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.

Analysis of fetal dose using Optically Simulated Luminescence Dosimeter and ion chamber in randophantom for various radiotherapy techniques.

To analyse the fetal dose in all three trimesters in patients treated for brain tumors during pregnancy, a modified rando phantom representing three different trimesters was used with provisions for insertion of ion-chamber and Optically Simulated Luminescence Dosimeter (OSLD). The measurement regions were chosen at the level of fundus, umbilicus and pubis. Seven different treatment plans with 6FF and 6FFF beam energies were generated. Treating pregnant patients with brain tumors is safe irrespective of planning modalities except 3DCRT plan where the dose is 10.24 cGy.

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.

Dosimetric evaluation of hybrid and volumetric-modulated arc therapy plan for left-sided chest wall irradiation in MONACO treatment planning system.

Background and Objective: Radiation induced toxicities in heart and lungs are diminishes the survival rate of cancer patients. The purpose of this study is to evaluate the dosimetric parameters of hybrid plans for chest wall irradiation of left breast carcinoma patients using Monaco treatment planning system and compare with the volumetric-modulated arc therapy (VMAT) treatment plans. Materials and Methods: Fifteen carcinoma left breast patients were randomly selected to evaluate the advantage of hybrid plan over VMAT. Hybrid plans were generated with 70% and 30% dose contribution from forward intensity-modulated radiotherapy and VMAT, respectively, whereas VMAT had been done with full prescription. Conformity and homogeneity indices were evaluated for target coverage between hybrid and VMAT plans. Results: Hybrid plan has proved its superiority over VMAT in terms of better organ at risk sparing and lesser low dose spillage and at the same time providing comparable target coverage. In low-dose spectrum, VMAT showed higher dose-volume than the hybrid plan. The maximum variation was found to be 44.75% at 7 Gy and the minimum dose difference was observed at 1 Gy (6.02%). Conclusion: This study suggests that the hybrid plan could be a better option for left-sided chest wall irradiation in regular clinical practice.

Dose Estimation Using Optically Stimulated Luminescence Dosimeter and EBT3 Films for Various Treatment Techniques in Alderson Rando Phantom and Estimation of Secondary Cancer Incidence for Carcinoma of Left Breast.

Aim: The aim of this study was to measure the dose to planning target and organ at risk (OAR) using Alderson Rando phantom for various treatment techniques in left breast radiotherapy and to estimate the secondary cancer incidence. Materials and Methods: Eleven different combinations of plans containing four techniques (three dimensional conformal radiotherapy, intensity-modulated radiation therapy [IMRT], volumetric modulated arc therapy [VMAT], and combination of 3DCRT and VMAT plans (HYBRID)) were created with 6 MV FF and 6 MV FFF (flattening filter and flattening filter-free) photon energies in phantom. Planned target volume and OAR doses in 23 different locations were measured using optically stimulated luminescence dosimeter (OSLD) and EBT3 films. Assuming the age of exposure as 30 years, lifetime attributable risk (LAR) was estimated based on excess absolute risk (EAR) models outlined in the Biological Effects of Ionizing Radiation VII report. Results: Film showed maximum deviations of 6.15% with IMRT_C_FF plan when compared with treatment planning system (TPS). The maximum percentage difference of 1.7% was found with OSLD measurement when compared with TPS for VMAT_T_FFF plan. EAR estimation was done for all the OARs including target. The LARs for left lung, right lung, and right breast were evaluated. The maximum LAR values of 2.92 ± 0.14 were found for left lung with VMAT_C_FFF plans. Conclusion: This study shows that both OSLD and EBT3 films are suitable for dose measurements using Rando phantom. OSLD shows superior results when compared with films, especially with relatively larger distances. Maximum LAR values were found with VMAT_C_FFF plans. Considering the secondary cancer risk associated with the patients treated in the younger age group, it is suggested that in vivo dose estimation should be a part of treatment quality audit whenever possible.

Determination of Collimator Helmet Factors for Leksell Gamma Knife 4C Unit Using GAF Chromic EBT3 Film and ImageJ Software.

BACKGROUND: Measurement of Collimator helmet factors (CHF) is an important quality assurance procedure to be performed on Leksell Gamma Knife unit at regular interval to make sure that the interchangeable collimator helmet fit into the source channels without any positional inaccuracy which leads to major treatment error. The primary aim of this study is to measure the CHFs for Elekta Leksell Gamma knife 4C helmets using GafChromic EBT3 film and Image J software. METHODS: GafChromic EBT3 film, EPSON expression 10000 XL scanner and Image J analysis software was used for this study. The calibration curve of GafChromic EBT3 film was generated with known dose values for 14 mm collimator helmet using ImageJ software. The collimator helmet factor (CHF) for 4mm, 8mm and 14 mm collimator helmets were measured by normalizing dose rates of 4mm, 8mm and 14 mm to the dose rate of 18 mm collimator helmet using the previously generated calibration curve. The measured CHF was compared to Elekta reference value and previously published mean values. RESULTS: The measured CHFs were 0.896, 0.958, and 0.986 for 4mm, 8mm and 14mm collimators respectively. The percentage difference obtained was 1.7 %, 0.21 %, 0.1 % between measured values and reference values. CONCLUSION: The measurement of CHFs in LGK 4C unit using GafChromic EBT3 film and ImageJ software is a reliable method to verify the manufacturer quoted CHFs in routine quality assurance procedures.

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.

Verification of Surface Dose for Flattening Filter and Flattening Filter Free Beams in Beam-Matched Medical Linear Accelerators.

BACKGROUND: The purpose of this study was to evaluate the surface dose (SD) of 6 and 10 MV flattening filter beam (FF) and flattening filter free (FFF) beam for different square field sizes in three Beam-matched medical linear accelerators using a parallel-plate ionization chamber. MATERIALS AND METHODS: The experiment was carried out in a phantom composed of 40×40 cm2 solid Water slabs of varying thickness. Further sheets of solid water phantom were added to take readings in the build-up region for both SSD and SAD technique. Surface doses are measured with a PPC-05 chamber and DOSE 1 electrometer, at measurement depth of 1 mm interval and all results are plotted relative to the dose measured at Dmax for various field sizes. Surface dose readings are therefore reported as relative surface dose. RESULTS: Surface dose increased linearly with field size for both FF and FFF photon beams in all three beam-matched linear accelerators in both SSD and SAD setup. The surface dose of FFF was higher than FF beams in all field sizes. For the given energy the surface dose difference (relative to 10x10 cm2 field size of 6FF) between FF and FFF beam was larger for large field size. For 6FF and 6FFF beam the surface dose difference for 5x5 cm2 is -5.27%, and for 30x30 cm2 it is 12.91%. The measured surface dose differences between linear accelerators are not statically significant (P>0.989). Similarly, the surface dose difference between SSD and SAD setup was also analysed and had no statistical significance (P>0.849). CONCLUSION: Study showed that the surface dose difference between beam-matched linear accelerators are insignificant. The surface dose difference between SSD and SAD setup were also found negligible. Most importantly, changing patients between beam-matched linear accelerators will not have any significant changes in surface dose in clinical setup.
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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.

Metal-Free, Graphene Oxide-Based Tunable Soliton and Plasmon Engineering for Biosensing Applications.

The quest for auxiliary plasmonic materials with lossless properties began in the past decade. In the current study, a unique plasmonic response is demonstrated from a stratified high refractive index (HRI)-graphene oxide (GO) and low refractive index (LRI)-polymethyl methacrylate (PMMA) multistack. Graphene oxide plasmon-coupled emission (GraPE) reveals the existence of strong surface states on the terminating layer of the photonic crystal (PC) framework. The chemical defects in GO thin film are conducive for unraveling plasmon hybridization within and across the multistack. We have achieved a unique assortment of metal-dielectric-metal (MDM) ensuing a zero-normal steering emission on account of solitons as well as directional GraPE. This has been theoretically established and experimentally demonstrated with a metal-free design. The angle-dependent reflectivity plots, electric field energy (EFI) profiles, and finite-difference time-domain (FDTD) analysis from the simulations strongly support plasmonic modes with giant Purcell factors (PFs). The architecture presented prospects for the replacement of metal-dependent MDM and surface plasmon-coupled emission (SPCE) technology with low cost, easy to fabricate, tunable soliton [graphene oxide plasmon-coupled soliton emission (GraSE)], and plasmon [GraPE] engineering for diverse biosensing applications. The superiority of the GraPE platform for achieving 1.95 pg mL-1 limit of detection of human IFN-γ is validated experimentally. A variety of nanoparticles encompassing metals, intermetallics, rare-earth, and low-dimensional carbon-plasmonic hybrids were used to comprehend PF and cavity hot-spot contribution resulting in 900-fold fluorescence emission enhancements on a lossless substrate, thereby opening the door to unique light-matter interactions for next-gen plasmonic and biomedical technologies.

Is High Definition MLC Dosimetrically Superior to Standard Definition MLC for SIB-SBRT for Carcinoma Prostate.

OBJECTIVE: The study was conducted to quantitatively evaluate the dosimetric effects of high definition (2.5 mm) and standard definition (5.0 mm) MLC on the quality of SBRT plans using SIB-IMRT and SIB-VMAT technique for carcinoma prostate and also to evaluate the dosimetric advantage of one technique over the other. MATERIALS AND METHODS: Seventeen annonymized planning CT data sets were used to generate plans for both VMAT and IMRT techniques using 2.5 mm and 5.0 mm MLC.The prescription to the nodule was 45Gy in 5 fractions and to the prostate was 35Gy in 5 fractions.CI, GI, D2%, D98%, D50% and V95% for target; D2%, Dmean, V80%, V20% for OAR's; V5% of the irradiated volume, and delivered MU's were analyzed.An independent t-test was used to compare the plans. Patient specific QA for all plans were also performed and analyzed. RESULTS: Minor difference in dosimetric indices was observed between 2.5mm and 5mm MLC VMAT plans, except D2% (PTV35) and D98% (GTV45) were better in 2.5mm MLC plans (p.

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 Validation of Volumetric Modulated Arc Therapy with Three 6MV Beam-Matched Linear Accelerators

Background: To avoid inconvenience to patients due to linear accelerator down time in busy radio-therapy departments, treatment plans can be switched between linear accelerators provided that all exhibit the same same dosimetric characteristics. In other words linear accelerators should be beam-matched. Aim: The aim of this study was to evaluate the clinical significance of beam-matching using VMAT plans. Materials and Methods: Dosimetric data with a 6MV beam from am Clinac 2100CD were taken as baseline values and other two units, a 2300CD and a Unique Performance, were factory tuned in accordance. An analysis of PDD data was performed for different field sizes to evaluate energy matching. Beam profiles for field sizes of 10×10 cm2 and 40 × 40 cm2 at depths of 1.5 cm and 10 cm were analyzed. The relative output factor and MLC dosimetric properties were compared with each machine to determine variability among the different models. Thirty patients from our database were selected, ten each for head and neck, thorax and pelvis sites. VMAT plans were created in the Eclipse treatment planning system for a Clinac 2100 CD for reference. and verification plans were created for each to compare point dose measurements. Results: The TPR 20/10 for 10 × 10 cm2 was well matched, showing no energy differences. Deviation of all point dose measurements fell within ±3%. Planar dose maps all showed greater than 95% of points with a passed area γ-value less than 1. Conclusion: Our study evaluation of beam matching with treatment planning modeling showed good agreement fior 6 MV beams across all three linear accelerators used in our clinical environment.

Dosimetric Validation of Volumetric Modulated Arc Therapy (VMAT) Using AAPM TG-119 Benchmark Plans in an Upgraded CLINAC 2100CD for Flattening Filter Free (FFF) Photon Beams

Background: Recently we have upgraded our Varian Clinac 2100CD with a 6MV FFF beam, this upgrade being the first of its kind in our country. Even though the dosimetric characteristics of FFF beams have been reported both in experimental and Monte Carlo studies, application in planning and delivery is complex. The aim of this study was to validate the commissioning of upgraded FFF beams dosimetrically using AAPM TG-119 bench mark plans for VMAT and to make a comparison with IMRT plans for both flattened filtered and FFF beams. Materials and Methods: AAPM TG-119 proposes a set of test clinical cases for testing the accuracy of IMRT planning and delivery systems. For these clinical cases we generated four treatment plans using IMRT FF, IMRT FFF, VMAT FF and VMAT FFF on a Varian Clinac 2100CD machine equipped with a millennium 120 MLC in Eclipse treatment planning system. Dose prescription and planning objectives were set according to the TG-119 goals and plans were scored based on planning objectives. Plans were compared using dose coverage, the conformity index and the homogeneity index. Point doses were measured at points recommended by TG-119 using a CC13 ion chamber. Planar dosimetry was accomplished using Imatrix and gamma evaluation was conducted using Omnipro IMRT software. Results: Dose distributions of FFF beam based plans were comparable to FF plans for both IMRT and VMAT. Our planning results matched TG-119 planning results. Measured point doses were within ±2% of planned doses and planar dosimetry gamma values were <1 for >95% of data points for all plans. Conclusion: We found a reduction of 40% treatment time for FFF against FF beams for sliding window IMRT. Upgraded FFF beams were in good agreement with TG-119 benchmark plans and goals.

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.

Response of Nanodot Optically Stimulated Luminescence Dosimeters to Therapeutic Electron Beams.

Response of Al2O3:C-based nanoDot optically stimulated luminescence (OSL) dosimeter was studied for the dosimetry of 6, 9, 12, 16, and 20 MeV therapeutic electron beams. With reference to ionization chamber, no change in the response was observed with the change in the energy of electron beams for the field size from 6 cm × 6 cm to 25 cm × 25 cm, dose rates from 100 MU/min to 600 MU/min, and the linearity in the response up to 300 cGy. The fading of the transient signal was higher for 20 MeV electron beam than that of 6 MeV electron beam by about 5% as compared to value at 20 min after irradiation. The depletion of OSL signal per readout in 200 successive readouts was also found to change with dose and energy of electron beam from 6 MeV (9% and 12% per readout at 2 and 10 Gy, respectively) to 20 MeV (9% and 16% at 2 and 10 Gy, respectively). The OSL sensitivity changed in the range from 2% to 6% with accumulated doses from 2 to 8 Gy and with electron energy from 6 to 20 MeV, but the sensitivity could be reset using an optical annealing treatment. Although negligible fading for postirradiation storage from 20 min to several months, acceptable precision and linearity in the desired range, and high reproducibility makes nanoDot dosimeters very attractive for the dosimetry of therapeutic electron beams, a note should be made for changes in sensitivity at doses beyond 2 Gy and electron beams energy dependence in reuse, short-term fading, and signal depletion on repeated readout.

Analysis of small field percent depth dose and profiles: Comparison of measurements with various detectors and effects of detector orientation with different jaw settings.

The advent of modern technologies in radiotherapy poses an increased challenge in the determination of dosimetric parameters of small fields that exhibit a high degree of uncertainty. Percent depth dose and beam profiles were acquired using different detectors in two different orientations. The parameters such as relative surface dose (D S), depth of dose maximum (D max), percentage dose at 10 cm (D 10), penumbral width, flatness, and symmetry were evaluated with different detectors. The dosimetric data were acquired for fields defined by jaws alone, multileaf collimator (MLC) alone, and by MLC while the jaws were positioned at 0, 0.25, 0.5, and 1.0 cm away from MLC leaf-end using a Varian linear accelerator with 6 MV photon beam. The accuracy in the measurement of dosimetric parameters with various detectors for three different field definitions was evaluated. The relative D S(38.1%) with photon field diode in parallel orientation was higher than electron field diode (EFD) (27.9%) values for 1 cm ×1 cm field. An overestimation of 5.7% and 8.6% in D 10 depth were observed for 1 cm ×1 cm field with RK ion chamber in parallel and perpendicular orientation, respectively, for the fields defined by MLC while jaw positioned at the edge of the field when compared to EFD values in parallel orientation. For this field definition, the in-plane penumbral widths obtained with ion chamber in parallel and perpendicular orientation were 3.9 mm, 5.6 mm for 1 cm ×1 cm field, respectively. Among all detectors used in the study, the unshielded diodes were found to be an appropriate choice of detector for the measurement of beam parameters in small fields.

Cranio Spinal Irradiation of Medulloblastoma Using High Precision Techniques - A Dosimetric Comparison.

Radiotherapy planning, delivery and junction dose verification remain exigent for Cranio Spinal Irradiation (CSI) in medulloblastoma patients. This study aims to evaluate high precision techniques such as Intensity Modulated Radiation Therapy (IMRT), Rapid Arc Therapy (RA) with and without flattening filter (FF) on the basis of dosimetric analysis. Five patients treated with jagged junction Intensity Modulated RadioTherapy (IMRT) using dynamic Multi Leaf Collimators (MLC) were randomly selected for this retrospective study. IMRT, Rapid Arc (RA) plans were simulated in the same CT data set with and without flattening filter. Total dose prescribed was 28.80 Gy in 16 fractions. An evaluation criterion of 98% of PTV receiving 100% of the prescription dose was followed in all plans. Twenty treatment plans with 260 Dose Volume Histograms (DVHs) was created. Dosimetric parameters such as Dmax, Dmin, Dmean, V95%, V107%, CI for PTV and Dmax, Dmean, V80%, V50%, V30%, V10% for Organs At Risk (OAR) were extracted from DVHs. Treatment delivery efficiency was also evaluated for total Beam On Time (BOT). FFF Rapid Arc therapy : 6F_RA) resulted in conformal doses throughout the cranio spinal axis. FF and FFF dynamic IMRT had minimal V107%, 1.23% and 2.88% compared to 49.15 and 66.36 of rapid arc therapy (with and without FF). 6F_IMRT resulted in lesser mean doses to eyes, liver, lungs and kidneys. Heart mean dose was less (3.08 Gy) with 6X_IMRT. Thyroid and esophagus doses could be reduced to about 41.2% and 10% respectively with 6F_RA. The BOT for the treatment techniques were 3.43 min (6X_IMRT), 1.59 min (6F_IMRT), 5min (6X_RA), 4.5 min (6F_RA). Removal of flattening filter in IMRT could improve dose coverage along the caniospinal axis and normal tissue sparing. A reduction of 46.3% BOT could increase treatment efficiency of 6F_IMRT compared to 6X_IMRT. CSI could be simpler since junction doses can be evaded in IMRT and RA techniques.