Dosimetric Methods Research Articles

Dosimetry for therapy ultrasound

Ultrasound is probably the only clinical therapy modality that does not as yet have a well established dosimetric parameter. There are a number of reasons for this. Unlike other energy forms, ultrasound produces cell killing by two distinct principle mechanisms: cavitation and heating. While dose parameters have been proposed for each mechanism, there has been no attempt to combine these into a single unit. Existing dosimetric methods will be reviewed and put into the context of other therapeutic methods. Biological and physical end points will be discussed.

EFFECT OF STRESS FACTORS ON THE STATE OF VESSELS OF HEART MICROCURCULAR CHANNEL IN RATS

Objective: analysis of the state of vessels of myocardium microcircular channel of rats, exposed to hypokinesia, incorporation of 137 Сs radionuclides. Material and methods. Myocardium of pedigreeless white rats was studied by means of histologic, electro-microscopic and dosimetric methods of analysis. Results. The study revealed considerable structural functional changes at tissue, cellular and subcellular levels in the vessels of the microcircular channel of the rats` myocardium. Conclusion. The degree of the appearing disorders depends on the type and duration of stress impact. The structural change of the vessels of the microcircular channel are mostly evident at the early stages of the experiment. Compensation mechanisms are realized in long-term hypokinesia, which partially stabilizes myocardium. The characteristic feature of radiation impact is dose-independent effect of the lesions. The processes of intercellular regeneration do not eliminate radioinduced subcellular disorders. The gradual decline in the functional potential of the vascular system of myocardium can serve as a precondition for development of heart pathology.

Monitoring of transient cavitation induced by ultrasound and intense pulsed light in presence of gold nanoparticles

One of the most important challenges in medical treatment is invention of a minimally invasive approach in order to induce lethal damages to cancer cells. Application of high intensity focused ultrasound can be beneficial to achieve this goal via the cavitation process. Existence of the particles and vapor in a liquid decreases the ultrasonic intensity threshold required for cavitation onset. In this study, synergism of intense pulsed light (IPL) and gold nanoparticles (GNPs) has been investigated as a means of providing nucleation sites for acoustic cavitation. Several approaches have been reported with the aim of cavitation monitoring. We conducted the experiments on the basis of sonochemiluminescence (SCL) and chemical dosimetric methods. The acoustic cavitation activity was investigated by determining the integrated SCL signal acquired over polyacrylamide gel phantoms containing luminol in the presence and absence of GNPs in the wavelength range of 400–500nm using a spectrometer equipped with cooled charged coupled devices (CCD) during irradiation by different intensities of 1MHz ultrasound and IPL pulses. In order to confirm these results, the terephthalic acid chemical dosimeter was utilized as well. The SCL signal recorded in the gel phantoms containing GNPs at different intensities of ultrasound in the presence of intense pulsed light was higher than the gel phantoms without GNPs. These results have been confirmed by the obtained data from the chemical dosimetry method. Acoustic cavitation in the presence of GNPs and intense pulsed light has been suggested as a new approach designed for decreasing threshold intensity of acoustic cavitation and improving targeted therapeutic effects.

A novel method for 4D measurement-guided planned dose perturbation to estimate patient dose/DVH changes due to interplay

As IMRT/VMAT technology continues to evolve, so do the dosimetric QA methods. We present the theoretical framework for the novel planned dose perturbation algorithm. It allows not only to reconstruct the 3D volumetric doe on a patient from a measurement in a cylindrical phantom, but also to incorporate the effects of the interplay between the intrafractional organ motion and dynamic delivery. Unlike in our previous work, this 4D dose reconstruction does not require the knowledge of the TPS dose for each control point of the plan, making the method much more practical. Motion is viewed as just another source of error, accounted for by perturbing (morphing) the planned dose distribution based on the limited empirical dose from the phantom measurement. The strategy for empirical verification of the algorithm is presented as the necessary next step.

Dosimetric quality assurance of highly conformal external beam treatments: from 2D phantom comparisons to 4D patient dose reconstruction

As IMRT technology continues to evolve, so do the dosimetric QA methods. A historical review of those is presented, starting with longstanding techniques such as film and ion chamber in a phantom and progressing towards 3D and 4D dose reconstruction in the patient. Regarding patient-specific QA, we envision that the currently prevalent limited comparison of dose distributions in the phantom by γ-analysis will be eventually replaced by clinically meaningful patient dose analyses with improved sensitivity and specificity. In a larger sense, we envision a future of QA built upon lessons from the rich history of "quality" as a science and philosophy. This future will aim to improve quality (and ultimately reduce cost) via advanced commissioning processes that succeed in detecting and rooting out systematic errors upstream of patient treatment, thus reducing our reliance on, and the resource burden associated with, per-beam/per-plan inspection.

SU‐E‐T‐222: Contribution Of In Vivo Dose Measurement To The Intensity Modulated Radiotherapy Technique In The Treatment Of Prostate Cancer

Purpose: The contribution of in‐vivo dose measurement of organs at risk by TLD and semiconductor detectors during Intensity‐modulated radiotherapy technique in treatment of prostate cancer was studied. Methods: Appropriate prostate cancer patients target and organs at risk contours that managed for IMRT planning were fusion on CT images of Rando phantom. For PTV1 and 2 the equally angel nine fields (within 200°− 160°)IMRT plans were designed using 6MV photon energy with 180cGy per fraction to total dose of 45 and 72Gy respectively. The point dose and two‐dimensional dose measurements for the quality control evaluation of IMRT plans were done and found in appropriate limits. The 3 sub‐slab of Rando phantom were re‐made from paraffin and rectum cavity was created in accordance with the probe for measuring rectal dose. Based on dosimetric method each dosimeter was selected and located on measurement region. Prostate IMRT plans were applied to the phantom. In‐vivo dose measurements were repeated several times by TLD and diode, obtained dose values for rectum and bladder compared with TPS. Results: Both semiconductor and TLD dose measurement values for bladder and rectum is generally in agreement with the TPS. The rectum dose measurement values were differing from TPS up to 30% in intense dose gradient regions. This is because when the coordinates of the reference points are in intense dose gradient regions, +/− 1 mm changes in position create +/− 3–10% difference between the calculated and measured doses. Conclusion: The intracavitary rectum and bladder probes used for brachytherapy dosimetry applications could be applicable in external radiation treatments. In‐vivo dosimetry has an important role in the quality assurance of radiotherapy, especially to preventing errors which may occur in every stage of treatment. In this context,for more complex treatment techniques like IMRT the traditional in‐vivo dosimetric methods using TLD and semiconductor diodes was found to be reliable.

SU‐E‐T‐432: Dosimetric Computation of Cyberknife SBRT Plans for Treatment of Kidney and Adrenal Gland

Purpose: It is well‐accepted that Monte Carlo algorithm (MCA) methods are superior to Ray‐Tracing algorithm (RTA), i.e., effective path length (EPL), methods for computing radiation dose, especially for structures with heterogeneous tissue compositions. In this study, the differences of dose distribution in target volumes (kidneys and adrenal glands) and adjacent organs at risk (OARs) were evaluated while the treatment plans were computed using MCA and RTA for Cyberknife™ stereotactic body radiotherapy (SBRT). Methods: A total of 14 renal tumor (10 kidney and 4 adrenal gland) patients who had prescriptions in the range of 24Gy–48Gy (mean=42Gy) were selected. Treatment plans were computed using RTA and MCA in Cyberknife MultiPlan™ (version‐3.5.2). First, dose was calculated using RTA and then was recomputed using MCA, keeping total MU the same. Liver, heart, stomach, spleen, bowel, and contralateral kidney were considered as OARs. Dosimetric parameters considered for the target and OARs were minimum dose, mean dose, and maximum point dose. Additionally, dose delivered to 95% and 5% of the target volumes were evaluated. Dose computed with two algorithms were compared and statistically analyzed using two‐tailed t‐tests. Results: This study revealed that differences (average) in mean dose (0.79Gy), maximum dose (2.02Gy), 95% (0.76Gy) and 5% (0.83Gy) of target coverage dose were not statistically significant (p‐value>0.05). Differences in minimum dose (average=0.04–3.75Gy) to OARs exhibited significance (p‐value< 0.05), though differences in minimum dose to target volumes (average=1.14Gy) showed no significant difference (p‐value>0.05). Conclusion: Although Monte‐Carlo is more accurate compared to Ray‐Tracing for dosimetric computation, especially tumors in heterogeneous tissue composition, dose computation and delivery using MCA are quite involved due to additional computational time and associated patient‐specific quality assurance. This study indicated that both RTA and MCA are almost equally effective as dosimetric computation methods for renal tumors, and RTA can be used without compromising dosimetric accuracy.

Assessing Human Exposure to Electromagnetic Fields From Wireless Power Transmission Systems

The strong reactive near-field wireless power transmission (WPT) systems induce electric fields in the body tissue of persons in their close vicinity. This may pose potential direct health hazards or indirect risks via interference with medical implants. In this paper, the safety guidelines and the fundamental coupling mechanisms of the human body with the electromagnetic near fields of WPT are reviewed as well as the methodology and the instrumentation for the demonstration of the safety of such systems operating between 100 kHz and 50 MHz. Based on this review, the advantages and shortcomings of state-of-the-art numerical and experimental techniques are discussed and applied to a generic WPT operating at 8 MHz. Finally, current research needs are identified which include: 1) the extension of safety guidelines for coverage of persons with implants; 2) more computationally efficient full-wave solvers; 3) higher quality human models which cover different population groups and include improved models of nerve tissue; 4) experimental dosimetric methods for the WPT frequency range; and 5) product standards to demonstrate safety of specific WPT.

SU-E-T-87: Commissioning of a 50-100 KV X-Ray Unit for Skin Cancer Treatment

Purpose: To provide dosimetric data for a 50–100 kV X-ray unit for skin cancer treatments. Discrepancy of measurements using two different types of chambers was investigated. Methods: Two superficial treatment units SRT-100 from Sensus were studied. The unit has three energies: 50, 70 and 100 kV x-ray and five cones from 1.5 to 5 cm diameters. Data collected during commission were: (a) HVL, (b) dose rate, cGy/minute at 15 cm target-to-skin distance in air under a 5 cm cone, and (c) cone size factors. A Farmer-type thimble chamber, PTW 23333, and a thin-window parallel plate (PP) ion-chamber, PTW23342 were used. Measured data were compared with vendor provided values. Results: Average HVL values for 50, 70 and 100 kV of the two units were 0.52, 1.15 and 2.2 mm Al, respectively. HVL values were 5–9 % lower if the thimble chamber was used for 70 and 100 kV. Dose rate were found 3–4% lower with the thimble chamber compared with the value from the PP chamber. As expected, the two type chambers measured the same cone size factors, from 0.97 to 1.0 for 1.5 to 5 cm cone. When the same PP chamber was used for another SRT-100 unit with a similar HVL value, the dose rates were found with a 2–9% difference for 50–100 kV between the two units. Dosimetric data from vendor should be used as a reference only. Conclusions: Data obtained from a thimble chamber underestimated the dose by 3–4% compared with measured by a PP chamber. Dose rate from two identical treatment units can differ by 9%. Commissioning of a soft X-ray machine should be done with proper dosimetric methods.

An organ-based approach to dose calculation in the assessment of dose-dependent biological effects of ionising radiation in Arabidopsis thaliana

There is a need for a better understanding of biological effects of radiation exposure in non-human biota. Correct description of these effects requires a more detailed model of dosimetry than that available in current risk assessment tools, particularly for plants. In this paper, we propose a simple model for dose calculations in roots and shoots of Arabidopsis thaliana seedlings exposed to radionuclides in a hydroponic exposure setup. This model is used to compare absorbed doses for three radionuclides, 241Am (α-radiation), 90Sr (β-radiation) and 133Ba (γ radiation). Using established dosimetric calculation methods, dose conversion coefficient values were determined for each organ separately based on uptake data from the different plant organs. These calculations were then compared to the DCC values obtained with the ERICA tool under equivalent geometry assumptions. When comparing with our new method, the ERICA tool appears to overestimate internal doses and underestimate external doses in the roots for all three radionuclides, though each to a different extent. These observations might help to refine dose–response relationships. The DCC values for 90Sr in roots are shown to deviate the most. A dose–effect curve for 90Sr β-radiation has been established on biomass and photosynthesis endpoints, but no significant dose-dependent effects are observed. This indicates the need for use of endpoints at the molecular and physiological scale.

Indigenously developed multipurpose acrylic head phantom for verification of IMRT using film and gel dosimetry

The purpose of this study was to validate the newly designed acrylic phantom for routine dosimetric purpose in radiotherapy. The phantom can be used to evaluate and compare the calculated dose and measured dose using film and gel dosimetric methods. In this study, a doughnut‐shaped planning target volume (8.54 cm3) and inner organ at risk (0.353 cm3) were delineated for an IMRT test plan using the X‐ray CT image of the phantom. The phantom consists of acrylic slabs which are integrated to form a human head with a hole in the middle where several dosimetric inserts can be positioned for measurement. An inverse planning with nine coplanar intensity‐modulated fields was created using Pinnacle TPS. For the film analysis, EBT2 film, flatbed scanner, in‐house developed MATLAB codes and ImageJ software were used. The 3D dose distribution recorded in the MAGAT gel dosimeter was read using a 1.5 T MRI scanner. Scanning parameters were CPMG pulse sequence with 8 equidistant echoes, TR=5600, echo step=22 ms, pixel size=0.5 times 0.5, slice thickness=2 mm. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), R2 images were converted to dose images. The dose comparison was accomplished using in‐house MATLAB‐based graphical user interface named “IMRT3DCMP”. For gel measurement dose grid from the TPS was extracted and compared with the measured dose grid of the gel. Gamma index analysis of film measurement for the tolerance criteria of 2%/2 mm, 1%/1 mm showed more than 90% voxels pass rate. Gamma index analysis of 3D gel measurement data showed more than 90% voxels pass rate for different tolerance criteria of 2%/2 mm and 1%/1 mm. Overall both 2D and 3D measurement were in close agreement with the Pinnacle TPS calculated dose. The phantom designed is cost‐effective and the results are promising, but further investigation is required to validate the phantom with other 3D conformal techniques for dosimetric purpose.PACS numbers: 87.53.Kn, 87.55.km, 87.56.N‐

Review of PhD thesis “Study on Dosimetric Methods of Intensity Modulated Radiotherapy”

Review of PhD thesis “Study on Dosimetric Methods of Intensity Modulated Radiotherapy”

Effective dose conversion coefficients for radionuclides exponentially distributed in the ground.

In order to provide fundamental data required for dose evaluation due to environmental exposures, effective dose conversion coefficients, that is, the effective dose rate per unit activity per unit area, were calculated for a number of potentially important radionuclides, assuming an exponential distribution in ground, over a wide range of relaxation depths. The conversion coefficients were calculated for adults and a new-born baby on the basis of dosimetric methods that the authors and related researchers have previously developed, using Monte Carlo simulations and anthropomorphic computational phantoms. The differences in effective dose conversion coefficients due to body size between the adult and baby phantoms were found to lie within 50 %, for most cases; however, for some low energies, differences could amount to a factor of 3. The effective dose per unit source intensity per area was found to decrease by a factor of 2–5, for increasing relaxation depths from 0 to 5 g/cm2, above a source energy of 50 keV. It is also shown that implementation of the calculated coefficients into the computation of the tissue weighting factors and the adult reference computational phantoms of ICRP Publication 103 does not significantly influence the effective dose conversion coefficients of the environment. Consequently, the coefficients shown in this paper could be applied for the evaluation of effective doses, as defined according to both recommendations of ICRP Publications 103 and 60.

WE‐A‐BRB‐05: Methods to Optimize COMS Plaque Loading for Improved Dose Homogeneity and Conformity

Purpose: To investigate the dosimetric optimization methods of varied intra‐plaque seed strength and radionuclide choice for COMS eye plaque brachytherapy. Methods: The COMS plaque Silastic insert arranges seeds about concentric circles (rings) of different radii. Thus, each plaque dose distribution may be considered as the superposition of its constituent rings. The MCNP5 radiation transport code was used to simulate a 16mm COMS plaque populated with 103Pd, 125I, and 131Cs sources. Each constituent source ring was then activated to determine its contribution to the total dose distribution while accounting for the dosimetric influences of inter‐seed attenuation and scatter. Tumor dose distributions were generated from >106 permutations of ring weightings and radionuclides for a given apical prescription dose. These dose distributions were analyzed for target dose uniformity and conformity, and compared to that of a uniformly‐loaded 125I plaque. Results: Using 131Cs seeds in only the plaque‘s outer ring yielded the most homogeneous dose distribution (+16% vs. 125I). The proximal sclera dose was 42% lower, yet center of the eye and opposite retina doses were 16% and 30% higher, respectively. Combinations with 103Pd in the plaque inner ring weighted to deliver >98% of the prescription dose yielded the most conformal dose distributions (>24% vs. 12 5I). Though doses to the eye center and opposite retina were >32% and >55% lower, respectively, the proximal sclera dose was >26% higher. Various combinations using only 103Pd with the outer ring contributing >50% of the prescription dose produced dose distributions both more homogeneous and more conformal than 125I. Proximal sclera, eye center, and opposite retina doses were typically 8‐12%, 11‐14%, and 38–40% lower, respectively, than using 125I. Conclusions: Intra‐plaque radionuclide and seed strength variations produced more homogeneous and conformal dose distributions than uniformly‐loaded COMS plaques. This optimization approach is generalizable to all plaque sizes and tumor dimensions.

Le radiophysicien, l’enfant et la pédiatrie

Estimation of the absorbed dose in nuclear pediatric is mandatory and necessary to assess the risk in radiation protection of each child. The radiophysicist plays an essential role in this process because he has the knowledge for this evaluation. This work aims to compare different dosimetric methods in the literature. All are based on the most used in this domain, “MIRD method”. The maximum relative deviations are 30% for three tests studied and do not set a reference.

Evaluation of Sonochemiluminescence in a Phantom in the Presence of Protoporphyrin IX Conjugated to Nanoparticles

Introduction When a liquid is irradiated with high-intensity and low-frequency ultrasound, acoustic cavitation occurs and there are some methods to determine and quantify this phenomenon. The existing methods for performing these experiments include sonochemiluminescence (SCL) and chemical dosimetric methods. The particles in a liquid decrease the ultrasonic intensity threshold needed for cavitation onset. In this study, a new nanoconjugate made up of Protoporphyrin IX (PpIX) and gold nanoparticles (GNP), i.e., Au-PpIX was used to provide nucleation sites for cavitation. The nonradiative relaxation time of PpIX in the presence of GNPs is longer than the similar time for PpIX without GNPs. This effect can be used in medical diagnostic and therapeutic applications. Materials and Methods The acoustic cavitation activity was investigated studying integrated SCL signal in the wavelength range of 400-500 nm in polyacrylamide gel phantom containing luminol using a cooled CCD spectrometer at different intensities of 1 MHz ultrasound. In order to confirm these results, a chemical dosimetric method was utilized, too. Results SCL signal level in gel phantom containing Au-PpIX was higher than the other phantoms. These results have been confirmed by the chemical dosimetric data. Conclusion This finding can be related to the existence of PpIX as a sensitizer and GNPs as cavitation nuclei. In other words, nanoparticles have acted as the sites for cavitation and have increased the cavitation rate. Another theory is that activation of PpIX has produced more free radicals and has enhanced the SCL signal level.

On Seeking the Worst Case of Human Exposure to the External EMF Radiation

Electromagnetic field (EMF) radiation is very controversial issue. For one hand, many therapeutic and diagnostic applications are proved effective; on the other hand, excessive exposure may cause adverse biological effects. In order to prevent the over-exposure, the worst results are frequently discussed for the safety limits. The basis of the issue is to establish the relationship between the external radiation field and the dose (SAR) distribution in the whole body and the specified tissues. External sources, dosimetric methods and the digital human models are the key elements which could determine that relationship. Because of lack of the non-invasive measurement techniques, numerical methods such as FDTD, FEM and MoM are frequently utilized to qualify the in-body dose distribution. Variability introduced for the numerical methods are insignificant. Given the external radiation field are fixed; the influential factors were include the tissue dielectric properties, postures/statures, anatomical structures, physique features and etc. Practically, they are often interwoven to each other. Many studies were focusing on the surrogate models and the morphing was available in order to find out the worst case with a certain external exposure.

Estimation of absorbed dose to the kidneys in patients after treatment with 177Lu-octreotate: comparison between methods based on planar scintigraphy.

BackgroundLu-[DOTA0, Tyr3]-octreotate (177Lu-octreotate) is used to treat neuroendocrine tumors with high somatostatin-receptor expression. 177Lu-octreotate is mainly excreted via the kidneys, but to some extent, accumulates in the kidney cortex due to, e.g., tubular reabsorption. Renal toxicity is one of the main limiting factors in 177Lu-octreotate treatment. Further knowledge of the biodistribution and dosimetry of 177Lu-octreotate in individual patients is needed. The aim of this study was to estimate the absorbed dose to the kidneys and compare the results obtained with planar imaging and different dosimetric methods: (1) conjugate-view (CV) method using patient-specific kidney sizes, (2) PA method, based on posterior images only, (3) CV method with reduced number of time points (CVreduced data), and (4) CV method using standard kidney sizes (CVstandard size).MethodsTotally, 33 patients each received 3.4 to 8.2 GBq of 177Lu-octreotate up to five times, with infusion of lysine and arginine to block the renal uptake. Whole-body planar gamma camera images were acquired on days 0, 1, 2, and 7. The 177Lu concentration in the kidneys was determined by the CV method, and the absorbed dose was estimated with patient-specific organ sizes. Comparison to the CV method was made using posterior images only, together with the influence of the number of time points and with standard organ sizes.ResultsLarge interindividual variations were found in the time-activity curve pattern and in the absorbed dose to the kidneys using the CV method: 0.33 to 2.4 Gy/GBq (mean = 0.80 Gy/GBq, SD = 0.30). In the individual patient, the mean deviation of all subsequent kidney doses compared to that of the first administration was 1% (SD = 19%) and 5% (SD = 23%) for the right and left kidneys, respectively. Excluding data for day 7 resulted in large variations in the absorbed dose.ConclusionLarge interindividual variations in kidney dose were found, demonstrating the need for patient-specific dosimetry and treatment planning.

Monte Carlo modelling of a silicon strip detector for microbeam radiation therapy

Abstract Microbeam radiation therapy is an experimental technique utilising synchrotron X-rays collimated into a planar array of microbeams. Due to the complex structure of the radiation field and high dose rate, this introduces dosimetric challenges. Current dosimetric methods are inadequate in that they lack either real-time readout, or high spatial resolution. A detector system, consisting of the Silicon Multi-Strip Detector and associated readout system was developed at the University of Wollongong. This system performs online, real-time dosimetry, and is designed for placement upstream of the patient as a transmission detector. The interaction of synchrotron radiation with this detector, both in terms of perturbative effects on the beam and interactions within the detector are investigated with the Monte Carlo simulation toolkit, Geant4. The interaction percentage of the detector with the traversing beam was determined as being (1.97 ± 0.01) %. Energy deposition was found to be uniform across the depth of the detector, negating the most proximal and distal 5-μm region, a phenomenon that may be attributed to a lack of charged particle equilibrium. The effect that the detector has on a traversing beam’s macroscopic dose deposition was determined as being a mean reduction in dose of (1.44 ± 0.15) % for depths in water between 5 and 35 mm.

SU‐E‐T‐127: Two Dosimetric Methods of Measuring Linac Beam Spot Position

Purpose: Two dosimetric methods of measuring the electron beam spot position of photon beams w.r.t. the Collimator Axis Of Rotation (CAOR) are described. Methods: Method 1 (SNG test) uses a cylindrical ion chamber (IC) (A1SL) in a phantom mounted on a jig co‐rotational with the collimator making the relationship among the chamber, phantom, jaws and CAOR fixed and independent of collimator angle. A jaw parallel to the IC axis is set to zero and the phantom adjusted so the IC signal is 65% of the open field value. cGy/mu is measured every 30 degrees of collimator rotation. The phantom is then translated normal to the IC axis until the signal is 35%. The shift is recorded to calculate sensitivity (%dose/mm). For a radially symmetric spot centred on CAOR, the IC signal is angularly constant. Deviation from constant indicates a displacement from the CAOR and a shape distortion. The displacement and spot shape are calculated using Fourier analysis and penumbra shape (sensitivity). The 2nd method uses a hard wedge instead of the jaw and sensitivity is obtained from wedge profiles. Test measurements were performed on an Elekta 6 MV: the In Plane beam spot position was measured before and after it was moved by changing bending magnet current (Bending Fine). Results: Independent beam spot measurement (Flexmap Image Registration) showed the initial beam spot, projected back to source, 0.29 mm from CAOR, moving to 0.91 mm (delta=0.62 mm) with steering. The SNG test reported the beam spot moving from 0.13mm to 0.92 mm from CAOR (delta=0.79 mm). The hard wedge test gave an initial position of 0.09 mm off axis and a final position of 0.79 mm (delta=0.7 mm) Conclusions: The methods described provide a quantitative measure of beam spot position, not requiring the user to independently define the CAOR.Jason Smale is an Employee of Elekta Canada