TLD as a tool for remote verification of output for radiotherapy beams: 25 years of experience

IAEA-CN-96-82 TLD as a tool for remote verification of output for radiotherapy beams: 25 years of experience J. Francisco Aguirre, Ramesh C. Tailor, Geoffrey S. Ibbott, Marilyn Stovall and William F. Hanson Department of Radiation Physics The University of Texas M. D. Anderson Cancer Center, Houston, Texas USA Houston, Texas USA Abstract: The University of Texas M.D. Anderson Cancer Center (UTMDACC) has extensive experience with thermoluminescent dosimetry (TLD) as a quality assurance tool for output and energy monitoring of radiation therapy beams. Over the past 25 years the TLD results of the monitored institutions, commissioning data for TLD readers, the characterization data of lithium fluoride TLD-100 powder and the records of a quality assurance program of the system have accumulated. Nearly 1600 TLD sessions over the past 7 years on a cobalt unit reveal an accuracy in dose determination of 0.9% (one standard deviation). This represents a measure of the best achievable accuracy for TLD measured therapy doses. Based on this experience the windows of acceptability may be tightened from 5% in dose to 3% and 5 mm in electron depth to 3 mm. 1. Introduction The UTMDACC has two separate groups that operate mailed thermoluminescent dosimetry services. One is the Radiological Physics Center (RPC) and the other is Radiation Dosimetry Services (RDS). Both groups monitor the output of megavoltage high energytherapeutic photon beams (cobaltcobalt- 60 to 25 MV) and electron beams (6-25 MeV) used in radiation therapy. The RPC, under a grant from the U.S. National Cancer Institute (NCI), monitors the quality of radiation dosimetry performed at institutions participating in NCI-funded cooperative clinical trials. This assures that the institutions participating in the trials have adequate QA procedures and that no major systematic dosimetry discrepancies exist. The methodology has been described in detail [1]. This program includes periodic monitoring of beam output for photon and electron beams, and electron beam energy. Agreement between the RPC TLD measurement and the output as stated by the institution is expected to be better than 5%. If the disagreement exceeds 5% the discrepancy is resolved through phone calls, correspondence, repeat TLD irradiation or an on-site visit where ion chamber measurements are performed. Radiation Dosimetry Services (RDS) has a similar program that shares the same TLD and techniques as the RPC. It offers its services for a fee to customers who can request verifications of photon or electron beams at any frequency. Discrepancies and acceptability criteria are very similar to those of the RPC without a clear option for an on-site review to resolve intractable discrepancies. The RPC system started photon beam verifications in 1977, and for electrons in 1982. RDS initiated its for-fee service in 1987. Between the two programs a total of ~about 3600 treatment units at megavoltage radiation therapy facilities are involved. Approximately 6000 x-ray beams and 7500 electron beams are monitored per year. In the past three years, on average 3% of photons and 7% of electrons are outside the 5% criterion and result in repeat irradiation. Most of them are resolved by communication and repeat TLD irradiation. 2. Materials and Methods Present address: University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Box 547, Houston, TX 77030, USA \\iaea_f1\izewska$\my Documents\Symposium papers\session 11 \Full papers\iaea-cn-96-82 Aguirre.doc Revised 10/8/2002 5:06:34 PM

The methodology of the remote audit TLD program within UTMDACC has been explained in detail [2]. A summary of the procedures is presented below. 2.1. Thermoluminescent material Both centers use lithium fluoride powder (Harshaw Chemical Co, TLD 100) provided in disposable polyethylene capsules. Each capsule holds approximately 25 mg of powder. A large number of the capsules are filled with the same single batch of TLD powder thereby assuring uniform characteristics. Prior to its use every batch is subjected to a commissioning process. 2.2. Phantom design For photon beams TLD capsules are placed in an acrylic mini-phantom that provides for electronic equilibrium. The mini-phantom is supported in air by a nearly massless stand during irradiation. For electrons, a larger acrylic, full-scatter, phantom is provided. Both types of phantoms have 3 TLD capsules placed at the depth of maximum dose, d max. For electrons a second set of 3 capsules is placed at a depth of 30% to 80% dose to monitor energy by measuring themeasuring the percent depth dose. 2.3. Instrumentation UTMDACC uses single sample TLD readers. All TLD are massed and the TL signal is normalized to the mass. Each instrument is commissioned prior to regular use by optimizing signal-to-noise ratios with respect to PMT voltage settings. This verifies its ability to reproduce TLD signal per mass within an acceptable standard deviation (1.2% @ 1 σ) and confirms that the operating parameters are appropriate and produce glow curves of an acceptable appearance. Each sample is read in a 2-minute cycle that aims at careful replication of the cycle. Today, the reading cycle includes preheat for 5 seconds at 110 o C after which the signal is acquired for 46 seconds with a temperature ramp of 5 C/s up to 320 C. Nitrogen gas flows throughout the entire session, beginning 30 minutes before a reading session starts, to eliminate chemi-fluorescencenon-radiation inducednon-radiation-induced signal. Each sample is massed weighted while the system cools from the previous reading and is placed on the planchet after it has cooled to below 50 C. TL readings are provided with a reading precision of 0.01 µc and weighting precision of 0.01 mg. The (typical reading is typically 15 to 508 µ? C for an approximately 25 mg sample. ) and mass measurements have a reading precision of 0.01 mg. 2.4. TLD Characterization Prior to the use of a new batch, a representative sample is tested in order to establish its reproducibility, dose response, fading characteristics and energy dependence. The last term combines corrections for two effects: The TLD powder dependence on beam energy, and the effects of attenuation and scatter, by the mini-phantom, which also vary with energyenergy-phantom dependence.. These tests are also performed on the batch in current use as a redundant check. 2.5. Dose calibration of the TLD system TLD system Standard dosimeters are irradiated in a cobalt-60 beam to a known dose of about 300 cgy. The cobalt - 60 unit is pre-calibrated with an ion chamber dosimetric system traceable to NIST. The current protocol used for calibration is the AAPM TG-51 protocol [3]. In each session, standard samples are read, three at the start and three at the end of the session. The dose per unit signal/mg corrected for linearity and fading is determined which is identified as the system sensitivity. 2.6. Dose determination from customer dosimeters The customer dosimeters are read between the two sets of standards. A session normally has around 12 sets of customer dosimeters, one-third of them for electrons that have six capsules. The signal per unit mass is obtained, corrected for fading, linearity and energy-block dependence and the dose to the 2

TLD is calculated. The dose is then adjusted to match with the institution s dose specification conditions by including backscatter, inverse square effects, percent depth dose adjustments, etc. Interspersed throughout the session, four additional dosimeters, identified as controls, are read. These controls are pre-irradiated on a second cobalt unit whose beam output is pre-calibrated with an ion chamber. Controls provide a check of the system s reliability to measure the dose from control dosimeters that are irradiated under very tightly reproducible setup. As such, the control TLDs are expected to provide the best results. The controls also serve to identify and measure drift in the reader s response during a session. 3. Quality Assurance of the system The expected quality of the results is maintained through a comprehensive quality-assurance program that includes the following aspects: Acceptance and commissioning of instrumentation Acceptance and commissioning of TL powder Tests per session. During the course of each reading session the TLD reader and the TLD results are system is tested for the following parameters. Background and test signals? Test signal Standard deviation of the signal from standard dosimeters (σ 1.2%) Standard deviation of the signal from control dosimeters (σ 1.2%) Agreement between measured and predicted dose to controls (σ 1%) Outliers in each set of 3 TLD s (3% criterion) Weighing scale s reproducibility (±0.05 mg) Institutional TLD results against their historical averages Monthly checks Several parameters that may indicate anomalous variations are reviewed every month. Those include System sensitivity Standard deviation of standards and controls per technician and per reading unit Measured to predicted dose to controls irradiation Intercomparisons intercomparison The RPC and RDS perform a quarterly intercomparison of cobalt-60, 6 and 18 MV photons and 6 and 12 MeV electrons. Yearly inter-comparisons are done with the International Atomic Energy Agency. Additional ccomparisons have also been performed with the Quality Assurance Network of the European Society for Therapeuticitc Radiology and Oncology (ESTRO-EQUAL)ESTRO EQUAL Program. Record keeping Records of results of reading sessions, QA, Records are maintained of all the sessions, quarterly and yearly results. Dedicated books for the recording of malfunctions, maintenance, and repairs are kept. 4. Results For the 25 years of TLD work, one thing that has not changed is the TLD material (Harshaw TLD100). Other equipment and proceduresparameters such as readers, weighing scales, duration of routines and heating cycles have changed over time. Most of the conclusions drawn here are based on the data accumulated since 1991. There are however some conclusions that could very well apply to longer periods of time. 4.1. Batch characteristics The signal per unit mass per unit dose (sensitivity) for a particular batch is dependent on the TLD reader and it varies over time as the planchet gets used. Two different batches of TLD powder, read simultaneously on the same reader, generally yield different sensitivity. Batch-to-batch sensitivity \\iaea_f1\izewska$\my Documents\Symposium papers\session 11 \Full papers\iaea-cn-96-82 Aguirre.doc Revised 10/8/2002 5:06:34 PM

changes of up to 15% have been observed. However, other characteristics such as dose linearity, fading and energy-phantom energy correction have been found to remain the same (within the limits of accuracy of the measurement) from batch to batch. 4.2. Precision of dose determination Statistics of 1646 TLD measurements over 7 years on a cobalt unit at UTMDACC are presented as a histogram in Fig.1. The data meas/calc along the x-axis represents the dose ratio measured by TLD versus measured by an ion chamber. The distribution approximates to Gaussian shape and shows a 0.9% standard deviation that correlates well with predicted precision [4] discussed in section 4.5. 90 80 70 N = 1646 Avg = 1.001 SD = 0.9% Frequency Fig 1.- Agreement between TLD measured dose and predicted dose from decay after ion chamber calibration. Each value is the average of 10 TLD samples per session. Since these data correspond to TLD irradiated routinely under tightly reproducible geometry at UTMDACC the 0.9% precision represents the best possible of our system. 4.3. Institutional results 60 50 40 30 20 10 0 0.965 0.972 0.979 0.986 0.993 1.000 1.007 1.014 1.021 1.028 1.035 TLD Measured / Predicted 7000 6000 5000 PHOTONS 27,631 beams Average TLD/Inst=1.006 Std.Dev.=1.9% ELECTRONS 22,653 beams Average TLD/Inst=1.003 Std.Dev.=2.2% Frequency 4000 3000 2000 Photons Electrons 1000 0 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Fig 2. Institutional results histogram excluding outliers (tld TLD / Ins > 7%) 1 1.01 1.02 (TLD/Inst) Fig 2 is the frequency distribution of TLD measurements of ~27000 photon beams and 23500 electron beams at participating institutions. These data are the results of measurements over a 7-year period (1993-2000). Over this period most institutions used the TG-21 Protocol [5]. Standard deviations of the distributions are 1.9% for photons and 2.2% for electrons, which are higher than that previously shown for the case of a cobalt unit. This higher standard deviation is a result of compounding of additional uncertainties among which the calibration measurements and the use of nominal dose rate instead of a measured value, drifts in the units between calibration and irradiation and setup uncertaitiesuncertainties are significant contributors. 1.03 1.04 1.05 1.06 1.07 1.08 100% BEAM CALIBRATION POST TG-51 Photon (TLD) Percent within Criterion 95% 90% 85% 80% 75% Photon (ion chamber) TG-21 Implementation Electron (ion chamber) Electron (TLD) TG-51 Implementation 4 70%

Fig. 3 Calibration verifications with ion chamber and with TLD. Percent within criteria (+/- 5% for TLD, +/- 3% for ion chamber) Fig 3 shows how the dose agreement measured by TLD versus the stated has improved over the past 25 years. The ordinate is percent of institutions that have an acceptable agreement with the TLD measurements. From this figure one can see an obvious steady improvement in dose agreement. We believe that this is in part due to the RPC s program whichprogram, which has rendered the monitored institutions on a continuous alert. PHOTONS: Beam-wise spread in oputput checks (TLD/Inst) # Beams 500 450 400 350 300 250 200 Photons Beams with 5 TLD No. of Beams=2,710 Avg.Std.Dev.=1.5% Avg. TLD/Inst=1.005 Fig 4. - Histogram of average standard deviation of 5 or more repeat results of TLD on institutional photon beamsindividual x-ray beams for units with 5 or more TLD measurements Fig 4 shows a distribution of standard deviations for photon beams that have been verified 5 times or more. The location of the peak shows the most probable standard deviation to be 1.5%. Based on this concept, the RPC may change its current window of acceptability from ±5% to ±3% (2σ). Based on similar arguments the RPC is considering to change its current window of ±5 mm in electron depthdose measurement to ±3 mm. 4.5 Uncertainty in dose determination 150 100 50 0 0.10 0.50 0.90 1.30 1.70 2.10 2.50 2.90 3.30 3.70 4.10 Std.Dev. The detailed analysis of uncertainty in dose determined from mailable TLD is published [4]. Dose D is determined form TLD signal/mass T using the following formula. Dose, D = T S K f kl ke Factors kf, kl, and ke are fading, linearity and energy (nominal beam energy) correction factors. Symbol S represents system sensitivity in terms of dose/signal, and is determined by using the above \\iaea_f1\izewska$\my Documents\Symposium papers\session 11 \Full papers\iaea-cn-96-82 Aguirre.doc Revised 10/8/2002 5:06:34 PM

equation applied to a cobalt unit, for which ke is 1 by definition and dose D is known by an ion chamber measurement. Standard error in dose is determined by compounding the standard errors in each of the factors in above equation. For a cobalt unit, this leads to a standard error of 0.9% based on an average reading of 6 TLD samples. This matches well with the measured standard deviation in Fig. 1. For linac generated beams, the estimated standard error is 1.5 based on an average of 3 TLD samples. REFERENCES [1] HANSON et al Dosimetry QA in the United States from the Experience of the Radiological Physics Center; QA in Radiotherapy Physics SSDL Newsletter No. 30, IAEA/WHO, Vienna, Austria (Apr 1991) [2] T. H. KIRBY, W. F. HANSON, R. J. GASTORF, C. H. CHU, and R. J. SHALEK, "Mailable TLD system for photon and electron therapy beams," Int. J. Radiat. Oncol., Biol., Phys. 12, 261 265 (1986). [3] TASK GROUP 51, RADIATION THERAPY COMMITTEE, AMERICAN ASSOCIATION OF PHYSICISTS IN MEDICINE: Protocol for clinical reference dosimetry of high-energy photon and electron beams, Med. Phys. 26: 1847-1870 (1999) [4] KIRBY et al Uncertainty analysis of absorbed dose calculations from thermoluminescent dosimeters, Med. Phys. 19 1427 (1992). [5] TASK GROUP 21, RADIATION THERAPY COMMITTEE AMERICAN ASSOCIATION OF PHYSICISTS IN MEDICINE. "A Protocol for the Determination of Absorbed Dose from High -energy Photon and Electron Beams", Med. Phys. 10:741-771 (1983) This work was supported by PHS Grant CA10953 awarded by the NCI, DHHS. 6