Phantom Scatter Research Articles

Analytical approach for determining beam profiles in water phantom of symmetric and asymmetric fields of wedged, blocked, and open photon beams.

Nowadays, in most radiotherapy departments, the commercial treatment planning systems (TPS) used to calculate dose distributions needs to be verified; therefore, quick, easy‐to‐use, and low‐cost dose distribution algorithms are desirable to test and verify the performance of the TPS. In this paper, we put forth an analytical method to calculate the phantom scatter contribution and depth dose on the central axis based on the equivalent square concept. Then, this method was generalized to calculate the profiles at any depth and for several field shapes — regular or irregular fields—under symmetry and asymmetry photon beam conditions. Varian 2100 C/D and Siemens Primus Plus linacs with 6 and 18 MV photon beam were used for irradiations. Percentage depth doses (PDDs) were measured for a large number of square fields for both energies and for 45° wedge, which were employed to obtain the profiles in any depth. To assess the accuracy of the calculated profiles, several profile measurements were carried out for some treatment fields. The calculated and measured profiles were compared by gamma‐index calculation. All γ–index calculations were based on a 3% dose criterion and a 3 mm dose‐to‐agreement (DTA) acceptance criterion. The γ values were less than 1 at most points. However, the maximum γ observed was about 1.10 in the penumbra region in most fields and in the central area for the asymmetric fields. This analytical approach provides a generally quick and fairly accurate algorithm to calculate dose distribution for some treatment fields in conventional radiotherapy.PACS number: 87.10.Ca

Recovery coefficients determination for partial volume effect correction in oncological PET/CT images considering the effect of activity outside the field of view

The partial volume effect (PVE) has a great impact in quantitative PET/CT imaging. Correction methods have been recently proposed by many authors to make the image quantification more accurate. This work presents a methodology for determining the recovery coefficients (RCs) for PVE correction in PET/CT images. It was taken into account the radioactivity outside the field of view (FOV), which is expected in a patient image acquisition. The NEMA image quality phantom and the NEMA scatter phantom were used. The phantoms were filled with (18)F-FDG for different sphere-to-background ratios. The RCs have been determined from image acquisitions in a Siemens Biograph 16 Hi-Rez PET/CT scanner with and without the scatter phantom. The RC values that ranged from 0.38 to 1.00 without the scatter phantom exhibited a wider variation when this latter was taken into account (from 0.27 to 1.02). This more realistic estimation must be considered if one takes into account that an incorrect SUV measure in tumors leads to errors in the evaluation of the response to therapy based on PET/CT images. The activity outside the FOV should be considered in RCs determination to improve the RC-based PVE correction method.

Investigations of peripheral dose for helical tomotherapy

PurposeWhenever treating a patient with percutaneous radiotherapy, a certain amount of dose is inevitably delivered to healthy tissue. This is mainly due to beam's entry and exit in the region of the target volume. In regions distant from the target volume, dose is delivered by leakage from the MLC and head scatter from the accelerator head and phantom scatter from the target volume (peripheral dose). Helical tomotherapy is a form of radiation therapy with a uniquely designed machine and delivery pattern which influence the peripheral dose. The goal of this work was to investigate peripheral dose in helical tomotherapy. The experiments were used to establish a complex characterization of the peripheral dose. Materials and methodsA 30*30*60cm3 slab phantom and TLD-100 (Lithium fluoride) were used for the experiments. Treatment procedures were generated with the tomotherapy planning system (TPS). Additionally, procedures were created on the Operator Station of the tomotherapy system without a calculation of the dose distribution. The peripheral dose which was produced by a typical tomotherapy treatment plan was measured. Furthermore, these procedures were used to differentiate the parts of the peripheral dose in phantom scatter dose and head scatter and leakage dose. Additionally, the relation between peripheral dose and treatment time and between peripheral dose and delivered dose was investigated. Additionally, the peripheral dose was measured in an Alderson phantom. ResultsDistances of 30cm or more resulted in a decrease of the peripheral dose to less than 0.1% of the target dose. The measured doses have an offset of approximately 1cGy in comparison to the calculated doses from the TPS. The separated head scatter and leakage dose was measured in the range of 1cGy for typical treatments. Furthermore, the investigations show a linear correlation between head scatter leakage dose and treatment time and between scatter dose parts and delivered dose. A peripheral dose of 0.28% of the target dose was measured in the Alderson phantom at a distance of 17.5cm from the edge of the target volume. ConclusionsThe peripheral dose delivered by a tomotherapy treatment is clinically unobjectionable. The measurements confirmed a linear correlation between head scatter and leakage and treatment time and between scatter dose and delivered dose.

MO‐D‐108‐06: Off‐Axis Ratio Based Correction for the Determination of TMR for Flattening‐Filter Free (FFF) Mode Beams

Purpose: Due to the nature of the beam fluence of the flattening‐filter free (FFF) mode from Varian TrueBeam linear accelerator, the tissue‐maximum ratio (TMR) conversion formalism from percent‐depth dose (PDD) may not be appropriate at large field sizes and large depths. The purpose of this work was to establish an analytical correction formalism which accounts for the differences in FFF‐mode beam profiles that corrects for small differences in TMR between the calculations and measurements. Methods: Two FFF‐mode beams (6‐MV and 10‐MV) were investigated using Varian TrueBeam linear accelerator. Initially, the analytical correction formalism starts out with the TMR conversion equation which is the product of the PDD, inverse square, and phantom scatter ratio. A ratio of two off‐axis ratios (OARs) at the surface and depth of interest is determined for a given field size. This OAR ratio is multiplied with the phantom scatter term in the TMR conversion equation to account for small differences in TMR values at large field sizes (>= 15 × 15 cm2) and depths (>= 15 cm). Results: For 10‐MV FFF‐mode, the difference between the measured and the standard TMR formalism ranged from ‐ 3.2% to ‐ 0.9% (mean = ‐ 1.8% and 1‐standard deviation = 0.7%) for field sizes >= 15 × 15 cm2 and >= 15 cm depth. When the proposed correction formalism was applied to the TMR conversion formalism, the difference between the measured and the corrected TMRs for field sizes >= 15 × 15 cm2 and >= 15 cm depth ranged from −0.7% to −0.2% (mean = −0.4% and 1‐standard deviation = 0.1%). Similar relationships were observed for 6‐MV FFF‐mode. Conclusion: The analytical correction formalism which accounts for the differences in the FFF‐mode beam profiles has been shown to correct for the small differences in TMR that were observed without the correction.

Study of Head Scatter Factor in 4 MV Photon Beam Used in Radiotherapy

The 4 MV photon beam offers equal build-up region behavior like Co-60 beam and it plays a major role in head and neck and pediatric radiotherapy. In this study an attempt is made to study the head scatter factor (SC) for 4 MV photon beam using locally designed PMMA and Brass miniphantoms. The SC is measured in combination of PMMA miniphantom with 0.6 cc chamber and Brass miniphantom with 0.6 cc and 0.13 cc chambers. The measured SC is compared with the literature data and it agrees within ± 1.98%. The study reveals that either 0.13 cc or 0.6 cc chamber with PMMA or Brass phantom materials can be used for SC measurements in a 4 MV photon beam. The variation of SSD does not alter the head scatter factor. The collimator exchange effect is found to be within 1, and it is less than that of other linear accelerators. It is also found that the presence of internal wedge has significant contribution to head scatter factor. The Phantom scatter factor is also calculated and it agrees within ±1% with published data.

SU‐E‐T‐393: A Study of Varian Linac Beam Data Equivalency for IMRT

Purpose: The purpose of this study was to compare the photon beam data for four Varian (2300iX) accelerators in six categories: beam quality, phantom scatter factor, output ratio in air, primary off axis ratio, electron disequilibrium factor and benchmark measurements including dose per MU ratio at selected points and a few IMRT plans. Methods: We compared the beam quality of these accelerators, characterizing the attenuation and beam hardening coefficients and percent depth dose (PDD) at 10cm depth. The phantom scatter factor (Sp) and output ratio in air (Sc) were measured in a full phantom and a miniphantom, respectively, at depth of 10cm and SAD = 100cm as a function of field sizes. Results: PDD agreed within 1% and the attenuation coefficients agreed within 3% for the same energy. The output ratio in air Sc is almost machine independent but was energy dependent and agreed within 0.3%. For the same nominal energy, the phantom scatter factor (Sp) measured at 10cm depth agreed within 1.4% regardless of the accelerator. We delivered 3 IMRT plans (prostate, H&N and lung) to study the difference in dose delivery among the four linacs. The dose delivered was compared for depths of 5, 10 and 20cm and to the calculated dose. All measurements were performed using a 2D diode array and analyzed using the distance to agreement (DTA) criteria of 3% and 3mm. The agreement among linear accelerators is much better than the agreement between measurement and calculation, and are better than 3.5% for all IMRT cases studied. Conclusion: Accelerators from the Varian manufacturer, with the same nominal energy and model, can be treated as identical to within 1.8% for conventional treatment as well as for IMRT treatment, provided that caution is given to the dosimetrical characteristics for OAR and small field dosimetry.

SU-E-T-117: Optically Stimulated Luminescence for Diagnostic and Therapeutic Low to Medium Energy X-Ray Beams Experimental Dosimetry

Purpose: Optically stimulated luminescence (OSL) using aluminium oxide crystals (Al2O3:C) is a dosimetry technique (1), already validated in radiation therapy (2). The aim of this study was to characterize the OSL detectors in terms of energy dependence using monochromatic beams in the low-medium energy range (25-85 keV); and to evaluate their potential for experimental dosimetry in therapeutic applications at these energies. Methods: The OSL system is composed of a multichannel reader and alumina crystals (Al2O3:C, 0.09 mm3) that are remotely read by laser light. The OSL signal is directly linked to the absorbed dose. Irradiations in were performed at 8 energy points from 25 to 90 keV using a synchrotron source. Absolute OSL dose measurements were also performed at 80 keV at 2 cm depth in water equivalent phantoms (3) (1-8 Gy range). The detectors have been read over 300 s to collect all the light available from the trapped electrons. Results: The OSL detector show a lower energy dependence (< 10%) in the 25 to 50 keV range than at higher energies (∼ 45% from 50 to 90 keV). OSL is a reproducible absolute dosimetry technique at 80 keV (sigma<5 %). The field size has no other influence on the OSL signal than the one due to phantom scatter output factors (3). Conclusion: We demonstrated that the OSL energy dependance decreases with the beam energy. The conventional models (4) of OSL response in function of beam energy and delivered dose were confronted to our results. The next step of this study is to use OSL for beam penumbra caracterisation, where conventional detectors exhibit a severe energy dependance due to low energy spectra. In vivo dosimetry applications in low-medium enregy radiotherapy are foreseen due to the good reproducibility in OSL signal for absolute dose measurements.

SU-E-T-557: Dosimetric Evaluation of Elekta-XiO Superposition Convolution Algorithm for Siemens MultiLeaf Collimators

Purpose: Elekta-XiO treatment planning system(TPS) uses the same machine layout to simulate both two level jaw system Linacs like Siemens and three level jaw system like Varian Linacs. The purpose of this study is to perform a systematic investigation of the dose calculated from TPS using superposition convolution algorithm and compare that to the measurements for these two types of Linacs. Method and Materials: Siemens Primus and Artiste accelerators with multi-leaf collimators as lower jaws and Varian Trilogy accelerators with tertiary multileaf collimators were used in this study. A series of specially-designed T-shaped MLC fields with different field-aperture-opening-ratios(FAOR) and several clinically used 3DCRT MLC-blocked fields were calculated using the XiO superposition convolution and Clarkson methods. These plans were delivered on the respective Linacs and measurements were compared to the calculations. Several TMR based dose calculations were performed as independent checks on both superposition convolution and Clarkson methods. Monte Carlo simulations were performed to verify the dose calculation and to study the sources of scattering contribution. Results: A larger discrepancy was found between the dose calculated by XiO using superposition convolution algorithm and the dose measured for the Siemens Linacs than Varian Linacs. An average difference of 1.8% between Siemens and Varian Linacs was found for the situation of FAOR >0.33, possibly resulted from modeling the non-existant lower jaws for Siemens Linacs in the XiO convolution algorithm. TMR based dose calculations using a combined collimator scatter and phantom scatter factors with a XiO back-projected-blocked equivalent square(BES) can estimate the delivered dose accurately(0.12% of measured values) including extreme cases for Siemens(FAOR<0.33). Conclusions: A correction of magnitude of 1.8% should be applied to Linacs that use MLCs to replace a pair of jaws in the XiO superposition convolution algorithm when FAOR is >0.33. For FAOR<0.33, the algorithm cannot predict the delivered doses accurately.

SU-E-T-09: Evaluation of Depth Dose Distribution with Limit Scatter Condition and Comparing with Treatment Planning System

Purpose: Beam commissioning data are acquired in full scatter condition but some body parts such as fingers, hands and legs are treated with limited scatter. An evaluation of depth dose distribution in carried out to study limited scatter situation and compared with treatment planning system.Material and Methods: Radiation scatter is considered as an important contribution of dose in patients. This experiment evaluated the depth dose distribution under different size of water phantoms and compared the dose calculated by Eclipse treatment planning software (AAA Algorithm). Measurements were made specially designed limited scatter phantom that were stackable with 6 and 16MV photon beams. The phantom was 3.5cmX12cmX5cm (deep) paper container filled with water was used as a water phantom unit cell that were added to achieve variability in scatter. For different scatter condition purpose, measurement was made by adding different number of phantom unit. An ion chamber reading is used to compare the dose. Results: The measurements show that the depth dose distribution at central axis has weakly dependent on lateral scatter condition. The dose from 6 and 16 MV beams at 1.0 cm depth has 1.02% and 2.70% difference from single unit (considered as 3.5cmX5cmX12cm water phantom) to 9 units (considered as 10.5cmX15cmX12cm water phantom). The difference at dmax for both 6x and 16x are less than 0.5%. Conclusion: The lateral scatter contributes relatively small on central axis doses when the phantom size is comparable to the field size. Advance algorithm used in TPS provides reasonable (<1%) good dose information evaluation in limited scatter condition.

Performance and limitations of positron emission tomography (PET) scanners for imaging very low activity sources

Emerging applications for positron emission tomography (PET) may require the ability to image very low activity source distributions in the body. The performance of clinical PET scanners in the regime where activity in the field of view is <1 MBq has not previously been explored. In this study, we compared the counting rate performance of two clinical PET/CT scanners, the Siemens Biograph Reveal 16 scanner which is based on lutetium oxyorthosilicate (LSO) detectors and the GE Discovery-ST scanner which is based on bismuth germanate (BGO) detectors using a modified National Electrical Manufacturers Association (NEMA) NU 2-2007 protocol. Across the activity range studied (2–100 kBq/mL in a 5.5 mL line source in the NEMA scatter phantom), the BGO-based scanner significantly outperformed the LSO-based scanner. This was largely due to the effect of background counts emanating from naturally occurring but radioactive 176Lu within the LSO detector material, which dominates the observed counting rate at the lowest activities. Increasing the lower energy threshold from 350 keV to 425 keV in an attempt to reduce this background did not significantly improve the measured NECR performance. The measured singles rate due to 176Lu emissions within the scanner energy window was also found to be dependent on temperature, and to be affected by the operation of the CT component, making approaches to correct or compensate for the background more challenging. We conclude that for PET studies in a very low activity range, BGO-based scanners are likely to have better performance because of the lack of significant background.

Using cavity theory to describe the dependence on detector density of dosimeter response in non-equilibrium small fields

The dose imparted by a small non-equilibrium photon radiation field to the sensitive volume of a detector located within a water phantom depends on the density of the sensitive volume. Here this effect is explained using cavity theory, and analysed using Monte Carlo data calculated for schematically modelled diamond and Pinpoint-type detectors. The combined impact of the density and atomic composition of the sensitive volume on its response is represented as a ratio, Fw,det, of doses absorbed by equal volumes of unit density water and detector material co-located within a unit density water phantom. The impact of density alone is characterized through a similar ratio, Pρ −, of doses absorbed by equal volumes of unit and modified density water. The cavity theory is developed by splitting the dose absorbed by the sensitive volume into two components, imparted by electrons liberated in photon interactions occurring inside and outside the volume. Using this theory a simple model is obtained that links Pρ − to the degree of electronic equilibrium, see, at the centre of a field via a parameter Icav determined by the density and geometry of the sensitive volume. Following the scheme of Bouchard et al (2009 Med. Phys. 36 4654–63) Fw,det can be written as the product of Pρ −, the water-to-detector stopping power ratio , and an additional factor Pfl −. In small fields changes little with field-size; and for the schematic diamond and Pinpoint detectors Pfl − takes values close to one. Consequently most of the field-size variation in Fw,det originates from the Pρ − factor. Relative changes in see and in the phantom scatter factor sp are similar in small fields. For the diamond detector, the variation of Pρ − with see (and thus field-size) is described well by the simple cavity model using an Icav parameter in line with independent Monte Carlo estimates. The model also captures the overall field-size dependence of Pρ − for the schematic Pinpoint detector, again using an Icav value consistent with independent estimates.

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.

Monte Carlo-based investigation of water-equivalence of solid phantoms at 137 Cs energy

Investigation of solid phantom materials such as solid water, virtual water, plastic water, RW1, polystyrene, and polymethylmethacrylate (PMMA) for their equivalence to liquid water at 137Cs energy (photon energy of 662 keV) under full scatter conditions is carried out using the EGSnrc Monte Carlo code system. Monte Carlo-based EGSnrc code system was used in the work to calculate distance-dependent phantom scatter corrections. The study also includes separation of primary and scattered dose components. Monte Carlo simulations are carried out using primary particle histories up to 5 × 109 to attain less than 0.3% statistical uncertainties in the estimation of dose. Water equivalence of various solid phantoms such as solid water, virtual water, RW1, PMMA, polystyrene, and plastic water materials are investigated at 137Cs energy under full scatter conditions. The investigation reveals that solid water, virtual water, and RW1 phantoms are water equivalent up to 15 cm from the source. Phantom materials such as plastic water, PMMA, and polystyrene phantom materials are water equivalent up to 10 cm. At 15 cm from the source, the phantom scatter corrections are 1.035, 1.050, and 0.949 for the phantoms PMMA, plastic water, and polystyrene, respectively.

Dosimetric properties of a beam quality‐matched 6 MV unflattened photon beam

The purpose of this study was to report the characteristics of an equivalent quality unflattened (eqUF) photon beam in clinical implementation and to provide a generalized method to describe unflattened (UF) photon beam profiles. An unflattened photon beam with a beam quality equivalent to the corresponding flat 6 MV photon beam (WF) was obtained by removing the flattening filter from a Siemens ONCOR Avant‐Garde linear accelerator and adjusting the photon energy. A method independent from the WF beam profile was presented to describe UF beam profiles and other selected beam characteristics were examined. The short‐term beam stability was examined by dynamic beam profiles, recorded every 0.072 s in static and gated delivery, and the long‐term stability was evidenced by the five‐year clinical quality assurance records. The dose rate was raised fivefold using the eqUF beam. The depth of maximum dose (dmax) shifted 3 mm deeper, but the percent depth dose beyond dmax was very similar to that of the WF beam. The surface dose and out‐of‐field dose were lower, but the penumbra was slightly wider. The variation in head scatter and phantom scatter with changes in field size was smaller; the variation in the profile shape with change in depth was also smaller. The eqUF beam is stable 0.072 s after the beam is turned on, and the five‐year beam stability was comparable to that of the WF beam. A fivefold dose rate increase was observed in the eqUF beam with similar beam characteristics to other reported UF beam data except for a deeper dmax and a slightly wider penumbra. The initial and long‐term stability of the eqUF beam profile is on parity with the WF beam. The UF beam profile can be described in the generalized method independently without relying on the WF beam profile.PACS number: 87.55.de

Poster - Thur Eve - 71: Improved dose accuracy for plan checking IMRT breast plans.

Dose verification as part of plan checking is a critical component of high quality patient care. IMSure QA is a software platform used at the BC Cancer Agency that facilitates dose verification for both conformal and IMRT plans. We have recently initiated treating breast tangents using IMRT at the Fraser Valley Centre and noted increased dose discrepancies (mean difference of -3%) between Eclipse and IMSure's QA module. We identified two potential sources of error: air flash and tissue heterogeneity. We extend our generated fluences 3cm past the breast contour and into air to account for breathing, set-up uncertainties and swelling. IMSure does not account for the fluence in air or air flash. We present an air-flash-correction factor based on the ratios of TMRs and Phantom Scatter Factors which use the field sizes of fields with and without the air flash. In addition, we present a method to improve the heterogeneity correction used by IMSure to better match that used by AAA. Effectively we remove the IMSure's inherent heterogeneity correction and manually apply a AAA-based heterogeneity-correction factor. We evaluated our correction factors on a sample of 8 patients (32 fields) using ANOVA methods to determine which dose corrections most accurately reproduce Eclipse's values. We found the air-flash correction coupled with IMSure's inherent-heterogeneity correction has the best dose accuracy (mean difference improved from -3% to 0.3%). The AAA-heterogeneity correction alone also improved the accuracy (mean difference improved from -3% to - 1.5%), which is acceptable for plan checking purposes.

SU-E-T-523: Modeling Beam Data for Flattening Filter Free (FFF) Photon Beams.

For flattening-filter free (FFF) photon beams, conventional algorithm based on equivalent-square to calculate dose per MU is invalid because of the non-uniform profile. In this study, an empirical algorithm is developed to calculate the dose accurately, which can be used for secondary MU check for IMRT using FFF beams. A kernel-based algorithm based on three parameters (a0, w0, d0) is used to quantify the phantom scatter characteristics of the photon beam. The model is modified to quantify the shape of the FFF at off-axis locations by fitting the primary off-axis ratio (POAR) by a linear function 1 - br, where b is a constant and r is the radial distance. The resulting parameters are used in a kernel-based dose calculation algorithm for dose calculation. It is found that the proposed model can fit the product of the fractional depth doses (FDD) and phantom scatter factors (Sp) for field sizes between 2 and 40 cm and depth between 0 and 40 cm to a max and standard deviations of 1.7% and 0.01% and 1.8% and 0.01%, respectively, for 6 and 10 MV FFF beams. The value of b is 0.025 and 0.0323 for 6 MV and 10 MV photons, respectively, from fitting the POAR. The resulting phantom scatter parameters are consistent with those obtained from MC simulation. If the slope is not taken into account (b = 0), then the model cannot fit the central-axis Sp*FDD accurately and resulted in a maximum error of 3% and 4% for 6 and 10 MV, respectively. We have demonstrated that the shape of POAR from FFF beam will impact on the dose calculation on the central-axis. Conventional equivalent square law concept will not be applicable for dose calculation for FFF beams.

Whole-Body Single-Bed Time-of-Flight RPC-PET: Simulation of Axial and Planar Sensitivities With NEMA and Anthropomorphic Phantoms

A single-bed, whole-body positron emission tomograph based on resistive plate chambers has been proposed (RPC-PET). An RPC-PET system with an axial field-of-view (AFOV) of 2.4 m has been shown in simulation to have higher system sensitivity using the NEMA NU2-1994 protocol than commercial PET scanners. However, that protocol does not correlate directly with lesion detectability. The latter is better correlated with the planar (slice) sensitivity, obtained with a NEMA NU2-2001 line-source phantom. After validation with published data for the GE Advance, Siemens TruePoint and TrueV, we study by simulation their axial sensitivity profiles, comparing results with RPC-PET. Planar sensitivities indicate that RPC-PET is expected to outperform 16-cm (22-cm) AFOV scanners by a factor 5.8 (3.0) for 70-cm-long scans. For 1.5-m scans (head to mid-legs), the sensitivity gain increases to 11.7 (6.7). Yet, PET systems with large AFOV provide larger coverage but also larger attenuation in the object. We studied these competing effects with both spherical- and line-sources immersed in a 27-cm-diameter water cylinder. For 1.5-m-long scans, the planar sensitivity drops one order of magnitude in all scanners, with RPC-PET outperforming 16-cm (22-cm) AFOV scanners by a factor 9.2 (5.3) without considering the TOF benefit. A gain in the effective sensitivity is expected with TOF iterative reconstruction. Finally, object scatter in an anthropomorphic phantom is similar for RPC-PET and modern, scintillator-based scanners, although RPC-PET benefits further if its TOF information is utilized to exclude scatter events occurring outside the anthropomorphic phantom.

SU-E-T-38: Are the Calculation Methods for Determining Tissue-Maximum Ratios from Percent Depth Dose Valid for Flattening Filter-Free Photon Beams?

Purpose: The calculation methods used to determine tissue‐maximum ratio (TMR) from percent depth dose (PDD) were derived by considering differences between TMR and PDD such as the geometry and field size. Phantom scatter factors or peak scatter factors are used to correct dosimetric variation due to field size differences. This relation matches well when the photon beam is flat. However, validity of this equation for flattening filter‐free (FFF) photon beams has never been evaluated. This study aims to evaluate the validity of the equation for FFF beams. Methods:Measured TMR's of 6FFF and 10FFF beams (TrueBeam, Varian) were compared to calculated TMR's. The TMR's were calculated from PDD's using scatter factors both from BJR Supplement 25 and from our own measurements. The same methods were repeated for standard flat 6MV and 10MV beams to confirm the soundness of the measurements and calculations. Results: for the flat beams, the calculated and measured TMR's agree within 1.2% over all field sizes and depths. For the FFF beams, differences are minimal up toa depth of 10 cm for all field sizes. For field sizes less than 20 cm, no noticeable differences are found down to 30 cm depth. The calculated and measured TMR's start to deviate around 15 cm depth for field sizes larger than 20 cm. Differences increase with depth and field size, resulting indeviations up to 3.2%. Conclusions: For field sizes smaller than 20 cm and depths shallower than 15 cm, traditional calculation models for TMR are acceptable for FFF beams; however, beyond those limits, the models start to deviate from measurement. We believe this is due to decreased lateralscatter of the FFF beam. For clinical FFF beams, TMR should either be measured directly, or a new calculation model should be developed.

SU-E-T-536: Is BJR Supplement #25 Recommendation for Megavoltage Energy Independent Scatter Factor Still Valid for Flattening Filter Free Photon Beams?

In the process of measuring and validating fundamental dosimetry data prior to the clinical use of a treatment unit, it is prudent to compare measurements with previously published equivalent data. During the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian True Beam 6 MV FFF and 10 MV FFF) we compared measured Phantom Scatter Factors (Sp) with the Normalized Peak Scatter Factors (NPSFs) from the British Journal of Radiology Supplement 25 (BJR #25). The purpose of this work was to determine whether the energy independent NPSFs BJR#25 are valid for comparison with FFF photon beams. All measurements were performed using a Varian TrueBeam linear accelerator with photon energies of 6-MV, 6-MV FFF and 10-MV FFF modes. For all measurements, a Scanditronix CC04 ionization chamber was used. Both water and in-air measurements were made to obtain NPSFs normalized to the 10 × 10 cm2 field size. For measurements in water, the chamber was positioned at 100 cm source-to-axis distance at the depth of dose maximum. For in-air measurements, the chamber was positioned at 100 cm source-to-axis distance with appropriate build-up cap. From BJR #25, NPSFs were obtained for comparison with the measurements. The NPSF agreement between the 6-MV and 6-MV FFF with the BJR#25 were all within ±0.5%. The agreement ranged from 0.996 to 1.004 and 0.995 to 1.002 for 6-MV and 6-MV FFF, respectively. We also found that 10-MV FFF showed very similar trend. The scatter factors reported in BJR #25 are valid for comparison for 6-MV, 6-MV FFF, and 10-MV. Additional investigation is needed to further understand the dosimetric characteristics of FFF mode.

SU-E-T-149: Scatter Factors Comparison of 6MV Flattened and 7MV Unflattened Beams.

To compare the Collimator scatter factor (Sc), Phantom scatter factor (Sp) and Total scatter factors (Sc,p) of 6MV flattened Beam (6MV FB) and 7MV Unflattened beams (7MV UFB). The flattening filter and primary collimator are the major sources of producing the scattered radiation. In this study, the field sizes from 5×5 cm2 to 40 × 40 cm2 compared for 6MV FB and 7MV UFB. We measured Sc,p with CC 13 chamber at the depth of 10 g/cm2 using IBA blue phantom and Sc measured with CC 13 chamber at the depth of 10 g /cm2 using columnar phantom (TG 74) for 6MV FB and 7MV UFB x-ray beams from a Siemens - ARTISTE linear accelerator. The Sp values derived from the Sc,p and Sc Values. All the values of Sc,p, Sc and Sp are normalized to 10 × 10 cm2 field size the measured values of Sc,p for 6MV FB and 7MV UFB varies from 0.9437 to 1.0651 and 0.9690 to 1.0283 respectively. The Sc values for 6MV FB and 7MV UFB varies from 0.9676 to 1.0212 and 0.9882 to 1.0075 respectively. The Sp values for 6MV FB and 7MV UFB varies from 0.9752 to 1.0429 and 0.9806 to 1.0206 respectively. Our study results shows that Sc, Sp & Sc,p for 7MV UFB for smaller fields up to 10 × 10 cm2 were higher than 6MV FB and for larger fields greater than 10 ×10 cm2 it is vice versa. A scatter factors are measured and compared for 6MV FB and 7MV UFB. A significant variation observed for Sc, Sp and Sc,p values of 6MV FB and 7MV UFB .This is may be due to flattening filter and beam quality.