IV. Appendix B: Sample Calculations JORDAN,KEVIN 13

14 V. Appendix C: References Burmeister, Jay. Radiation Dosimetry Coursepack Rakowski, Joe. Radiation Therapy Physics Course Notes JORDAN,KEVIN 14