Index of content:
Volume 32, Issue 7, July 2005
- FIFTY‐FIRST ANNUAL MEETING OF THE CANADIAN ORGANIZATION OF MEDICAL PHYSICISTS AND THE CANADIAN COLLEGE OF PHYSICISTS IN MEDICINE
- CCPM Symposium: Optical Diagnostics and Therapeutics
CCPM Symposium I ‐ 01: Anatomic, functional, and molecular imaging using optical coherence tomography32(2005); http://dx.doi.org/10.1118/1.2030967View Description Hide Description
Optical Coherence Tomography (OCT) is a novel biomedical imaging technique which uses low‐coherence optical interferometry to obtain micron‐scale resolution tomographic images of sub‐surface tissue structure noninvasively. OCT has become a standard diagnostic tool in clinical ophthalmology, and is under investigation for other clinical applications including cancer detection and evaluation of cardiovascular disease. Within the past few years, dramatic technology advances have increased the performance of OCT systems many‐fold, and have also demonstrated the potential for micron‐scale functional and molecular imaging in living systems for the first time. We have developed spectral domain OCT scanners capable of imaging up to several times video rate, and applied them for real‐time two‐dimensional and near real‐time three dimensional imaging in human and small animal models. The applications of this new technology for high‐throughput noninvasive phenotyping and rapid 3D imaging in small animals and developmental biology models is particularly compelling. In addition, we have developed novel functional imaging extensions to OCT which take advantage of the altered spectral content of elastic and inelastically backscattered light to provide enhanced image contrast. These include the first demonstrations of molecular imaging with OCT, in which an imaging form of pump‐probe spectroscopy has been used to image the distributions of genetically expressed proteins with micron resolution in living animals, with sensitivity comparable to multiphoton microscopy.
CCPM Symposium I ‐ 02: Diffuse optical imaging of the neuro‐metabolic‐vascular relationship during brain activation32(2005); http://dx.doi.org/10.1118/1.2030968View Description Hide Description
The ability and interest in functional imaging of the human brain has grown with the advent of positron emission tomography(PET) and functional magnetic resonancetomography (fMRI). These imaging techniques are leading to a better understanding of the healthy, diseased, and injured functioning brain. Diffuse optical imaging is a non‐invasive, portable, and relatively inexpensive method that complements PET and fMRI with the ability to continuously monitor the hemodynamic, metabolic, and possibly neuronal activity state of the brain, and to measure populations of subjects and paradigms not amendable to PET or fMRI. All of these methods are predominantly sensitive to the hemodynamic parameters of the brain which arise from the neuronal and metabolic activity. During this talk, I will discuss the contributions of optical imaging to understanding the relationship between neuronal, metabolic, and vascular activity within the brain. Better knowledge of this relationship will guide the development of better treatments and improve the utility of diagnostic imaging methods.
CCPM Symposium II ‐ 01: Imaging breast tumor tissue in vivo with diffuse light: Tumor tissue characterization and monitoring32(2005); http://dx.doi.org/10.1118/1.2030969View Description Hide Description
Diffuse optical tomography with near‐infrared light has allowed characterization of breast tumortissue with a number of different constituent parameters, which could have relevance for diagnosis and therapy. Multi‐spectral tomography provides quantification of hemoglobin, oxygen saturation, water, and scatterer particle size and density. These parameters are shown for normal and diseased breast tissue, with an eye toward their pathobiological interpretation. The images of tumors present in breast cancer show significant increases in hemoglobin, water and scattering relative to the corresponding normal tissue, and the results of ongoing clinical trials are presented. The scattering particle size is shown to be correlated to the pathologically measured particle sizes in excised breast tissue, and further model‐based interpretation of the scatter signal may yield important structural information at the nanometer level in tissue, as measured macroscopically with NIR tomography.Imaging of fluorescence from tissue is also possible and is presented, along with a demonstration of how the technological design can be altered to allow video‐rate imaging similar to an ultrasound scanner.
32(2005); http://dx.doi.org/10.1118/1.2030970View Description Hide Description
Photodynamic therapy (PDT)‐ the use of light‐activated drugs‐ continues to develop as a viable treatment for solid tumors and dysplasias and for non‐oncologic applications. The underlying optical technologies for light generation, delivery and dosimetry are described. The last, in particular, remains challenging. This, together with the measurement of photosensitzer levels in tissue,tissue oxygenation and the biological effects of PDT, has required development of several different techniques and corresponding clinical and pre‐clinical instruments. These include fiberoptic‐based optical probes, near‐infrared luminescencemeasurement of excited singlet oxygen generated in PDT,measurements of photosensitzer photobleaching, and the use of bioluminescence to assess both tumor (or cell) destruction and gene regulation. Fundamentally new approaches to PDT include the use of ultrafast pulsed lasers for 2‐photon excitation of photosensitizers, the use of ‘metronomic’ (i.e. low dose‐rate) drug and light delivery, and tumor‐specific targeting using phototherapeutic ‘molecular beacons’. Each of these techniques requires multidisciplinary research and development spanning physics, optical engineering, photobiology and clinical specialties.
- J.R. Cunningham Young Investigator's Symposium
32(2005); http://dx.doi.org/10.1118/1.2031023View Description Hide Description
An important step in the image guided adaptive radiotherapy (IGAR) process is the registration of medical images.Image registration has been used in clinically for a number of years; however registration systems have been restricted to linear or rigid registration, meaning that they cannot take into account soft tissue or organ motion with respect to rigid bony structures. Among its applications, non‐linear or deformable registration will allow for more accurate delineation of tumours and critical structures by correcting for organ motion and patient miss‐alignment from image study to study. Since deformable registration is still in its infancy, a standard protocol for the validation of these systems does not exist. A comprehensive protocol to assess the accuracy of deformable registration systems over a wide range of clinical and research applications has been developed. The protocol has been applied to the Reveal‐MVS Fusion Workstation from Mirada Solutions Ltd. It consists of a preliminary phantom study designed to assess the registration of images with well‐defined objects that have known positions, sizes, and shapes. In addition, a collection of novel and established metrics are used to determine image registration accuracy for both, real and simulated patient images. Results show that the Reveal‐MVS system is well suited for some applications of non‐linear image registration, but not applicable for others. Results will be used to further refine and improve upon existing non‐linear image registration algorithms.
32(2005); http://dx.doi.org/10.1118/1.2031024View Description Hide Description
Dosimetry of high‐energy photon beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. This study investigates the wall correction factor, Pwall, in high‐energy photon beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. For cylindrical chambers, dosimetry protocols use an empirical formula to determine Pwall despite high quality experimental evidence that there are problems with this formula. Wall corrections are not provided for parallel‐plate chambers in photon beams due to a lack of information available regarding the correction factors for these chambers. Monte Carlo calculations are carried out using the EGSnrc user‐code CSnrc to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. Calculations of the wall correction are performed for a variety of chambers at the reference depth in photon beams, using realistic beam spectra from clinical accelerators, ranging in nominal energy from to 24 MV. For cylindrical chambers, Pwall values differ by as much as 0.8% from the predicted values. This discrepancy is used to resolve previous experimental results that pointed to problems with the Pwall formalism. Pwall values are also shown for parallel‐plate chambers in high‐energy photon beams and have corrections up to 2%. These data should allow parallel‐plate chambers to be used in photon as well as in electron beams.
Sci‐YIS Fri ‐ 03: Calculation of singlet oxygen dose from photosensitizer photobleaching during mTHPC or Photofrin photodynamic therapy in vitro32(2005); http://dx.doi.org/10.1118/1.2031025View Description Hide Description
Photodynamic therapy(PDT) is emerging as a treatment option for various malignant conditions. PDT damage is caused by the generation of singlet oxygen. This process is dependent on the complex interaction between the photosensitizer (PS), treatment light, and oxygen. Since these parameters may be highly variable among patients and may change during treatment, it is difficult to predict therapeutic outcome based on administered PS and delivered light dose alone. An implicit dose metric model has been proposed in which singlet oxygen dose is monitored by the decrease in PS fluorescence during treatment caused by reactions between PS and singlet oxygen. To investigate this, MatLyLu (MLL) rat prostate adenocarcinoma cells were incubated with Foscan (mTHPC) or Photofrin and treated with the appropriate wavelength of light. Fluorescence was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony formation assay. Singlet oxygen dose models were developed based on measurements of PS fluorescence and reaction kinetics. Cell survival correlated well to calculated singlet oxygen dose, independent of initial PS concentration, treatment fluence rate, and oxygenation. These results indicate that investigation of photobleaching is warranted as an in vivo dose metric. We found 1.5 ± 0.3 and 0.45 ± 0.09 mM of singlet oxygen was required to reduce the survival fraction by 1/e for mTHPC and Photofrin respectively.
Sci‐YIS Fri ‐ 04: Clinical impact of seed density and prostate elemental composition on permanent seed implant dosimetry32(2005); http://dx.doi.org/10.1118/1.2031026View Description Hide Description
The purpose is to evaluate the impact of inter‐seed attenuation and prostate elemental composition in clinical prostate treatment plans with 6711 125I permanent seed implants using the Monte Carlo(MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. The MC toolkit Geant4 is used to perform the simulations. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG43‐based calculation, superposition MC (SMC), full MC with water prostate (MCW), and full MC with realistic prostate tissue (MCP). The SMC method is a technique for which a shifted one‐source MC distribution is added to the total dose distribution for each source position. It was found that seed density has a definite influence on inter‐seed attenuation. A typical low seed density (42 0.6 mCi seeds in a 26 cc prostate) yields a 0.7% drop in the CTV D90 value when comparing SMC to MCW while a drop of 2.5% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate composition is similar for all plans. When comparing MCW to MCP, the difference in total dose deposited in the CTV is 2.9 +/− 0.2%, while it is 3.6 +/− 0.3% for the D90 parameter. Overall, the effect on D90 is ranging from 4.3% to 9.3% when comparing TG43 to MCP. This effect is clinically significant.
32(2005); http://dx.doi.org/10.1118/1.2031027View Description Hide Description
The Electron Sealed Water (ESW) calorimeter is a new device designed for measurements in clinical electron beams. This work presents the calorimeter design, preliminary measurements and evaluation of correction factors. The calorimeter contains a 30×30×20 cm3 water phantom surrounded by a cooling system to allow for operation at 4°C. Two thermistor probes measuretemperature change inside a glass vessel filled with high‐purity nitrogen‐saturated water. Correction factors for glass perturbation were calculated using Monte Carlo simulations. Corrections for thermal conduction were simulated using Femlab software. Resistance of the thermistor probes was measured using a lock‐in amplifier and an AC bridge circuit. Measurements were done for 6, 9, 12, 16 and 20 MeV electron beams from a Varian Clinac 21EX. Irradiations of 667 MU were done at 1000 MU/min for an irradiation time of 40 s. Measurements were also taken with a PTW Roos ion chamber inside the calorimeter phantom. For 30 runs, the standard error on the mean temperature change was less than 0.2% for all energies. When normalized to the 12 MeV measurements, values of k′R‐50 for the Roos chamber calculated from calorimetermeasurements for the 9, 16 and 20 MeV beams agreed with TG‐51 values within 0.7 %. Water calorimetry in electron beams has previously been regarded as unfeasible as high dose gradients were thought to provoke unmanageable temperature gradients. Using the ESW calorimeter, we have shown for the first time that reproducible measurements can be performed in electron beams with energies as low as 6 MeV.
Sci‐YIS Fri ‐ 06: High spatial resolution magnetic resonance spectroscopic imaging of the brain at 3T for external beam radiation therapy planning32(2005); http://dx.doi.org/10.1118/1.2031028View Description Hide Description
Recent research has shown that Magnetic Resonance Spectroscopic Imaging(MRSI) has good prospects for cancer detection and staging. Recent advancements in hardware in the form of RF coils and gradient sets have resulted in a good outlook for MRSI to be used in detection, localization, and classification of the lesions. Work done by other groups has shown that merging molecular imaginginformation from MRSI with anatomic scans used for treatment planning proved beneficial in delineating the tumor volume. MRSI involves defining an MRimage volume which can be divided into a matrix of 3‐D sub‐volumes. Spectroscopic data is then acquired for all these sub‐volumes simultaneously. This leads to the quantification and localization of different levels of metabolites such as Choline, Creatine and N‐ acetylaspartate that are highly useful in detecting and possibly staging lesions. We focused on performing MRSI on the brain using a head transmit/receive (T/R) coil. Utilizing a reduced 2‐Dimensional Point Resolved Spectroscopy (2‐D PRESS) Turbo Spectroscopic Imaging (TSI) sequence to improve the spectral resolution of different metabolite peaks and the spatial resolution of the spectroscopic scan, good spectral/spatial information can be acquired revealing more accurate bio‐chemical imaging. Thus, an MRSI study could be easily used for improved treatment planning. Using 3T MRI we have improved the spatial resolution of the MRSI scans to 5×5×10mm3/voxel, meanwhile maintaining a good signal to noise ratio. This would increase the spatial relevance of the spectra acquired in the scan and in turn provide more information in the treatment planning phase.
32(2005); http://dx.doi.org/10.1118/1.2031029View Description Hide Description
Survival curve behaviour and degree of correspondence between Linear Quadratic model1 (LQ) and experimental data in an extensive dose range for high dose rate was analyzed. Detailed clonogenic assays with irradiation given in 0.5Gy increments and a total dose range varying from 10.5 and 16Gy were performed. The cell lines investigated were: CHOAA8, hamster fibroblast cells; U373MG, human glioblastoma cells; and human prostate carcinoma cell lines CP‐3, and DU‐145. The analyses were based on χ2 statistic and Monte Carlo simulation of the experiment. A decline of fit quality at very low doses (<2Gy) is observed. This result can be explained by the hypersensitive effect observed in CHOAA8 and U373MG data and an adaptive type response in CP3 cell line. A clear improvement is discerned at slightly higher doses. This could be a result of the linearity existent in the trend of survival curve at low doses, that will affect the total fit in a range from 0Gy to final dose in linear quadratic region. The impact of including low dose data is shown through α/β ratios showing relative differences of 42, 40 and 23% for CHOAA8, CP3 and U373MG. LQ model cannot explain survival at high doses. This is shown as a deterioration of goodness of fit for high doses. A comparison of Linear Quadratic Linear model2 (LQL) with the LQ in fitting the experimental data at high doses was also performed.
Sci‐YIS Fri ‐ 08: Regional change in brain perfusion after fractionated stereotactic radiotherapy (FSRT) at 4 months and 3 years follow‐up32(2005); http://dx.doi.org/10.1118/1.2031030View Description Hide Description
Purpose: The change in hemodynamic parameters, such as mean transit time and cerebral blood volume, reflect the damage to vasculature. A relationship between the change in hemodynamic parameters and radiationdose delivered would help predict the degree and nature of damage, and would be most beneficial for patients with a long life‐expectancy who are at risk of long‐term radiation‐induced injury. Method and Materials: We applied the relative perfusion weighted MRI technique, currently used in strokeimaging, to calculate the relative regional mean transit time (rrMTT) and relative regional cerebral blood volume (rrCBV). We acquired data for one patient. We used a 3.0 T magnet at the Seaman Family MRI Centre in Calgary and a single‐shot echo‐planar imaging (EPI) sequence following the injection of a paramagnetic contrast agent (Gd‐DTPA‐Magnevist; Berlex, Wayne, NJ). These images have been processed to yield rrMTT and rrCBV. The patient had previously been treated with surgery, but had received no chemotherapy. The percentage change in rrMTT and rrCBV was correlated to the spatial distribution of radiationdose delivered using the Pinnacle® radiation treatment planning system. Results: Our preliminary results show that with a follow‐up time of 4 months and 3 years after receiving approximately 5000 cGy/25 fractions, rrMTT and rrCBV change significantly in normal tissue and tumour. The most important normal tissue changes occur in the near‐target area. Conclusion: The change in rrMTT and rrCBV indicates response to treatment. Perfusion weighted MRI can be used to assess the change in hemodynamic measures after radiotherapy.
32(2005); http://dx.doi.org/10.1118/1.2031031View Description Hide Description
We present a novel matrix dosimeter with high sensitivity, good precision and reproducibility based on plastic scintillation dosimetry. This dosimeter possesses excellent water equivalence and linearity to dose, dose rate and energy. It was built by coupling scintillating fibers, chosen for their high collection efficiency, to clear optical fiber. Light measurements were performed with a color CCDcamera in order to compare two techniques to remove the stem effect: background subtraction and chromatic filtering. Chromatic filtering uses light at two different wavelengths (green and blue channels of the CCD) to remove the undesired Cerenkov radiation. Irradiations were performed at 6 MV for various doses and field sizes. Stem effect caused by Cerenkov radiation ranged from 5 % (5×5 field) to 35 % (30×30 field) of the total signal. Chromatic filtering and background subtraction both allow to extract from the signal the stem effect with a similar precision. Linearity of the system was validated down to 2.5 cGy (1.31 % standard deviation) with 0.0055 cm2 probes. At 10 cGy the standard deviation dropped below 0.6 %. Depth dose curves were also measured with a precision below 1 % compared to ionization chambermeasurements. Chromatic filtering removes the necessity of a second optical fiber for background subtraction, therefore increasing the spatial resolution while maintaining a precision below 1 % for most of the dose range. The CCDcamera allows more than 150 detectors in its field of view, which can be used in water, in phantom or in vivo.
Sci‐YIS Fri ‐ 10: Tomographic composition analysis of intact urinary calculi by x‐ray coherent scatter32(2005); http://dx.doi.org/10.1118/1.2031032View Description Hide Description
Knowledge of urinary stone composition and structure provides important insights in guiding treatment and preventing recurrence. No present method can successfully provide information relating structure and composition of intact stones. We are developing a tomographic technique that uses measures of coherently scattered diagnostic x rays to yield stone composition and structure. Coherent‐scatter (CS) properties depend on molecular structure and are, therefore, sensitive to material composition. For powdered, amorphous or polycrystallinematerials with no significant parallel crystal orientation, CS patterns are azimuthally symmetric. In materials with preferred crystallite orientation, such as urinary stones, bright spots appear in their CS patterns. This may compromise a composition analysis based on comparing CS measurements from urinary calculi to a library of CS signatures from powdered chemicals. We show that a tomographic reconstruction of CS measurements (CSCT) effectively eliminates bright spots and yields CS patterns equivalent to powders. This allows for direct comparison with a powdered chemical reference library and provides more accurate material identification. Validation was achieved using an aluminium rod phantom, which exhibits bright spots much like calculi. CSCT composition analysis was performed on intact stones deemed chemically pure by infrared spectroscopy. Computed tomographic reconstruction of CS signals allowed the generation of composition maps, showing the distribution of components. These images provide strong evidence that current laboratory techniques risk missing critical stone components in their analysis due to inadequate sampling. This supports the development of CS analysis as a stone analysis technique both in the laboratory and possibly in situ.
- Poster Session and Reception
32(2005); http://dx.doi.org/10.1118/1.2030980View Description Hide Description
The purpose of this project was to design, fabricate and test the data acquisition timing control, precision rotary stage control, and analog data multiplexer unit for a prototype megavoltage computed tomography (MVCT) detector. An 80‐element prototype detector array is made with (element size 0.275 × 0.8 × 1 cm3) scintillators and photodiodes on an arc (radius of 110 cm). In addition to designing and fabricating an in‐house data acquisition system (front‐end integrators, data multiplexer unit, and timing control), a precision rotary stage and its control are added to create a third generation MVCT scanner.Data acquisition is synchronized with radiation pulses from a linac. Response of detector as a function of dose rate was studied by varying the source to detector distance. A narrow slit beam, at five locations, was used to measure the pre‐sampled MTF. Detector signal in open beam was measured for a number of radiation pulses to use the periodogram method for NPS estimation. Using the measured MTF, NPS, and photon fluence impinging on detector,DQE was calculated. Detector response is linear as a function of dose rate, however shows a non‐linear component while measuring the attenuation by solid water due to the polyenergetic spectrum. Beam‐hardening correction is necessary before image reconstruction. MTF at the Nyquist frequency (0.16 mm−1) is 0.48. Zero‐frequency DQE in 6 MV at 21% is higher than any experimental MVCT detector. The basic performance of the prototype detector is satisfactory for producing reasonable low contrast resolution in MVCT images with low dose.
32(2005); http://dx.doi.org/10.1118/1.2030981View Description Hide Description
The objective of this work was to study the feasibility of acquiring mega‐voltage cone‐beam CT (MV CBCT)images of sufficient quality for setup verification from 2 D projection images. The generation of mega‐voltage cone‐beam CT with a standard radiation therapy Primus linear accelerator and an amorphous silicon electronic portal imaging device has been attempted in this work. 2 D projection images of the head section of a Rando phantom were acquired at an interval of 2° from gantry angle of 258° to 102°. Thus 103 2D projection images of the Rando phantom were acquired with 6 MV beam with 1 MU/projection exposure at 300MU/minute dose rate. The estimated dose to the center of the phantom placed at the isocentre of the linac was about 75cGy. The cone angle was only 14.2° and hence for simplicity the images were reconstructed assuming a parallel beam geometry using the IRADON function implemented in MATLAB. High contrast objects are clearly seen on the reconstructed images thus showing the potential of MV CBCT for patient alignment during external beam radiotherapy. From the available image data set, reconstructions were also performed with fewer projections i.e with 52 and 26. Image quality was acceptable with 103 and 52 projections. Reconstruction with Feldkamp cone beam algorithm or iterative algorithm such as Algebraic Reconstruction Techniques (ART) could further improve the image quality.
Po‐Poster ‐ 03: A comparison of intensity‐modulated radiotherapy versus 3D conformal radiotherapy for prostate cancer patients whose rectal reaction to prior conventional radiotherapy was known32(2005); http://dx.doi.org/10.1118/1.2030982View Description Hide Description
Background and purpose: To evaluate the difference in biological and physical endpoints between intensity‐modulated radiotherapy(IMRT) plans and conformal plans for thirteen prostate cancer patients whose rectal reaction to prior radiation therapy was known (five patients experienced Grade 0 late rectal complications, six Grade 1, and two Grade 2). Materials and methods: Conformal plans were generated for each patient with Eclipse version 7.1.59 while the Helios inverse optimization component within Eclipse was applied for computation of the IMRT plans. Physical endpoints applied to compare the IMRT and conformal plans included the target dose statistics (minimum, maximum, and mean dose) as well as dose‐volume histogram results for the normal tissues (rectum, bladder, and femoral heads). Biological endpoints used for comparison of the IMRT and conformal plans were tumour control probability (TCP), normal tissue complication probability (NTCP), and probability of uncomplicated local control (P+). Rectal NTCP was computed for both RTOG Grade 3 or lower complications and severe rectal complications (severe proctitis, necrosis, stenosis, and fistula). Results: On average, IMRT increased the TCP by 2.8% and increased the TDI by 7.7%. The average NTCPs for the Grade 3 or lower rectal complications were 15.1% and 4.4% for the conformal and IMRT plans, respectively. For severe rectal complications the average NTCP decreased from 2.1% (conformal) to 1.2% (IMRT). The average P+ increased from 59.4% (conformal) to 69.9% (IMRT).Conclusions:IMRT was found to reduce the rectal NTCP for RTOG Grade 3 or lower complications to ∼0.36 times the NTCP for the conformal plan.
32(2005); http://dx.doi.org/10.1118/1.2030983View Description Hide Description
A daily test was developed to assure intensity modulated fields are delivered with a high degree of accuracy and precision using the dynamic multileaf collimator delivery mode. Quality assurance of the MLC, particularly for dynamic mode of delivery, requires positional accuracy of individual leafs, as well as gap width defined by opposing leaf pairs. The daily test was designed to be quick, simple to use, and easy to analyze. With the secondary collimating jaws at 25 cm wide and 39 cm long, a 5mm sliding gap over 5 cm at isocenter placed between two static fields 4 cm off axis was measured dosimetrically using Kodak XV film. The sliding gap allows us to test dosimetrically the positional accuracy defined by Varian in motor encoder counts, as a function of beam on time. The speed of the leaves is controlled by the MUs delivered. Relative filmdosimetry with respect to a baseline film obtained after MLC alinment is performed daily and analyzed using RIT software. In order to facilitate the registration of the QA film to the baseline film, a mounting device utilizing the upper wedge block was used. The QA test was tested using beam shaper files with controlled errors introduced and dose differences of 5% are easily detected. In addition to the daily film QA, as part of morning photon output checks a sliding gap output is measured and these results will be presented as well.
Po‐Poster ‐ 05: An evaluation of treatment dose error due to beam attenuation from a carbon fiber table top32(2005); http://dx.doi.org/10.1118/1.2030984View Description Hide Description
The emergence of carbonfiber materials for use in radiation therapy was largely due to its high mechanical strength, low specific density, and its perceived radio‐translucence. These characteristics made it an ideal material for the patient support assembly utilized during treatments. Modern radiation therapy commonly employs beams delivered at oblique angles. With the introduction of carbon fiber table tops the attenuation of the couch is often ignored during treatment planning and there is little effort to avoid intersection of the beam with the table during patient setup. The perception that carbon fiber is relatively radio‐translucent has permitted it to be used while neglecting to consider the effects it may have on the dose to the patient. In this study we have measured the attenuation of the couch under various conditions for 6 and 18 MV photons. We have found dose reductions in phantom of greater than 7%. We further investigate the ability of a commercial treatment planning system (Theraplan Plus) to properly model this effect during the planning stage. Our results show that incorporating the carbon fiber couch in the patient model reduces the dose error to less than 2%. These results reveal that it is worthwhile addressing this real clinical problem in such a manner that it can be routinely considered for all patient treatments. Thus, practical suggestions are proposed for the incorporation of the treatment tabletop into patient treatment planningdose calculations.
32(2005); http://dx.doi.org/10.1118/1.2030985View Description Hide Description
Megavoltage cone‐beam computed tomography (MV‐CBCT) is a volumetric imaging method that can improve patient setup verification techniques. MV‐CBCT utilizes the treatment beam to obtain projections at every 1–2° around the patient. For this to be clinically acceptable, total dose received by the patient from all imaging sessions must be kept to a minimum and typically should be ⩽5% of the prescribed dose. This necessitates the use of extremely low doses (<<1MU) in the acquisition of each projection. At such low dose levels beam spot instability is known to exist and can compromise image quality. The purpose of this work is to quantify the beam spot motion of Siemen's accelerator for a conventional 6 MV beam and 5.4 MV “imaging beam” generated with a beryllium target. This was accomplished by using an a‐Si flat panel detector to image a cone‐beam geometric calibration phantom and using a calibration algorithm to derive the spot motion in reference frames fixed in space and/or attached to the gantry. Motion of the beam spot was observed immediately after beam startup primarily in the gun‐target direction. The maximum fixed motion of the 6MV beam spot was 1.1±0.3 mm and is similar to that observed for the low Z beam (1.2±0.1 mm). However, the beam spot position of the latter stabilized at about 0.5 MU compared to 6 MU for the 6 MV beam and had much less fluctuations once stabilized. The beam spot position of the conventional beam was much less reproducible than the low Z beam.