Standard CTDI Phantoms Research Articles

Visualization and comparison of CT dose distribution between axial and helical acquisitions.

It is challenging to assess the accuracy of volume CT Dose Index (CTDIvol ) when the axial scan modes corresponding to a helical scan protocol are not available. An alternative approach was proposed to directly measure using helical acquisitions and relatively small differences (<20%) from CTDIvol were observed. To visually demonstrate the 3D dose distribution for both axial and helical CT acquisitions and quantitively compare and CTDIvol . 3D dose distribution within the standard CTDI phantoms (16 and 32cm diameter) from a single CT projection, Dp (x,y,z) was first generated using Monte Carlo simulation (GEANT4) with 9×108 photons per combination of tube voltage (80-140kV), collimation width (1-8cm), and z-axis location of the central ray of the x-ray beam, with a spatial resolution of 1 mm3 . These dose distributions from one single projection were analytically ensembled to simulate 3D dose volumes DA (x,y,z) and DH (x,y,z) for axial and helical scans, respectively, with different helical pitches (0.3-2) and scan lengths (100-150mm). 2D planar dose distributions were obtained by integrating the inside 100mm of the dose volumes. CTDIvol and were calculated using the planar dose data at corresponding pencil chamber locations and the percentage differences (PD) were reported. High spatial resolution 3D CT dose volumes were generated and visualized. PDs between and CTDIvol had strong dependency on scan length and peripheral chamber locations, with subtle dependency on collimation width and pitch. PDs were mostly within the range of±3% for a scan length of 150mm with four peripheral chamber locations. With a scan length covering the entire phantom length, directly measured from helical scans can serve as an alternative to CTDIvol only if all four peripheral locations were measured.

Technical Note: Evaluating automatic tube current modulation in CT using the standard CTDI dosimetry phantom.

To assess the utility of the standard body CTDI phantom in characterizing the operation scheme of tube current modulation (TCM) systems in CT. The body CDTI phantom was used to characterize two TCM systems: TCM1 and TCM2 , implemented in scanners from different vendors. The phantom was aligned at the gantry isocenter in two configurations. In configuration A, the facet planes of the phantom were parallel to the patient table, while in configuration B they were vertical to the patient table and parallel to the patient's long axis. Acquisitions were performed using the routine abdominal examination protocol. mA(z) profiles were recorded from images' DICOM header. The water equivalent diameter (dw ) and oval ratio (OR) were calculated as a function of z-axis location. Image noise was defined as the standard deviation (SD) of the mean Hounsfield unit value measured in a region of interest at the center of the phantom's image. Regression analysis was performed to modulated mA and SD vs dw and OR. The spatial concordance between the change in phantom size and change in mA (SCmA ) was calculated as the percent difference in the slope of mA(z) change between the 1st and 2nd half of the phantom. The corresponding spatial concordance between the change in phantom size and change in image noise (SCnoise ) was calculated. Modulated mA(z) along the z-axis did not substantially differentiate between configurations A and B. Correlation between ln(mA) and OR was found to be higher compared to correlation between ln(mA) and dw . SCmA was 48% for TCM1 and 33% for TCM2 . The corresponding SCnoise was 29% for TCM1 and 16% for TCM2 . Apart from routine CT dosimetry evaluations, the standard CTDI phantom positioned in configuration A or B may additionally be used by medical physicists to evaluate the performance of TCM operational characteristics.

Influence of cone beam CT (CBCT) scan parameters on size specific dose estimate (SSDE): a Monte Carlo study

The CT dose index (CTDIvol) is the dosimetric quantity used for multi-slice CT (MSCT) with beams ⩽4 cm. Conversion factors (f size) based on patient size are applied to CTDIvol to adjust for differences in patient size and derive size-specific dose estimates (SSDE) relating to patient dose. The aim of this study is to: (1) investigate use of a similar technique to provide SSDE values for cone beam CT (CBCT) scans, (2) determine whether factors derived for narrow beam MSCT are suitable for CBCT, and (3) investigate the influence of CBCT parameters on f size values.Monte Carlo simulations were used to model an on-board imager system integrated into a Varian Truebeam linear accelerator, and to assess doses for imaging. The CTDIvol is unsuitable for CBCT dosimetry, thus the assessments were applied to the modification CTDIIEC, recommended by the international electrotechnical commission (IEC). Conversion factors (f size,CBCT) were derived for CBCT to allow adjustment of values for the absorbed dose in water (AD) averaged over the beam width in the middle of the phantoms. Values of AD were evaluated as for CTDI300 measured with a 30 cm long chamber in centres and peripheries of long water phantoms with diameters of 10–40 cm for head and body protocols. Three beam widths 8, 16, and 24 cm were used with tube potentials ranging from 80–140 kV, for full and partial rotation modes. In order to derive f size,CBCT values, calculated values for AD were normalized with respect to the weighted CTDIIEC assessed in standard CTDI phantoms and free-in-air.Variations in f size,CBCT with beam width were minimal, 1%–5%, but those with tube potential were greater for 80 kV at small diameters reaching 11%. Acquisition mode affected f size,CBCT values by up to 7%. Best-fit curves were derived from the f size,CBCT values and compared to those reported by AAPM TG–204 for MSCT. Conversion factors estimated from these curves for 120 kV were within ±8% and ±13% of the MSCT values over head and body diameters, respectively, representing the majority of the adult population. Therefore, the use of MSCT factors to convert CTDIIEC for CBCT scans may lead to under/overestimation of doses to patients by 5% and 6%, on average, for the head and body protocols, respectively. Best-fit curves of the results from this study provide values that could be used to convert CTDIIEC for specific water-equivalent diameter (Dw) to a SSDE.

A study of the midpoint dose to CTDIvol ratio: Implications for CT dose evaluation.

In multidetector CT, the volume CT dose index (CTDIvol) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scanner-independent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length. A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDIL(water)/CTDIw (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose DL(0) (water) to CTDIvol ratio in a CT scan series with a scan length equal to L. For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of DL(0)/CTDIvol from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDIvol to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, DL(0)/CTDIvol decreased considerably as L decreased. DL(0)/CTDIvol was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, DL(0)/CTDIvol decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm. The midpoint dose to CTDIvol ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT.

Automated Calculation of Water-equivalent Diameter (DW) Based on AAPM Task Group 220.

The purpose of this study is to accurately and effectively automate the calculation of the water‐equivalent diameter (DW) from 3D CT images for estimating the size‐specific dose. DW is the metric that characterizes the patient size and attenuation. In this study, DW was calculated for standard CTDI phantoms and patient images. Two types of phantom were used, one representing the head with a diameter of 16 cm and the other representing the body with a diameter of 32 cm. Images of 63 patients were also taken, 32 who had undergone a CT head examination and 31 who had undergone a CT thorax examination. There are three main parts to our algorithm for automated DW calculation. The first part is to read 3D images and convert the CT data into Hounsfield units (HU). The second part is to find the contour of the phantoms or patients automatically. And the third part is to automate the calculation of DW based on the automated contouring for every slice (DW,all). The results of this study show that the automated calculation of DW and the manual calculation are in good agreement for phantoms and patients. The differences between the automated calculation of DW and the manual calculation are less than 0.5%. The results of this study also show that the estimating of DW,all using DW,n=1 (central slice along longitudinal axis) produces percentage differences of −0.92%±3.37% and 6.75%±1.92%, and estimating DW,all using DW,n=9 produces percentage differences of 0.23%±0.16% and 0.87%±0.36%, for thorax and head examinations, respectively. From this study, the percentage differences between normalized size‐specific dose estimate for every slice (nSSDEall) and nSSDEn=1 are 0.74%±2.82% and −4.35%±1.18% for thorax and head examinations, respectively; between nSSDEall and nSSDEn=9 are 0.00%±0.46% and −0.60%±0.24% for thorax and head examinations, respectively.PACS number(s): 87.57.Q‐, 87.57.uq‐

SU‐F‐J‐52: A Novel Approach to X‐Ray Tube Quality Assurance for CBCT Systems in Order to Better Assess the Patient Imaging Dose in a Large, Multi‐Unit Treatment Facility

Purpose:To standardize the tube calibration for Elekta XVI cone beam CT (CBCT) systems in order to provide a meaningful estimate of the daily imaging dose and reduce the variation between units in a large centre with multiple treatment units.Methods:Initial measurements of the output from the CBCT systems were made using a Farmer chamber and standard CTDI phantom. The correlation between the measured CTDI and the tube current was confirmed using an Unfors Xi detector which was then used to perform a tube current calibration on each unit.Results:Initial measurements showed measured tube current variations of up to 25% between units for scans with the same image settings. In order to reasonably estimate the imaging dose, a systematic approach to x‐ray generator calibration was adopted to ensure that the imaging dose was consistent across all units at the centre and was adopted as part of the routine quality assurance program. Subsequent measurements show that the variation in measured dose across nine units is on the order of 5%.Conclusion:Increasingly, patients receiving radiation therapy have extended life expectancies and therefore the cumulative dose from daily imaging should not be ignored. In theory, an estimate of imaging dose can be made from the imaging parameters. However, measurements have shown that there are large differences in the x‐ray generator calibration as installed at the clinic. Current protocols recommend routine checks of dose to ensure constancy. The present study suggests that in addition to constancy checks on a single machine, a tube current calibration should be performed on every unit to ensure agreement across multiple machines. This is crucial at a large centre with multiple units in order to provide physicians with a meaningful estimate of the daily imaging dose.

TU‐H‐207A‐01: Development of Conversion Factors to Account for Different CTDI Phantom Setups On a Wide‐Beam CT Scanner

Purpose:A fundamental measure performed during an annual physics CT evaluation confirms that system displayed CTDIvol nearly matches the independently measured value in phantom. For wide‐beam (z‐direction) CT scanners, AAPM Report 111 defined an ideal measurement method; however, the method often lacks practicality. The purpose of this preliminary study is to develop a set of conversion factors for a wide‐beam CT scanner, relating the CTDIvol measured with a conventional setup (single CTDI phantom) versus the AAPM Report 111 approach (three abutting CTDI phantoms).Methods:For both the body CTDI and head CTDI, two acquisition setups were used: A) conventional single phantom and B) triple phantom. Of primary concern were the larger nominal beam widths for which a standard CTDI phantom setup would not provide adequate scatter conditions. Nominal beam width (160 or 120 mm) and kVp (100, 120, 140) were modulated based on the underlying clinical protocol. Exposure measurements were taken using a CT pencil ion chamber in the center and 12 o'clock position, and CTDIvol was calculated with ‘nT’ limited to 100 mm. A conversion factor (CF) was calculated as the ratio of CTDIvol measured in setup B versus setup A.Results:For body CTDI, the CF ranged from 1.04 up to 1.10, indicating a 4–10% difference between usage of one and three phantoms. For a nominal beam width of 160 mm, the CF did vary with selected kVp. For head CTDI at nominal beam widths of 120 and 160 mm, the CF was 1.00 and 1.05, respectively, independent of the kVp used (100, 120, and 140).Conclusions:A clear understanding of the manufacturer method of estimating the displayed CTDIvol is important when interpreting annual test results, as the acquisition setup may lead to an error of up to 10%. With appropriately defined CF, single phantom use is feasible.

OPTIMIZATION OF PARAMETERS IN 16-SLICE CT-SCAN PROTOCOLS FOR REDUCTION OF THE ABSORBED DOSE

Introduction In computed tomography (CT) technology, an optimal radiation dose can be achieved via changing radiation parameters such as mA, pitch factor, rotation time and tube voltage (kVp) for diagnostic images. Materials and Methods In this study, the brain, abdomen, and thorax scaning was performed using Toshiba 16-slice scannerand standard AAPM and CTDI phantoms. AAPM phantom was used for the measurement of image-related parameters and CTDI phantom was utilized for the calculation of absorbed dose to patients. Imaging parameters including mA (50-400 mA), pitch factor (1 and 1.5) and rotation time (range of 0.5, 0.75, 1, 1.5 and 2 seconds) were considered as independent variables. The brain, abdomen and chest imaging was performed multi-slice and spiral modes. Changes in image quality parameters including contrast resolution (CR) and spatial resolution (SR) in each condition were measured and determined by MATLAB software. Results After normalizing data by plotting the full width at half maximum (FWHM) of point spread function (PSF) in each condition, it was observed that image quality was not noticeably affected by each cases. Therefore, in brain scan, the lowest patient dose was in 150 mA and rotation time of 1.5 seconds. Based on results of scanning of the abdomen and chest, the lowest patient dose was obtained by 100 mA and pitch factors of 1 and 1.5. Conclusion It was found that images with acceptable quality and reliable detection ability could be obtained using smaller doses of radiation, compared to protocols commonly used by operators.

TH‐C‐134‐01: Computed Tomography 2: Dose Basics and Getting the Most From Your Dose

TH‐C‐134‐01: Computed Tomography 2: Dose Basics and Getting the Most From Your Dose

TU‐A‐217BCD‐01: Introductory CT Dosimetry (and What We Can Learn from Making Measurements on Cylinders)

A variety of methods are used to describe and control the radiation output of a CT scanner. Different approaches may serve to emphasize one facet of a particular scan over another, favor the lexicon and style of a specific manufacturer or simply reflect history. The multiplicity of terms often adds confusion to what should be a simple task: to characterize the output of a CT machine in terms of the radiation dose to a cylinder. We will begin by examining radiation dose to cylinders with an emphasis on isolating superfluous parameters.We start, as in TG111, with an infinite helical scan of an infinitely long cylindrical phantom. During such a scan the dose at any point in the cylinder will asymptotically build up to its ultimate value Deq. The spatial average of this value, Deq, can serve as an indicator of scanner output and along with the total number of rotations N determine Etot, the total energy absorbed by the cylinder. If a finite cylinder is long enough (≥ Leq), a scan through it from one end to the other will yield these values in its central plane.The approach to Deq can conveniently be described by h(L), a robust function of scan length L which depends only weakly on z axis collimation and tube potential. This function facilitates the simple calculation of the lower dose to the central plane resulting from a finite scan. If the length of the cylinder is finite, as it is for the standard CTDI phantom, the dose is reduced further in a predictable fashion resulting in good correlation between Deq,bar and CTDIvol which in turn has been shown to be a useful tool in equating the radiation output between scanners differing in model and vendor.Though CTDIvol (and Deq,bar) are useful for characterizing CT machine output, we need to include the effect of patient size in assessing patient dose. TG204 has used measurements on phantoms of varying size to determine a size specific dose estimate (SSDE) which serves to accomplish this task. The SSDE has also proven to be very helpful in designing protocols for patients of varying size, especially pediatric patients.Learning Objectives:1. To describe the radiation dose to a cylinder in uncomplicated terms, starting with the particularly simple case of an infinite scan to an infinite cylinder.2. To extend the description to finite scans to infinite cylinders in terms of the h(L) function and then to the standard CTDI phantoms.3. To describe some of the subtleties in making reliable CTDI measurements.4. The demonstrated fact that dose indices (e.g., CTDIvol) provide a reliable indicator of machine output will be used to show, for example, how patient scans from machines of varying model and manufacturer can be compared.5. To describe how characterizing the dose to cylinders of varying size can be used both to provide a reasonable index of patient dose and as a useful tool in the design of pediatric protocols.

TU‐G‐217BCD‐01: Investigation of a Commercial OSLD System for CT Dosimetry

Purpose: With an increasing use of computed tomography (CT) in diagnostic procedures, there is a growing need for more accurate and patient‐specific dosimetry techniques. Optically stimulated luminescent dosimetry (OSLD), well‐suited for in‐vivo dosimetry with little if any impact on diagnostic image quality, offers a potential solution. We have characterized a commercially available OSLD system, provided CT calibration protocols, quantified uncertainties, and tested the system against ion chamber measurements for various CT applications. Methods: Landauer nanoDots were fully characterized through both measurements and theoretical approaches to quantify the effects of dose linearity, signal depletion, fading, angular dependence, and energy dependence specific to a CT environment. Three calibration approaches that included calibration (1) relative to a 60Co source, (2) relative to a 120kVp CT spectrum in air, and (3) using the vendor provided method were investigated along with their associated uncertainty. The OSL system and calibration procedures were tested and compared to ion chamber measurements using standard CTDI phantoms. The defined calibration protocols were used to measure an anthropomorphic head phantom's lens dose. Results: We determined the OSLD characteristics and calibration techniques appropriate for CT dosimetry. Most correction factors were less than 5% with the exception of energy dependence which, depending on the calibration method and specific scan parameters, was as large as 400% due to the OSL over‐response at low energies. The uncertainty budget for the dose calculations showed the uncertainties (2‐sigma) to range from 11% to 20% for a properly corrected system. More rigorous calibration approaches resulted in lower dosimetric uncertainties. Conclusion: The OSLD calibration protocols and correction factors described provide a precise and accurate patient dosimetry and documentation method for routine monitoring of CT patients, as well as for special procedures such as CT perfusion. Work supported by grant CA10953(NCI, DHHS)

SU-E-I-10: Putting Teeth into Your CT Dosimetry Program: Approaches to Cone- Beam Dental/Maxillofacial CT Dosimetry.

To review and compare different approaches to the problem of dosimetry for limited field-of-view (FOV) cone beam CT devices for dental and maxillofacial applications. The determination of patient doses from specialized, cone-beam CT devices for dental and maxillofacial work requires medical physicists to re-evaluate their dosimetry methods. These devices work in cone-beam geometry, with an axial field dimension on the order of the lengths of the standard head CTDI phantom and pencil ionization chamber. They may also utilize less than 360 degree scans, resulting in asymmetrical radiation distributions. This operating regime is far from that for which conventional CT dosimetry was designed, and alternative approaches must be considered. The alternatives include extensions of conventional CT dosimetry currently used for large axial FOV scanners (e.g. the extended CTDI parameter (CTDIe) for the Toshiba Aquillion One with 160 mm axial FOV) and the new method based on point dosimetry measurements recently formalized in AAPM Report TG-111. Conventional, modified-conventional, and TG-111 dosimetry measurements are used in two CT dose phantoms (adult head and pediatric head) to obtain dose indices for the Planmeca ProMax 3D Max dental CT scanner. Surface dose maps are generated using radiochromic film for correlation with the chamber dosimetry. Results for the three dosimetry approaches are compared for the specific case of the ProMax 3D Max scanner. Strengths and weaknesses of the three measurement paradigms for this type of application are compared. The increasing availability of specialized scanners operating in full cone-beam mode will require the clinical medical physicist to be conversant with extensions to the CT dose index methodology suitable for this equipment.

Estimated pediatric radiation dose during CT

State-of-the-art CT scanners typically display two dose indices: CT dose index (CTDI(vol) [mGy]) and dose length product (DLP [mGy-cm]) based on one of two standard CTDI phantoms (16- or 32-cm diameter) used in the calculation of CTDI(vol). CTDI(vol) represents the radiation produced by the CT scanner, not the radiation dose to an individual patient. Pediatric radiologists, aware of this discrepancy, have requested a method to estimate the CT patient dose based on the size of the pediatric patient or small adult. This paper describes the method developed by AAPM Task Group 204 to provide a better estimate of CT patient dose. These improved estimates of patient dose provide radiologists with a practical tool to better manage the radiation dose their patients receive. In the future, size-specific dose estimates (SSDE) received by the patient should be included in the patient's electronic medical record to help radiologists better assess risk versus benefit for their patients.

Radiation safety summit – method to estimate radiation dose to pediatric patients from CT scans

State-of-the-art CT scanners typically display two dose indices, CTDIvol (mGy) and DLP (mGy-cm), along with which two standard CTDI phantoms, 16or 32-cm diameter, are used to estimate the two indices. CTDIvol is based on radiation dose measurements on an individual scanner completed by a medical physicist with a pencil ionization chamber and either a 16or 32-cm diameter CTDI cylindrical plastic phantom. CTDIvol represents the radiation dose delivered to a standard plastic phantom from a specific model CT scanner using specified scan parameters: tube voltage, tube current, rotation time, pitch and selected bowtie filter. CTDIvol provides a standardized method to estimate and compare the radiation output of different CT scanners. CTDIvol is defined to be a dose index of CT scanners, not an indication of patient dose. The inability of CTDIvol to accurately reflect pediatric patient dose is illustrated in Fig. 1. Three cylindrical phantoms with diameters of 10 cm, 16 cm, and 32 cm (representing the smallest pediatric patient to an adult) were scanned with the same scanner using the same scan parameters. While this delivered the same radiation energy to each phantom, the measured CTDIvol in the phantoms (top row of data in figure) ranged from 47 mGy to 18 mGy. This clearly indicates that the dose to various-size patients (represented by the three phantoms) is a function of the energy produced by the scanner (determined by scan techniques) and the size of the patient. The superficial tissues of the large patient attenuate the radiation beam, reducing the dose delivered to the core of the large patient. In the figure, the center of the 10-cm phantom received 47 mGy of dose while the center of the 32-cm phantom received a dose of only 10.8 mGy for the same scan techniques. While the measured dose index in the previous example with an ionization chamber, CTDIvol, is very dependent on patient size when scan parameters for all three phantoms are identical, the displayed CTDIvol on a CT scanner is independent of changes in patient size by definition. The displayed scan index assumes the actual scan parameters but by definition assumes the patient is either a 16-cm or 32-cm diameter cylindrical plastic phantom. If the 16-cm phantom is assumed, the scanner will display 37 mGy for CTDIvol for each of the three represented patient sizes (second row of data in Fig. 1); this overestimates the dose to adult patients and underestimates dose to infants. If the 32-cm phantom is assumed, the scanner displays 18 mGy for all patients (third row of data in figure); this underestimates the dose to all patients smaller than adults. In either case, the displayed CTDIvol does not represent the pediatric patient dose and can mislead the technologist or radiologist. A task group of the American Association of Physicists in Medicine (AAPM), co-chaired by John Boone and Keith Strauss, is currently preparing a report that will provide scaling factors as a function of patient size that can be used by the technologist or radiologist to more accurately estimate the radiation dose to the tissues of the pediatric patient that are directly irradiated by the primary X-ray beam of the CT scanner. These scaling factors will be based on attenuation data collected from numerous CT scanners by multiple investigators. Hopefully, these scaling factors will be available by the time of this presentation. These scaling factors should allow the operator of the CT scanner to estimate the radiation dose to the Disclaimer Dr. Strauss has no financial interests, investigational or off-label uses to disclose.

TU‐C‐304A‐08: A Method to Adjust CTDI to Better Represent the Dose for Pediatric Patients

Purpose: CT manufacturers are required to report dose using standardized CTDI phantoms. However, CTDI can be confusing when applied to pediatric patients due to variations in patient size. We propose a simple method to scale standard CTDI to better represent a pediatric patient's dose by providing universal conversion tables that are indexed by patient size ranges. Method and Materials: The Medical Imaging and Technology Alliance (MITA) and Image Gently Pediatric CT Physics Work Group have obtained CTDI ratios between the 32 cm, 16 cm and 10 cm phantoms for current 16 and 64 slice scanners. X‐ray attenuation was measured for the CTDI phantoms and retrospectively determined from pediatric and adult pre‐scan projection images. Regression relationships between patient dimensions, attenuation and dose ratios were used to create tables of CTDI scale factors as a function of patient dimensions. Results: The range of attenuation ratios of the 2nd and 3rd quartiles for pediatric heads compared to the standard CTDI phantoms was 0.85 to 0.95 (16 cm) and 0.46 to 0.51 (32 cm). The range for pediatric bodies was 1.03 to 1.35 (16 cm) and 0.85 to 0.95 (32 cm). Individual measurement errors relative to the mean CTDI ratio due to manufacturer, scanner generation and kVp or bowtie filter selections were minimal. Preliminary results show errors increased from 0 % to 14 % with increasing difference between the attenuation of the pediatric patient and CTDI phantom. Conclusion: Universal dose conversion tables can be provided based on dimensions or can be automatically calculated from a patient's attenuation determined from a pre‐scan projection image with reasonable accuracy to scale CTDI or its future replacement to better represent dose for pediatric patients.

Concepts for dose determination in flat-detector CT

Flat-detector computed tomography (FD-CT) scanners provide large irradiation fields of typically 200 mm in the cranio-caudal direction. In consequence, dose assessment according to the current definition of the computed tomography dose index CTDIL=100 mm, where L is the integration length, would demand larger ionization chambers and phantoms which do not appear practical. We investigated the usefulness of the CTDI concept and practical dosimetry approaches for FD-CT by measurements and Monte Carlo (MC) simulations. An MC simulation tool (ImpactMC, VAMP GmbH, Erlangen, Germany) was used to assess the dose characteristics and was calibrated with measurements of air kerma. For validation purposes measurements were performed on an Axiom Artis C-arm system (Siemens Medical Solutions, Forchheim, Germany) equipped with a flat detector of 40 cm × 30 cm. The dose was assessed for 70 kV and 125 kV in cylindrical PMMA phantoms of 160 mm and 320 mm diameter with a varying phantom length from 150 to 900 mm. MC simulation results were compared to the values obtained with a calibrated ionization chambers of 100 mm and 250 mm length and to thermoluminesence (TLD) dose profiles. The MCs simulations were used to calculate the efficiency of the CTDIL determination with respect to the desired CTDI∞. Both the MC simulation results and the dose distributions obtained by MC simulation were in very good agreement with the CTDI measurements and with the reference TLD profiles, respectively, to within 5%. Standard CTDI phantoms which have a z-extent of 150 mm underestimate the dose at the center by up to 55%, whereas a z-extent of ⩾600 mm appears to be sufficient for FD-CT; the baseline value of the respective profile was within 1% to the reference baseline. As expected, the measurements with ionization chambers of 100 mm and 250 mm offer a limited accuracy, whereas an increased integration length of ⩾600 mm appeared to be necessary to approximate CTDI∞ in within 1%. MC simulations appear to offer a practical and accurate way of assessing conversion factors for arbitrary dosimetry setups using a standard pencil chamber to provide estimates of CTDI∞. This would eliminate the need for extra-long phantoms and ionization chambers or excessive amounts of TLDs.

WE‐E‐332‐01: A Unified Dose Indicator for Tomographic Acquisition Modalities

Introduction : Tomographic techniques in diagnostic and interventional radiology or radiation therapy are in rapid development. The CTDI formalism that is currently used to assess dose has been challenged by several groups. The goal of this contribution is to evaluate the required scan length to reach equilibrium on various MDCT units and estimate the average dose delivered within a slice when using systems having a cone beam geometry.Method and Materials: Measurements were performed on three MDCT systems (GEMS 8, 16 and 64‐row), a flat panel fluoroscopy (Allura — Philips), an IGRT (Synergy, Elekta ltd) and on a tomotherapy (TomoTherapy Inc.) using two standard CTDI phantoms and a home made phantoms (PMMA cylinder of 30 cm filled with water) with a conventional small volume ion chamber (0.6 cc Farmer type) and a standard pencil ion chamber. Dose profiles were also recorded at various positions within the slice to study the impact of scatter as a function of the distance between the centre of the phantom and its periphery. Results: For CT units a theoretical length of 400 mm is required to reach the equilibrium at the center of the phantom. The measurements show that a dose plateau was reached after 420 mm. The average dose within a slice measured in our home made phantom for standard abdominal CT (120 kV, 210 effective mAs) was 15.3 mGy; 26.9 mGy for the fluoro‐CT (122kV, 274mAs, 20.7s); 39.0mGy for IGRT system (120kV, 219mAs, 27×27cm2 field size, one rotation) and 13.1 mGy for the Tomotherapy system (3.5 MV, pitch=0.8). Conclusion: The CTDI concept should be replaced by a more generic methodology, such as the one presented here, that could be used in all cone beam geometries. Measurements should be done using water equivalent phantoms.

TU-FF-A4-04: Experimental Confirmation of Near Parabolic Shape of Dose Profile in Cylindrical Phantom for Dual Source CT

Purpose: To measure the radial dose distribution in a cylindrical CT phantom for both a single and dual source CT and to characterize its shape in order to determine whether the near parabolic shape used to justify CTDI volume calculations with equal weighting of the center and peripheral CTDI values is applicable for a two tube device. Method and Materials: A cylindrical phantom, the same diameter (32 cm) as the standard CTDI phantom was made with acrylic plastic. When assembled, it consisted of a cylinder with a cut in the transverse plane. Landauer optically stimulated luminescence(OSL) dots and Kodak X‐OMAT V films were sandwiched in the transversal slit. The assembled cylinder was scanned using a clinical protocol over a length that extended well beyond its endpoints at 120 kVp using a Siemens Dual Source Definition CT.Results: For a single tube, the radial dose distribution as measured using both the OSL detectors and film is close to parabolic. (The drop off in scatter close to the surface is not well accounted for by this simple curve.)Somewhat surprisingly,deviation from a parabolic shape near the surface when two tubes are used is only marginally different than for one tube. Under the conditions of the scan, equal weighting results in errors of only a few %, due mostly to the drop off at the surface. The 1/3*center+2/3*peripheral weighting used in standard calculations stems from a linear fit and results in errors of up to 11%. Conclusion: Measurements of the dose profile in a cylindrical phantom show that the shape is close to parabolic for both single and dual source machines except for drop off at the edges. A parabolic shape results in equal weighting coefficients for volumetric integral dose calculations using only center and peripheral CTDI measurements.

TU‐C‐L100J‐10: Combining Measurement and Monte Carlo Methods for Dose Assessment in Flat‐Detector CT

Purpose: Dose assessment in flat‐detector CT (FD‐CT) combining measurements and Monte Carlo (MC) simulations. Material and Methods: FD‐CT scanners provide large irradiation fields of typically 100 mm to 250 mm in the longitudinal direction. In consequence dose assessment according to the current definition of the CTDI would demand larger ionization chambers and phantoms which are not practical. We propose a method which includes a measurement in air or in a phantom with an integrating dosimeter to assess the dose at that point and to combine it with MC simulations to assess 3D dose distributions and integral dose for arbitrary objects and geometries. For validation purposes measurements were performed on a C‐arm system (Siemens Medical Solutions, Forchheim, Germany) equipped with a flat‐detector of 40 × 30 cm2. Dose was assessed for various tube voltages in cylindrical PMMA phantoms of 16 cm and 32 cm diameter with a varying z‐extent from 15 to 60 cm. The MC results were compared to the values obtained with calibrated ionization chambers of 100 mm and 250 mm length and to TLD dose profiles along the complete z‐extent of the phantoms. Additionally a comparison to measurements of dose distribution in an anthropomorphic phantom was performed. Results: The MC simulation was in agreement with the reference TLD measurements to within better than 10%. Standard CTDI phantoms with a z‐extent of 15 cm underestimate the dose at the center by up to 20% whereas a z‐extent of 300 mm appears to be sufficient for FD‐CT. The 100 mm chamber underestimates the measured CTDI value by over 40%. The MC tool can be used to calibrate the measurements with the ionization chamber of 100 mm. Conclusion: The combination of measurement and validated MC tool appears to be a flexible solution to assess arbitrary dose characteristics.

Imaging doses from the Elekta Synergy X-ray cone beam CT system

The Elekta Synergy is a radiotherapy treatment machine with integrated kilovoltage (kV) X-ray imaging system capable of producing cone beam CT (CBCT) images of the patient in the treatment position. The aim of this study is to assess the additional imaging dose. Cone beam CT dose index (CBDI) is introduced and measured inside standard CTDI phantoms for several sites (head: 100 kV, 38 mAs, lung: 120 kV, 152 mAs and pelvis: 130 kV, 456 mAs). The measured weighted doses were compared with thermoluminescent dosimeter (TLD) measurements at various locations in a Rando phantom and at patients' surfaces. The measured CBDIs in-air at the isocentre were 9.2 mGy 100 mAs(-1), 7.3 mGy 100 mAs(-1) and 5.3 mGy 100 mAs(-1) for 130 kV, 120 kV and 100 kV, respectively. The body phantom weighted CBDI were 5.5 mGy 100 mAs(-1) and 3.8 mGy 100 mAs(-1 )for 130 kV and 120 kV. The head phantom weighted CBDI was 4.3 mGy 100 mAs(-1) for 100 kV. The weighted doses for the Christie Hospital CBCT imaging techniques were 1.6 mGy, 6 mGy and 22 mGy for the head, lung and pelvis. The measured CBDIs were used to estimate the total effective dose for the Synergy system using the ImPACT CT Patient Dosimetry Calculator. Measured CBCT doses using the Christie Hospital protocols are low for head and lung scans whether compared with electronic portal imaging (EPI), commonly used for treatment verification, or single and multiple slice CT. For the pelvis, doses are similar to EPI but higher than CT. Repeated use of CBCT for treatment verification is likely and hence the total patient dose needs to be carefully considered. It is important to consider further development of low dose CBCT techniques to keep additional doses as low as reasonably practicable.