Experiences of the Secondary Standard Dosimetry Laboratory of the CPHR in the implementation and validation of the Postal Quality Audit in Radiotherapy, using TLDs Stefan Gutiérrez Lores 1, Gonzalo Walwyn Salas 1, Daniel Molina Pérez 1, Raudel Campa Menéndez 2. 1 Centro de Protección e Higiene de las Radiaciones. Calle 20 N 4113 e/ 41 y 47 Playa. C.P.: 11300. C. Habana. Cuba Telef: (537) 579571, (537) 531803 Fax: (537) 203 0165, (537) 2041188 E-mail: stefan@cphr.edu.cu Abstract. The use of the radiotherapy implies the necessity of rigorous quality standards in its different components, aimed to provide the best possible treatment and avoid potential patient s risks, that could even causing him the death. It has long been recognized that accurate knowledge of the dose in radiotherapy is vital to ensure safe and effective radiation treatments. To achieve this goal, comprehensive quality assurance programmes should be established to cover all steps from dose prescription to dose delivery. These programmes should include internal checks performed by the radiotherapy centers and external audits made by independent external bodies. This paper presents the methodology and experience of the SSDL for the implementation and validation of the Postal Quality Audit in Radiotherapy using TLDs. A National External Audits Group in radiotherapy (EAG) was constituted to performing quality audit for radiotherapy dosimetry, whose aim is to ensure adequate precision in the dosimetry of clinical beams. This National EAG is in charge or is responsible for the execution of the quality audits to the radiotherapy service, through visits as well as with TLDs postal way. Under this program were bought 200 LiF N(Mg,Ti) TL Rods type JR 1152F made in China, with dimensions of 6 mm x 1 mm x 1 mm. All of these rods were identified individually with a consecutive number made over one of its sides, using a fine tip of graphite. The method used to determinate the individual sensibility of the TL detectors was: irradiating a group of them, with the same history of irradiation and readout (100 rods approximately), four serial times in the same geometrical conditions, to read them out and to attribute to each of them a sensitivity factor <Si>. This sensitivity factor is equal to average for the 4 cycle of irradiation and readout of the quote between the TL readout from dosimeter i and the mean of all values for each cycle. This factor expresses the response variation of each individual dosimeter around the mean. Although in practice these mean vary from irradiation to irradiation, <Si> should remain constant because all dosimeter are subject to the same variations. A table with the sensitivity factors obtained is presented, individual detector variations coefficients of <Si> were less than 2% for all TLDs, obtained for 4 cycle of irradiation- readout. 1. Introduction In 1994 a group of consultants was invited to advise the Agency on expansion of the IAEA/WHO TLD [1] postal dose check service for radiotherapy hospitals by the transfer of methodology to national level. The consultants advised the Agency to initiate the Co-ordinate Research Programme (CRP) to adapt the IAEA well established TLD procedures to the country where existing resources enabled the set up of the External Audit Groups nationally recognized groups in charge of operating external quality audits for radiotherapy dosimetry. The External Audit Groups (EAG) include the SSDL, a TLD Measuring Centre (MC) and Medical Physics Group (MPG); these groups work in close co-operation during all steps of implementation of the TLD audits. The pilot countries, which have chosen for the CRP was Algeria, Argentina, China and India. Due to increasing interest in the programme, the Czech Republic, Israel and Malaysia joined the CRP in 1996. The first four countries, whose contracts started in 1995, have successfully completed their projects and continue the national TLD programme on routine basis. Three participants, whose contracts started in 1995, continue the implementation of their national TLD programmes without further financial assistance from the Agency. The Agency preserves the links with national EAGs in these countries to co-ordinate their activities and to update their activities and to update their techniques and procedures. On the request by Member States to further expand this CRP, five new countries joined the project in 1998. These are Colombia, Cuba, Philippines, Poland and Viet Nam. 1
Quality Audit is a review of the quality control system and dosimetry audit is a review of absorbed dose determination performed by independent person or body who is not responsible for the performance of the product or process under review. The audit should, in the final stage of its implementation, cover all steps of the radiotherapy procedure, i.e. the final aim should be to ensure that the prescribed dose is given to the patient. The quality and dosimetry audit can be established by three different ways: (1) postal measurement (i.e. by mailed dosimeters) (2) site visits (3) combination of postal measurements and site visits. Audit by site visits is the most efficient method, since all interesting parameters can be easily checked and the necessary actions can immediately be started. During a site visit all aspects of the work can be discussed with local personnel and a comprehensive review of the overall accuracy and possible problems can be attained. However, site visits to each clinic of the area serviced by the audit centre may become relatively expensive, especially in bigger or developing countries, and a suitable combination of postal measurements and site visits can be more appropriate [2]. 2. Materials and methods 2.1. TLD system. Lithium fluoride thermoluminescent dosimetry rods (TLD-100) type JR 1152F made in China (LiF: Mg, Ti) with dimensions of 6 mm x 1mm x 1mm. All of these rods were identified individually with a consecutive number made over one of its sides, using fine tip graphite. TLDs of LiF in its purest form exhibit relatively little thermoluminescence, which is the phenomenon of photon emission subsequent to heating. The presence of impurities, i.e, magnesium and titanium in TLD-100 LiF, appears to be necessary for radiation-induced thermoluminescence. When a crystalline TLD is irradiated, a minute fraction of the absorbed energy is stored in the crystal lattice. The energy stored in the irradiated TLD is recovered as visible light by placing it on a planchet heater in a commercially available TLD reader and heating the TLD in a light-tight chamber. A photomultiplier tube (PMT) placed in the TLD reader detects the light emitted from the TLDs. Characteristics of TLD that are particularly important for radiation dosimetry include variability of the TL response vs readout temperature, referred to as the glow curve, the variability of TL response related to annealing procedures and nonlinearity of the Tl dose response [3]. The TLD signal was read using universal TLD reader system Toledo type 654 E5 Vinten instruments. TLD rods placed singly on the planchet were heated 150 C ( 30 s prereadout) to 260 C heated in 30 s (10 C/s). 2.2. Annealing procedures. Different variants of annealing [4] (thermal treatment) of the TL detectors to use its before were examined. The annealing procedures consisted of two steps, high temperature and low temperature prior irradiation. High temperature annealing was accomplished using a furnace to 400 C (RADOS Nabertherm L/5, RADOS technology OY). The automatic temperature control provided a stable temperature to within ± 5 C. TLD rods, in aluminum tray, were kept inside the furnace for 1 h. At the end of 1 h annealing at 400 C, TLDs were maintained inside of the furnace to room temperature. Low temperature annealing was accomplished using a furnace to 100 C (FJ-411). The automatic temperature control provided a stable temperature to within ± 3 C. At the end of 2 h annealing at 100 C, TLD rods were maintained inside of the furnace to room temperature. Four cycles of annealing in a different way among groups and irradiation using identical geometry and same conditions were done. The variants used for the annealing were: Group 1: 1 h to 400 C with cooling inside the oven (slow cooling) Group 2: 1 h to 400 C with slow cooling plus 2 h to 100 C with cooling outside of the oven (quick cooling) Group 3: 1 h to 400 C with quick cooling Group 4: 1 h to 400 C plus 3 h to 100 C with quick cooling 2
2.3. Tld irradiation. Before the audits started, the reading conditions of the TLD system were established and the adequate parameters determined: calibration coefficient for 60 Co radiation, dose response linearity factor, fading and energy correction factors as well as the reader s drift correction. The determination of absorbed dose to water in a 60 Co gamma ray beam by TL-rods is carried out calibrated the TL-rods, by means of a ionization chamber with the described method in Derreumaux et al [5]. The dose measurements were performed with a 0.6 cm 3 NE 2571 ionization chamber that was coupled with the PTW UNIDOS 10002 electrometer. The chamber together with the electrometer was calibrated by the IAEA. The absorbed dose to water was determinate, by measuring ionization chamber and the procedure described in the code of practice for dosimetry based on standards of absorbed dose to water [6]. The TL- rods used in the dose determination of the capsules irradiated in the SSDL s beams was carried out using the SSDL s reference value of absorbed dose rate to water measured to 5 cm depth in 2528/3A phantom with dimensions 20 x 20 x 10 cm, 80 cm to SSD and a 10 cm x 10 cm cobalt 60 beam. The capsules were irradiated employed the machine time (corrected for transit time) calculated with the absorbed dose rate to water measured in the above full scatter phantom. 2.4. Individual sensitivity factors for TL-rods. The method used to determinate the individual sensibility of the TL detectors was: irradiating a group of them, with the same history of irradiation and readout (100 rods approximately), four serial times in the same geometrical conditions, to read them out and to attribute to each of them a sensitivity factor <Si> equal for the cycle of irradiation and readout of the quote between the TL readout from dosimeter i and the mean of all values for each cycle. This factor expresses the response of each individual dosimeter around the mean. Although in practice this mean vary from irradiation to irradiation, <Si> should remain constant because all dosimeter are subject to the same variations. The sensitivity factors, individual detector variations coefficients of <Si> is presented in Table 5 were less than 2% for all TLDs, obtained for 4 cycle of irradiation-readout. 2.5. Documentation of Quality Assurance (QA) manual. The Quality documentation of the service that has been implemented with this CRP consists on the following procedures, records and forms: Quality Manual that contain: quality objective, quality politic, service organizational outline and responsibilities system, control of the documentation and data, control of the certificates, treatment of non conformities. Work instruction and records: Preparation of the TL-detectors Quality control and periodic maintenance of the system TLD Annealing of TL-rods Irradiation of reference dosimeters Readout of Tl-rods Dose determination and results emission Forms: TLDs user s instruction sheet Data sheet for irradiation with Co-60 beam Form for the control of TLD dispatch, reception and evaluation form Result certificate to the user Application form to TL-detector s irradiation at SSDL s cobalt beam 3
2.6. External trial using IAEA TLD service. On 10 March 2000 was carried out a reference irradiation at IAEA s Dosimetry Laboratory. In this event were irradiated 15 capsules with TL-detectors using the IAEA s Co-60 beam. The first 10 capsules were irradiated for calibration propose with dose between 1.5 and 2.5 Gy and other 5 capsules were irradiated with unknown dose to test our effectiveness in the dose determination. On 21 March we sent a message with the results obtained for readouts of the TLDs irradiated in the IAEA s reference irradiation. The overall mean of 5 evaluated capsule was 1.99 Gy with a combined standard uncertainty (1σ) of 2%. 2.7. Carried out trial TLD audits of Cobalt units in Radiotherapy Service in the Country. Until the moment have been carried out five dose determinations in different radiotherapy centers. That is to say, the out put of five units of cobalt 60 has been verified, of the 8 in total that actually are being operated for clinical use in all services of radiotherapy of the country. All radiotherapy services of the country have implemented a monthly control of the output constancy of the cobalt 60 beams using their dosimetry system that includes a 2528/3A phantom, which allows an easy and quick positioning of the ionization chamber in the reference position. The constancy of the treatment unit beam output is locally verified regarding the measure carried out under equal conditions in the date that the beam output used in clinical practices of the installation was measured (with a full scatter water phantom). The capsules irradiation for the user was made using the phantoms (2528/3A) available in each radiotherapy department using some acrylic inserts for the IAEA s capsules, made for this aim, with the shape of the phantom cavity (like a Farmer chamber). In the figure 1 is presented a graph that shows the dose determination results in the five postal audits done. In all case the differences obtain between user stated dose and the dose measured by the measuring center were less than ± 5%. 3. Results and discussion 3.1. Annealing procedures It was to possible to select a optimum variant for the annealing to be used in preparation of the TL dosimeters to their irradiation. We achieved that using a second heating of 100 C for 2 hours after the first one to 400 C during 1 hour allows reducing significantly the dispersion of detectors readout regarding the average of group in successive irradiations. Also we concluded that there is not a significant variation among the reproducibility of the detector readout if they cool down inside or outside of the oven, the second variant was selected because less time is used for the execution of this procedure. The results summarized obtained in the response of 4 TL-detectors groups are presented in Table I. 4
Table I The results summarized obtained in the response of 4 TL-detectors groups Irradiation N / Dose [Gy] Group 1 1 2 3 4 Average 2.000 2.001 2.001 2.000 Stand Dev 0.000 0.046 0.065 0.044 % Stand dev 0.0 2.3 3.3 2.2 Group 2 Average 2.000 2.000 1.999 1.999 Stand Dev 0.000 0.036 0.034 0.034 % Stand dev 0.0 1.8 1.7 1.7 Group 3 Average 2.000 2.001 2.058 2.002 Stand Dev 0.000 0.054 0.064 0.088 % Stand dev 0.0 2.7 3.1 4.4 Group 4 Average 2.000 2.000 2.000 2.000 Stand Dev 0.000 0.034 0.025 0.029 % Stand dev 0.0 1.7 1.2 1.4 3.2. Documentation of Quality assurance Manual A draft of Quality Assurance Manual, including work instructions and forms that complies with ISO/IEC/17025 requirements have been established trough this project. 3.3. Successful results in an external trial carried out using the IAEA TLD service. On March 21 2000 we sent a message with the results obtained for readouts of the TLDs irradiated in the IAEA s reference irradiation. The overall mean of 5 evaluated capsule was 1.99 Gy with a combined standard uncertainty (1σ) of 2%. In two days latter we received an encouraging message with the value of the dose delivered to these capsules (1.999 Gy), which means that the deviation of measured and stated doses was 0.5%. The maxim S.D. obtained for the mean of 3 samples (a capsule) was 1.6%. This result was very satisfactory for the implementation and validation of the service. We can summarize the following results for doses evaluated, see Table II Capsule Dw (Gy) 11 1.97 ± 0.04 12 1.98 ± 0.03 13 2.01 ± 0.04 14 1.99 ± 0.03 15 1.98 ± 0.03 Control 2.03 ± 0.03 Table II The results summarize for doses evaluated in an external trial carried out using the IAEA TLD service 3.4. Perform TLD quality audits of Cobalt units in Radiotherapy Service in the country. Coordinated by the National External Audits Group in radiotherapy (EAG), until this moment, have been carried out five postal dose audits with TLDs to equal number of cobalt 60 beams of radiotherapy services of the country, what constitutes 62% of the total of beams of this energy available in clinical practices. The parameter checked was the beam output using the configuration used for periodical 5
dose constancy check, whose reference value is obtained during the annual machine calibration. For the five beams verified the agreement obtained between user stated dose and the measured dose were less than ± 5%. The calibration of the TL- rods used in the dose determination of the capsules irradiated in the user s beams was carried out using the SSDL s reference value of absorbed dose rate to water measured to 5 cm depth in an IAEA water tank, 80 cm to SSD and a 10 cm x 10 cm cobalt 60 beam. The capsules were irradiated in 2528/3A phantom employed the machine time (corrected for transit time) calculated with the absorbed dose rate to water measured in the above full scatter phantom. For all case the agreement obtained between user-stated dose and the measured dose were less than ± 5%, which is considered satisfactory. 4. Conclusions Establishment of a Quality Assurance Programme in laboratories following ISO/IEC 17025 standard is one of the most effective internationally recommended way to increase user and organizational confidence into the quality of works done. Quality Assurance Programme can not be simply copied from other organization. It shall reflect own organizational specificity, directives and policies. However common experience to laboratories with similar tasks, equipment and responsibilities can be identified and properly used during design of QA Programme and QA Manual. The experience with the TL- rods acquired in this project demonstrates that with an appropriate identification of the individual sensibility of these it is possible to obtain a reproducibility of its response inside 2%, and that this dose audits are an useful tool to use for the EAG to contribute an adequate precision in the dosimetry of clinical beams. References 1. Izewska J., et.al Development of a Quality assurance Programme for a radiation therapy dosimetry in developing countries. SSDL Newsletter N 44. IAEA January 2001. 2. Altonen P. The role of SSDL-Helsinki for dosimetry and quality audit in radiotherapy. IAEA- TECDOC-896. IAEA, August 1996. 3. C. Yu and G. Luxton. TLD dose measurement: A simplified accurate technique for the dose range from 0.5 cgy to 1000 cgy. Med. Phys. 26, 1010-1016 (1999). 4. Horowitz, Y. S. The annealing characteristics of LiF: Mg, Ti. Radiat. Prot. Dosim. 30, 219-230 (1990). 5. Derreumaux S, Chavandra J, Bridier A. Rossetti V and Dutreix A. An European quality assurance network for radiotherapy: dose measurement procedure. Phys. Med. Biol. 1995: 40; 1191-1209. 6. IAEA International Atomic Energy Agency. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. Technical Report Series n 398, Vienna, IAEA, 2001. 6