TLD Capsules Research Articles

The IAEA/WHO TLD Audit Program. The results of the TLD postal dosimetry audits in the National Centre for Radiotherapy and Nuclear Medicine Department, Korle-Bu, Accra (1998 to 2012)

During therapeutic procedures, the patient is always protected by the quality assurance program legitimately required for medical exposures. This comprehensive quality assurance programs, or quality management systems, include all components of radiation therapy practice, and has proven to be necessary in reducing the likelihood of dose misadministration. The aim of this paper is to gives a summary of various studies performed during the period of 15 years (1998-2012), and provides the results of a survey conducted by the IAEA. The methodology involve checking the beam output with its dosimeters, and to irradiate two TLD capsules in sequence in such a way that the irradiation time or monitor units calculation would deliver 200 cGy to a ‘tumour’ whose centre (the TLD capsule) is at 5 cm depth. Here, the irradiations were done in reference conditions using the agency’s Perspex holder, and the delivered dose is not a ‘given dose’ at the depth of maximum dose, but a dose at depth. The results of this study demonstrate that the QUATRO audit programme is a very useful tool for reviewing practices at our Centre, and shown beyond doubt to be effective in assuring the quality of dose determination at the Centre. Our study further indicates that it is possible to have the dose determination within acceptance limits by implementing correct procedures and cautiously performing dose calculations and measurements

Mailed dosimetric audit of therapeutic proton beams using thermoluminescence MTS-N (LiF:Mg,Ti) powder – First results

Institute of Oncology in Warsaw provides mailed dosimetric service of radiotherapy units in Poland according to the procedure implemented by IAEA. The service applies the dosimetric thermoluminescence (TLD) system with detectors in form of capsules filled with MTS-N (LiF: Mg, Ti) powder. In these studies dosimetric properties of the TLD system were determined to adapt it for mailed dosimetric audits of proton radiotherapy units. Dose response of TLD capsules filled with MTS-N powder was studied for doses between 1.75 Gy and 2.25Gy in the middle of 5 × 10 × 10 cm3 Spread Out Bragg Peak (SOBP). The SOBP was located at the different position (depth) in the water phantom in order to determine the energy dependence of the system. All exposures were performed at the isocentric gantry with Pencil Beam Scanning field at the Bronowice Cyclotron Center, Institute of Nuclear Physics (IFJ PAN). It was found that for TLD system with MTS-N powder the correction for the supralinear dose response should be applied for doses exceeding 2 Gy. No significant influence of response on the position (depth) of SOBP was observed. The preliminary results show that the standard mailed TLD dosimetry system based on MTS-N (LiF:Mg,Ti) can be adapted for auditing proton radiotherapy units.

SU‐G‐TeP2‐12: IROCHouston and MDAPL SRS Anthropomorphic Phantom Results

Purpose:To report the results of SRS phantom irradiationsMethods:Anthropomorphic SRS head phantoms were sent to institutions participating in NCI sponsored SRS clinical trials and institutions interested in verifying SRS treatment delivery. The phantom shell was purchased from Phantom Laboratory and altered to house dosimetry and imaging inserts. The imaging insert has 1.9 cm diameter spherical target. The dosimetry insert holds two TLD capsules and radiochromic film in the coronal and sagittal planes through the center of the target. Institutions were asked to image, plan and treat the phantom as they would an SRS patient. GammaKnife, CyberKnife and c‐arm accelerator institutions were asked to cover the target with 15 Gy, 20 Gy and 25 Gy, respectively. Following these guidelines and typical planning protocols for these three types of machines gives roughly 30 Gy to the center of the target for all units. Submission of the DICOM digital data set was required for analysis. Criteria of 5% for TLD results and 85% of pixels passing 5%/3mm gamma analysis were applied beginning in 2013.Results:The phantom was analyzed 269 times between the beginning of 2013 to present. The pass rate is 81%. Nineteen of the irradiation results failed only the TLD criteria, 19 failed only the film criteria and 12 failed both. Irradiations included 32 CyberKnife 23 GammaKnife, 3 TomoTherapy and 211 c‐arm units. Planning systems included Eclipse, Ergo, GammaPlan, Hi‐Art, iPlan, Monaco, MultiPlan, Pinnacle, RayStation, XiO and XKnife. Irradiations that were not accompanied with DICOM data were not included in this analysis.Conclusion:The phantom is a valuable end‐to‐end test used to independently verify the accuracy of SRS treatment delivery.This investigation was supported by IROC grant CA180803 awarded by the NCI.

SU‐E‐J‐101: Retroactive Calculation of TLD and Film Dose in Anthropomorphic Phantom as Assessment of Updated TPS Performance

Purpose:To demonstrate a quick and comprehensive method verifying the accuracy of the updated dose model by recalculating dose distribution in an anthropomorphic phantom with a new version of the TPS and comparing the results to measured values.Methods:CT images and IMRT plan of an RPC anthropomorphic head phantom, previously calculated by Pinnacle 9.0, was re‐computed using Pinnacle 9.2 and 9.6. The dosimeters within the phantom include four TLD capsules representing a primary PTV, two TLD capsules representing a secondary PTV, and two TLD capsules representing an organ at risk. Also included were three sheets of Gafchromic film. Performance of the updated TPS version was assessed by recalculating point doses and dose profiles corresponding to TLD and film position respectively and then comparing the results to reported values by the RPC.Results:Comparing calculated doses to reported measured doses from the RPC yielded an average disagreement of 1.48%, 2.04% and 2.10% for versions 9.0, 9.2, 9.6 respectively. Computed doses points all meet the RPC's passing criteria with the exception of the point representing the superior organ at risk in version 9.6. However, qualitative analysis of the recalculated dose profiles showed improved agreement with those of the RPC, especially in the penumbra region.Conclusion:This work has demonstrated the calculation results of Pinnacle 9.2 and 9.6 vs 9.0 version. Additionally, this study illustrates a method for the user to gain confidence upgrade to a newer version of the treatment planning system.

TU‐G‐103‐04: The Accuracy of Monte Carlo Based Dose Estimates Compared to In‐Vivo Dose Measurements

Purpose: The purpose of this study was to extend the validation of Monte Carlo (MC) simulation based dose estimates by comparing simulated values with in‐vivo measurements from clinical patient scans. Methods: The Institutional Review Board approved the acquisition of in‐vivo rectal dose measurements in a pilot study of 9 patients undergoing CT Colonography on an MDCT (LightSpeed VCT, GE Healthcare). Per patient, two scans, prone and supine, were acquired both using a fixed mAs technique. In‐vivo dose measurements were obtained using TLD capsules that were affixed to the inner lumen of rectal catheters. Dose from the TLDs were determined taking into account their energy response. Simulations were performed using voxelized models of the patients and the TLDs based on CT image data. A previously developed MC based model was used to simulate the scanner and each patient scan. Dose to the TLD was estimated for each patient scan. The measured and simulated in‐vivo TLD dose values were compared for all 9 patients and the Root Mean Square (RMS) error was calculated as well as the correlation. Additionally the simulated results were compared with calculated Size Specific Dose Estimates (SSDEs) for each patient. Results: The RMS error between TLD measurements and MC simulations was 12.2% with a maximum of 20.3% and minimum of ‐ 19.7%. Comparison of simulations and SSDE resulted in an RMS of 17.0% with a minimum of ‐ 22.9% and maximum of 22.8%. Conclusion: The results of this study demonstrated that MC simulations using voxelized patient and equivalent source model Result in reasonably accurate doses compared to actual measured doses. Several possible improvements to the performance of the MC model were identified (including reconstructing images at the biggest field of view for full coverage of the anatomy, etc.) Dr. Michael McNitt‐Gray; Institutional research agreement, Siemens AG; Recipient research support Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski, LLC; Maryam Khatonabadi and Kyle McMillan ripient research support Siemens AG

SU-GG-T-442: Dose Verification of SRS Monte Carlo Plan with a Moving Anthropomorphic Phantom

Purpose: Dose verification of Stereotactic Radiosurgery plan using Monte Carlo for tissue heterogeneity correction with a moving anthropomorphic phantom Method and Materials: An anthropomorphic lung phantom has gold fiducials, TLD capsules and GAFChromic films imbedded in a target in the left lung, TLD capsules are also inserted in heart and cord structures. The phantom was scanned, CT images were sent to Cyberknife Multiplan for planning. Target and critical structures were contoured. A SRS treatment plan with no tissue heterogeneity correction was generated to give 6Gy to 95% of the target, 5.4Gy to 99% of the target, no point > 2cm from target > 3.5Gy, conformai index < 1.2, cord < 1.8Gy, heart < 3Gy, 10% of whole lung < 2Gy. The plan was recalculated with Monte Carlo with 2% uncertainty for tissue heterogeneity correction for dose delivery. During delivery the phantom moved in superior/inferior and anterior/posterior direction, Cyberknife Synchrony tracking system took orthogonal x-ray images of the phantom while infra-red camera tracked LED diodes placed on the phantom to track phantom motion in real time. After the fiducials were identified by the tracking program and correlated with the motion of LED diodes, a motion model of the target was built; radiation started with the Cyberknife robot following the predicted target motion and adjusted its position during irradiation. Throughout delivery, the motion model was updated and adjusted with new x-ray images of the fiducials. Results: Target TLD was 98.3% of Monte Carlo dose; heart and cord TLD were within 7% acceptability. Profiles from 3 orthogonal films were displaced L/R 0mm/3mm, P/A 1mm/5mm and I/S 1mm/2mm from treatment plan profiles, within 5mm acceptability. Conclusion: Cyberknife Multiplan with Monte Carlo accurately predicts dose in heterogeneous tissue; Cyberknife Synchrony tracking system delivers dose accurately to a target in a moving phantom.

The use of LiF (TLD‐100) as an out‐of‐field dosimeter

The commonly used thermoluminescent dosimeter TLD‐100 (Harshaw Chemical Company, Solon, OH) responds not only to photons and electrons, but also to neutrons that are produced during high‐energy therapies. As a result, TLD‐100 measurements outside of the treatment field are suspect when high‐energy radiation is used. Although alternatives such as TLD‐700 do not respond to neutrons, specialty dosimeters of this kind are expensive and are not routinely used in most clinics. In the current study, we examined the accuracy of TLD‐100 in measuring the out‐of‐field photon dose as a function of treatment energy.To determine the accuracy of TLD‐100 as compared with TLD‐700, TLD‐100 was irradiated outside of the treatment field by medical accelerators operated at 6, 10, 15, and 18 MV. In an effort to eliminate the response of TLD‐100 to neutrons, TLD capsules were encased in varying thicknesses of cadmium foil (0.25 – 0.75 mm) before being irradiated at 18 MV. The out‐of‐field TLD‐100 was found to be accurate at 6 MV and 10 MV, but to be substantially over‐responsive at 15 MV and 18 MV (by up to 1063% relative to TLD‐700). By wrapping the TLD‐100 in up to 0.75 mm of cadmium, it was possible to drastically reduce (down to 39% on average) the over‐response of the TLD‐100; however, total removal of the over‐responsiveness was not possible. Although TLD‐100 is well suited for measuring out‐of‐field dose at energies as high as 10 MV, at higher energies (15 MV or greater), this dosimeter over‐responds substantially and should not be used. Although encasing the TLD in cadmium minimized over‐response to a degree, the reduction was not sufficient to make TLD‐100 viable for measuring out‐of‐field dose at high treatment energies.PACS numbers: 87.52.‐g, 87.53.‐j, 87.53.‐Dq

Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer

With the increased use of cone beam CT (CBCT) for daily patient setup, the accumulated dose from CBCT may be significantly higher than that from simulation CT or portal imaging. The objective of this work is to measure the dose from daily pelvic scans with fixed technical settings and collimations. CBCT scans were acquired in half-fan mode using a half bowtie and x-rays were delivered in pulsed-fluoro mode. The skin doses for seven prostate patients were measured on an IRB-approved protocol. TLD capsules were placed on the patient's skin at the central axis of three beams: AP, left lateral (Lt Lat) and right lateral (Rt Lat). To avoid the ring artefacts centred in the prostate, the treatment couch was dropped 3 cm from the patient's tattoo (central axis). The measured AP skin doses ranged 3–6 cGy for 20–33 cm separation. The larger the patient size the less the AP skin dose. Lateral doses did not change much with patient size. The Lt Lat dose was ∼4.0 cGy, which was ∼40% higher than the Rt Lat dose of ∼2.6 cGy. To verify this dose asymmetry, surface doses on an IMRT QA phantom (oval shaped, 30 cm × 20 cm) were measured at the same three sites using TLD capsules with 3 cm table-drop. The dose asymmetry was due to: (1) kV source rotation which always starts from the patient's Lt Lat and ends at Lt Lat. Gantry rotation gets much slower near the end of rotation but dose rate stays constant and (2) 370° scan rotation (10° scan overlap on the Lt Lat side). In vivo doses were measured inside a Rando pelvic heterogeneous phantom using TLDs. The left hip (femoral head and neck) received the highest doses of ∼10–11 cGy while the right hip received ∼6–7 cGy. The surface and in vivo doses were also measured for phantoms at the central-axis setup. The difference was less than ∼12% to the table-drop setup.

TH‐D‐VaIB‐02: Skin and Body Dose Measurements for Varian Cone‐Beam CT (CBCT) During IMRT for Prostate Cancer

Purpose: With the increased use of CBCT for daily patient setup, kV dose delivered to patient should be investigated. This study is to measure skin and body dose from Varian daily CBCT for prostate patients. Methods and Materials: CBCT scans were acquired in half‐fan and pulsed‐fluoro mode with a half bow‐tie mounted. A technical setting of 125kV, 80mA and 25ms was used. Skin and body doses were first measured for a Rando pelvic and an IMRT QA phantom, set centrally, with TLD and a cylindrical chamber. Then skin dose for 7 prostate patients undergoing daily CBCT was measured. To avoid the ring artifacts centered at prostate, the treatment couch was dropped 3cm from patient's tattoo. TLD capsules were placed on patient's skin at 3 sites: AP, Lt Lat and Rt Lat. Phantom measurement was also made for this setup. The absorbed dose was determined by the air‐kerma‐based calibration method recommended by TG61. Results: For phantoms set centrally, measured skin dose was ∼6 cGy, ∼5.6 cGy, ∼3.7cGy at AP, Lt Lat, and Rt Lat, respectively. Body dose at the center was ∼3–4 cGy. With table dropping (TD), only AP skin dose was increased ∼12%. Patient AP skin dose varied with separation, ranging 4–6 cGy for thicker patients (AP 23 – 33 cm) and 6 – 8 cGy for thinner patients. Minimum changes were observed on lateral dose for patients with different size. Lt Lat skin (4cGy) and bone (9cGy) doses were higher than Rt Lat skin (3cGy) and bone dose (6cGy) Conclusions: Daily CBCT provides better patient setup but it increases skin and body dose. The dose can range from 120 – 330 cGy for skin and 120 – 380 cGy for body during the 42 daily fractions delivered for IMRT prostate patients.

Stem corrections for ionization chambers.

Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem efffect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 60Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25- and 100-R chambers), a Capintec 192 dosimeter with a Farmer 0.6-cm3 probe, a PTW transit dose probe, and an EG and G IC-18 probe with a Keithley 610-B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.