Effect of Detector Orientation and Influence of Jaw Position in the Determination of Small-field Output Factor with Various Detectors for High-energy Photon Beams.

Background: Accurate dose measurements are difficult in small fields due to charge particle disequilibrium, partial source occlusion, steep dose gradient, and the finite size of the detector. Aim: The study aims to determine the output factor using various detectors oriented in parallel and perpendicular orientations for three different tertiary collimating systems using 15 MV photon beams. In addition, this study analyzes how the output factor could be affected by different configurations of X and Y jaws above the tertiary collimators. Materials and Methods: Small field output factor measurements were carried out with three detectors for different tertiary collimating systems such as BrainLab stereotactic cones, BrainLab mMLC and Millennium MLC namely. To analyze the effect of jaw position on output factor, measurements have been carried out by positioning the jaws at the edge, 0.25, 0.5, and 1.0 cm away from the tertiary collimated field. Results: The data acquired with 15 MV photon beams show significant differences in output factor obtained with different detectors for all collimating systems. For smaller fields when compared to microDiamond, the SRS diode underestimates the output by up to -1.7% ± 0.8% and -2.1% ± 0.3%, and the pinpoint ion chamber underestimates the output by up to -8.1% ± 1.4% and -11.9% ± 1.9% in their parallel and perpendicular orientation respectively. A large increase in output factor was observed in the small field when the jaw was moved 0.25 cm symmetrically away from the tertiary collimated field. Conclusion: The investigated data on the effect of jaw position inferred that the position of the X and Y jaw highly influences the output factors of the small field. It also confirms that the output factor highly depends on the configuration of X and Y jaw settings, the tertiary collimating system as well as the orientation of the detectors in small fields.

Evaluation of International Atomic Energy Agency Technical Report Series-483 Detector-specific Output Correction Factor for Various Collimator Systems.

Aim: In this study, a 6MV flattening filter (FF) and 6MV FF Free (FFF) photon beam small-field output factors (OF) were measured with various collimators using different detectors. The corrected OFs were compared with the treatment planning system (TPS) calculated OFs. Materials and Methods: OF measurements were performed with four different types of collimators: Varian Millennium multi-leaf collimator (MLC), Elekta Agility MLC, Apex micro-MLC (mMLC) and a stereotactic cone. Ten detectors (four ionization chambers and six diodes) were used to perform the OF measurements at a depth of 10 cm with a source-to-surface distance of 90 cm. The corrected OF was calculated from the measurements. The corrected OFs were compared with existing TPS-generated OFs. Results: The use of detector-specific output correction factor (OCF) in the PTW diode P detector reduced the OF uncertainty by <4.1% for 1 cm × 1 cm Sclin. The corrected OF was compared with TPS calculated OF; the maximum variation with the IBA CC01 chamber was 3.75%, 3.72%, 1.16%, and 0.90% for 5 mm stereotactic cone, 0.49 cm × 0.49 cm Apex mMLC, 1 cm × 1 cm Agility MLC, and 1 cm × 1 cm Millennium MLC, respectively. Conclusion: The technical report series-483 protocol recommends that detector-specific OCF should be used to calculate the corrected OF from the measured OF. The implementation of OCF in the TPS commissioning will reduce the small-field OF variation by <3% for any type of detector.

Determination of small-field output factors for beam-matched linear accelerators using various detectors and comparison of detector-specific output correction factors using IAEA Technical Report Series 483 protocol.

Background: Beam matching is widely used to ensure that linear accelerators used in radiotherapy have equal dosimetry characteristics. Small-field output factors (OF) were measured using different detectors infour beam-matched linear accelerators and the measured OFs were compared with existing treatment planning system (TPS) Monte Carlo algorithm calculated OFs. Materials and methods: Three Elekta Versa HDTM and one Elekta InfinityTMlinear accelerators with photon energies of 6 MV flattening filter (FF), 10 MVFF, 6 MV flattening filter free (FFF) and 10 MVFFF were used in this study. All the Linac'swere beam-matched, Dosimetry beam data were ± 1% compare with Reference Linac. Ten different type of detectors (four ionizationchambers and six diode detectors) were used for small-field OF measurements. The OFs were measured for field sizes of 1 × 1 to 10 × 10 cm2, and normalized to 10 × 10 cm2 field size. The uncorrected and corrected OFs were calculated from these measurements. The corrected OF was compare with existing treatment planning system (TPS) Monte Carlo algorithm calculated OFs. Results: The small-field corrected and Uncorrected OF variations among the linear accelerators was within 1% for all energies and detectors. An increase in field size led to a reduction in the difference between OFs among the detectors, which was the case for all energies. The RSD values decreased with increasing field size. The TRS 483 provided Detector-specificoutput-correction factor (OCF) reduced uncertainty in small-field measurements. Conclusion: It is necessary to implement the OF-correction of small fields in a TPS. Special care must be taken to incorporate the corrected small-field OF in a TPS.

Characteristics Performance of Newly Developed Parallel Plate Chamber in Electron Beams.

Background: To investigate the dosimetric performance of newly developed parallel plate chamber in electron beams. Materials and Methods: Rosalina Instruments India Private Limited (Mumbai, Maharashtra, India) has designed and fabricated PRATT2 parallel plate chamber. The various dosimetric characteristics, including pre- and post-irradiation leakage, stability, polarity effect, chamber response with bias voltage, dose linearity, dose rate effect, and chamber absorbed dose calibration, were performed for the developed chamber. The electron beam energies of 4, 6, 8, and 15MeV were used in this study. Results: The pre- and post-irradiation leakage of the developed chamber was within the acceptable limit. The chamber shows good stability over the electron beams used in this study. The maximum error in polarity effect was 0.7% for the developed chamber. The chamber shows the good linear response with dose, and its response is independent of the dose rate for all electron beams. The beam quality correction factor (kQ, Q0) was determined for the all electron beam energies, which was used for determination absorbed dose in electron beams. Discussion: The developed parallel plate chamber (PRATT2) is suitable for dosimetry of electron beams in radiotherapy. The chamber is cost effective and shows precise and reproducible response. The study carried out confirms that the newly designed and fabricated ion chamber can be used in the measurement of absorbed dose for therapeutic electron beams.

Validation of American Association of Physicists in Medicine TG 43 Dosimetry Data in Commercial Treatment Planning System.

AIMS: This study aimed to validate the dosimetric data of low-energy photon-emitting low-dose rate (LE-LDR) brachytherapy seed sources in commercial treatment planning system (TPS). MATERIALS AND METHODS: The LE-LDR seed sources dosimetric data were published in the American Association of Physicists in Medicine (AAPM) Task Group reports TG-43 (1995), TG-43U1 (2004), TG-43U1S1 (2007), and TG-43U1S2. The Bhabha Atomic Research centre (BARC) 125I Ocu-Prosta seed dosimetry data are also available in the literature. The commercially available TPSs are using both two-dimensional (cylindrically symmetric line-source) and one-dimensional (1D) (point source) dose-calculation formalisms. TPS used in this study uses only 1D dose-calculation formalism for permanent implant dosimetry. The point-dose calculation, dose summation, isodose representation, and dose-volume histogram quality assurance tests were performed in this study. The point-source dose-calculation tests were performed for all the available sources in the literature. The others tests were performed for the I-125 BARC Ocu-Prosta seeds. The TPS-calculated doses were validated using manual calculation. RESULTS AND DISCUSSION: In point-source calculation test, the TPS-calculated point-dose values are within ±2% agreement with manually calculated dose for all the seeds studied. The agreement between the TPS and manually calculated dose is 0.5% for the dose summation test. The isodose line pass through the grid points at an equal distance was verified visually on the computer screen for seed used clinically. In dose-volume histogram test, the TPS-determined volume was compared with the real volume. CONCLUSION: Misinterpretation of the TPS test and/or misunderstanding of the TG-43 dose-calculation formalism may cause large errors. It is very important to validate the TPS using literature provided dosimetric data. The dosimetric data of BARC 125I Ocu-Prosta Seed are validated with other AAPM TG-43-recommended seeds. The dose calculation of Best® NOMOS permanent implant TPS is accurate for all permanent implant seeds studied.

Prostate permanent implant brachytherapy with BARC I-125 Ocu-Prosta seeds.

AIM: The aim of this study is implementation and establishment of standard operating procedure for permanent prostate implant brachytherapy technique using BARC I-125 Ocu-Prosta seeds. MATERIALS AND METHODS: The transrectal ultrasound (US)-guided procedure was used for permanent implant procedure. The Best® Sonalis™ US Imaging System and Best NOMOS™ Treatment Planning system was used for volume study and implant procedure. The BARC provided I-125 Ocu-Pro radioactive seeds were implanted into the patient with help of Mick@ Applicator. The implant was performed based on pre-implant dosimetry and intraoperative planning performed during implant procedure. RESULTS: The necessary quality assurance tests were performed for US system before clinical use. The boost dose of 110 Gy was prescribed to the prostate volume of 34.71 cc. About 48 seeds with activity of 0.35 mCi (each) implanted into the prostate volume with reference to intraoperative planning. At the end of procedure, the patient underwent abdomen fluoroscopic examination, to ensure the seed counts in the prostate volume. The day after the implant, the patient was discharged. One month later a planning computed tomography and treatment planning was performed for seed position and dose verification. CONCLUSIONS: Low dose rate permanent implant brachytherapy has the advantage of being a one-time procedure and the existing long term follow-up supports its excellent outcome and low morbidity. BARC-BRIT is supplying the loose 125I seeds. These seeds can be easily implanted into the patient using Mick applicator. However, the pre-implant seed preparation and implant procedure may result some radiation exposure to staff involved. The radiation dose can be minimized with good practice. This report is one patient pilot study and intended to test the implant systems and standard operative procedure henceforth for permanent implant brachytherapy procedure.

Evaluation of optically stimulated luminescence dosimeter for exit dose in vivo dosimetry in radiation therapy.

AIM: The aim of this study was to assess and analyze the exit dose in radiotherapy using optically stimulated luminescence dosimeter (OSLD) with therapeutic photon beams. MATERIALS AND METHODS: Measurements were carried out with OSLD to estimate the exit dose in phantom for different field sizes, various phantom thicknesses, and with added backscatter material. The data obtained were validated with ionization chamber data where applicable. A correction factor was found to determine the actual dose delivered at the exit surface using measured and theoretical dose. RESULTS: The exit dose factor with Co-60, 6 MV, and 18 MV beams for 10 cm phantom thickness was found to be 0.752 ± 0.38%, 0.808 ± 0.34%, and 0.882 ± 0.42%. The dose enhancement factor with field size was ranging from 3% to 7.7% for Co-60 beam, from 2.6% to 6.6% for 6 MV, and from 2.5% to 4.7% for 18 MV beams at 10 cm depth of the phantom with 20 cm backscatter. The percentage reduction in exit dose with no backscatter material at 25 cm depth with field size of 10 cm × 10 cm was 5.6%, 4.4%, and 4.0%, less than the dose with full backscatter thickness of 20 cm for Co-60 beam, 6 MV, and 18 MV beam. CONCLUSIONS: The promising results confirm that accurate in vivo exit dose measurements are possible with this potential dosimeter. This technique could be implemented as a part of quality assurance to achieve quality treatment in radiotherapy.

Dosimetric characterization of optically stimulated luminescence dosimeter with therapeutic photon beams for use in clinical radiotherapy measurements.

AIM: The modern radiotherapy techniques impose new challenges for dosimetry systems with high precision and accuracy in in vivo and in phantom dosimetric measurements. The knowledge of the basic characterization of a dosimetric system before patient dose verification is crucial. This incites the investigation of the potential use of nanoDot optically stimulated luminescence dosimeter (OSLD) for application in radiotherapy with therapeutic photon beams. MATERIALS AND METHODS: Measurements were carried out with nanoDot OSLDs to evaluate the dosimetric characteristics such as dose linearity, dependency on field size, dose rate, energy and source-to-surface distance (SSD), reproducibility, fading effect, reader stability, and signal depletion per read out with cobalt-60 (60 Co) beam, 6 and 18 MV therapeutic photon beams. The data acquired with OSLDs were validated with ionization chamber data where applicable. RESULTS: Good dose linearity was observed for doses up to 300 cGy and above which supralinear behavior. The standard uncertainty with field size observed was 1.10% ± 0.4%, 1.09% ± 0.34%, and 1.2% ± 0.26% for 6 MV, 18 MV, and 60 Co beam, respectively. The maximum difference with dose rate was 1.3% ± 0.4% for 6 MV and 1.4% ± 0.4% for 18 MV photon beams. The largest variation in SSD was 1.5% ± 1.2% for 60 Co, 1.5% ± 0.9% for 6 MV, and 1.5% ± 1.3% for 18 MV photon beams. The energy dependence of OSL response at 18 MV and 60 Co with 6 MV beam was 1.5% ± 0.7% and 1.7% ± 0.6%, respectively. In addition, good reproducibility, stability after the decay of transient signal, and predictable fading were observed. CONCLUSION: The results obtained in this study indicate the efficacy and suitability of nanoDot OSLD for dosimetric measurements in clinical radiotherapy.

The effect of influence quantities and detector orientation on small-field patient-specific IMRT QA: comparison of measurements with various ionization chambers.

Intensity-modulated radiation therapy (IMRT) requires a patient-specific quality assurance (QA) program to validate the treatment plan and a high level of dosimetric accuracy in the treatment delivery. Dosimetric verification generally consists of both absolute- and relative-dose measurements in a phantom using ionization chambers. Measurements were carried out with three different ionization chambers (Scanditronix FC 65G, Exradin A18, and PTW PinPoint 31014) to assess the effects of influence quantities such as the stability, pre- and post-irradiation leakage, stem effect, polarity, and ion recombination on the IMRT point-dose verification with two different orientations. The Exradin A18 and PTW PinPoint ion chambers demonstrated noticeable leakage to magnitudes of 0.6 and 1.2%, whereas negligible leakage was observed with FC 65G ion chamber. Maximum deviations of 0.5 and 0.6% were noticed for the smallest field owing to the ion recombination effect with the PTW PinPoint ion chamber in the parallel and perpendicular orientations, respectively. The calculated total uncertainties of all influence quantities for the FC 65G, A18, and PTW PinPoint ion chambers were 0.5, 0.7, and 1.3%, respectively. The uncertainties determined for each chamber were incorporated into the point-dose measurements of 30 head and neck patient-specific QA plans, and the variation was found to be within ±3%. The magnitude of the leakage in a small-volume ion chamber indicated the significance of incorporating the correction factors in the absolute-dose measurement. A paired t test analysis indicated that the influence quantities significantly affect the point-dose measurements in the patient-specific IMRT QA.

Dosimetric evaluation of newly developed well-type ionization chamber for use in the calibration of brachytherapy sources.

The well-type ionization chamber has been designed for convenient use in brachytherapy source strength calibration. The chamber has a volume of 240 cm3, weight of 2.5 kg, and is open to atmospheric conditions. The well-type ionization chamber dosimetric characteristics such as leakage current, stability, scattering effect, ion collection efficiency, and nominal response with energy were studied. The evaluated dosimetric characteristics of well-type ionization chamber were compared with two other commercially available well-type ionization chambers. The study shows that the newly developed well-type ionization chamber is reliable for air-kerma strength calibration. The results obtained confirm that this chamber can be used for the calibrations of high-dose rate brachytherapy sources.

Dosimetric Performance of Newly Developed Farmer-Type Ionization Chamber in Radiotherapy Practice.

Dose measurement with ionization chamber is essential to deliver accurate dose to the tumor in radiotherapy. The cylindrical Farmer-type ionization chamber is recommended by various dosimetry protocols for dose measurement of radiotherapy beams. The air-equivalent graphite wall Farmer-type ionization chamber (FAR 65 GB) of active volume 0.65 cm3 with aluminum as the central electrode material was fabricated. Various dosimetric parameters were studied for the newly developed ionization chamber in cobalt-60, 6 and 18 MV photon beams. The preirradiation and postirradiation leakage of the chamber was within 0.08%. The long-term stability and the stem effect of the chamber were within 0.07% and 0.3%, respectively. The sensitivity of the ionization chamber was found to be 22.15 nC/Gy. The chamber shows linear response with dose for cobalt-60, 6 and 18 MV photon beams. The ion recombination correction factor increases with increase in bias voltage. For all energies and field sizes, the polarity correction factor is almost closer to unity. The ion recombination and polarity correction measurements show that the polarizing potential and polarity recommended during the calibration of ionization chamber should be used for routine measurement to avoid the uncertainty. The chamber response is independent of dose rate and energy. The chamber is cost-effective and shows precise and reproducible response. The study carried out confirms that the newly fabricated ion chamber can be used in the measurement of absolute dose for high-energy photon beams.

Comparison of individual and composite field analysis using array detector for Intensity Modulated Radiotherapy dose verification.

AIM: To compare the measured and calculated individual and composite field planar dose distribution of Intensity Modulated Radiotherapy plans. MATERIALS AND METHODS: The measurements were performed in Clinac DHX linear accelerator with 6 MV photons using Matrixx device and a solid water phantom. The 20 brain tumor patients were selected for this study. The IMRT plan was carried out for all the patients using Eclipse treatment planning system. The verification plan was produced for every original plan using CT scan of Matrixx embedded in the phantom. Every verification field was measured by the Matrixx. The TPS calculated and measured dose distributions were compared for individual and composite fields. RESULTS AND DISCUSSION: The percentage of gamma pixel match for the dose distribution patterns were evaluated using gamma histogram. The gamma pixel match was 95-98% for 41 fields (39%) and 98% for 59 fields (61%) with individual fields. The percentage of gamma pixel match was 95-98% for 5 patients and 98% for other 12 patients with composite fields. Three patients showed a gamma pixel match of less than 95%. The comparison of percentage gamma pixel match for individual and composite fields showed more than 2.5% variation for 6 patients, more than 1% variation for 4 patients, while the remaining 10 patients showed less than 1% variation. CONCLUSION: The individual and composite field measurements showed good agreement with TPS calculated dose distribution for the studied patients. The measurement and data analysis for individual fields is a time consuming process, the composite field analysis may be sufficient enough for smaller field dose distribution analysis with array detectors.

Plan evaluation and dosimetric comparison of IMRT using AAPM TG119 test suites and recommendations.

In order to verify intensity modulated radiotherapy quality assurance procedure and to establish the practical base line commissioning, American Association of Physicists in Medicine-Task Group 119 test suite DICOM-RT images and structure were downloaded for planning and dosimetric comparison. The square slab phantom of water equivalent plastic was used for the measurement. This phantom can permit point dose measurement with ionization chamber by placing the chamber at 7.5 cm depth in the slab phantom. The planar dose measurements were carried out by positioning the Matrixx detector at 10 cm depth. The planning and measurements were performed as per AAPM TG119 guidelines. The test suite includes AP:PA field, band test, multitarget, prostate, head and neck and C-shape. The ion chamber measurements were within 3% of the planned dose for target and avoidance structure region. The ion chamber measurement results are in good agreement with the TG119 recommendation of ±3% for all the test suites. The planar dose measurements were performed with Matrixx for individual fields at the planned gantry angle. The results show that the pass criteria for γ ≤ 1 were between 93 to 97% for all the test cases. Our results are in good agreement with the TG119 recommendation. The present study aimed to compare the measured dose with the planned dose using computer planning system. The test suites were used to assess the planning and delivery systems so as to provide the basis for IMRT commissioning and QA.

Comparison of dosimetric characteristics of physical and enhanced dynamic wedges.

BACKGROUND: Wedge filters can be used as missing tissue compensators or wedge pairs to alter the shape of isodose curves so that two beams can be angled with a small hinge angle at a target volume without creating a hotspot. AIM: In this study the dosimetric properties of Varian Enhanced Dynamic Wedge (EDW) and physical wedges (PW) were analyzed and compared. MATERIALS AND METHODS: Ionometric measurements of open field output factor, physical wedge output factor, physical wedge factor and EDW factor for photon beams were carried out. A 3D scanning water phantom was used to scan depth dose and profiles for open and PW fields. The 2D ionization matrix was used to measure profiles of physical and EDW wedges. The isodose curves of physical and EDW angles were obtained using a therapy verification film. RESULTS AND DISCUSSION: The PW output factors of photons were compared with the open field output factors. The physical and EDW factors were compared. The difference in percentage depth dose for open and PW fields was observed for both photon beams. The measured isodose plots for physical and EDW were compared. CONCLUSION: The wedge field output factor increases with field size and wedge angle compared to that of the open field output factor. The number of MU to deliver a particular dose with the EDW field is less than that of the PW field due to a change in wedge factor. The dosimetric characteristics, like profile and isodose of EDW, closely match with that of the PW.

Dosimetric study of 2D ion chamber array matrix for the modern radiotherapy treatment verification.

Intensity-modulated radiotherapy treatment demands stringent quality assurance and accurate dose determination for delivery of highly conformal dose to the patients. Generally 3D dose distributions obtained from a treatment planning system have to be verified by dosimetric methods. Mainly, a comparison of two-dimensional calculated and measured data in several coplanar planes is performed. In principle, there are many possibilities to measure two-dimensional dose distributions such as films, flat-panel electronic portal imaging devices (EPID), ion chambers and ionization chamber arrays, and radiographic and radiochromic films. The flat-panel EPIDs show a good resolution and offer a possibility for real-time measurements: however to convert the signal into dose, a separate commercial algorithm is required. The 2D ion chamber array system offers the real-time measurements. In this study, dosimetric characteristics of 2D ion chamber array matrix were analyzed for verification of radiotherapy treatments. The dose linearity and dose rate effect of the I'matriXX device was studied using 6 MV, 18 MV photons and 12 MeV electrons. The output factor was estimated using I'matriXX device and compared with ion chamber measurements. The ion chamber array system was found to be linear in the dose range of 2-500 cGy and the response of the detector was found to be independent of dose rate between 100 MU/min to 600 MU/min. The estimated relative output factor with I'matriXX was found to match very well with the ion chamber measurements. To check the final dose delivered during IMRT planning, dose distribution patterns such as field-in-field, pyramidal, and chair tests were generated with the treatment planning system (TPS) and the same was executed in the accelerator and measured with the I'matriXX device. The dose distribution pattern measured by the matrix device for field-in-field, pyramidal, and chair test were found to be in good agreement with the calculated dose distribution by TPS both for 6 and 18 MV photons (gamma < or = 1: 96%, criteria 3%, 3 mm). Two 7-field IMRT plans (one prostate, one head and neck) dose distribution patterns were also measured with I'matriXX device and compared with film dosimetry. The measurements and evaluation proves that I'matriXX can be used for quantifying absolute dose. Moreover, using I'matriXX as absolute dosimeter in IMRT field verification, avoids the time-consuming procedure of making ionometric measurement for absolute dose estimation and film for dose distribution verification. The I'matriXX can also used for routine quality assurance checks like flatness, symmetry, field width, and penumbra of the linear accelerator beam.