Dose uncertainty due to aperture effects in dynamic fields.

Dosimetry of intensity modulated radiation therapy requires accurate modeling of the beamlets that comprise each treatment segment. Planning systems such as Varian Eclipse and Philips Pinnacle recommend measuring dose distributions and output factors for fields as small as possible, generally down to at least 2 x 2 cm2. Conventionally, we perform these measurements for regular fields, defined by the secondary collimators. In practice, it is the multileaf collimation system (MLC) that defines the intensity map and provides dynamic dose modulation in either a moving window or segmented step-and-shoot mode. For this review we have only considered the latter delivery mode. Using this method, we have studied aperture motion effects on the dynamic collimator scatter (S(c)), total scatter (S(c,p)), and phantom scatter (S(p)) factors for various combinations of collimator settings (4 x 4-14 x 40 cm2) and dynamically stepped leaf gaps (0.1 to 1.0 cm) in comparison with those for static field factors. For two different Varian linear accelerators, we found similar results in a systematic dependence of collimator scatter on gap width and collimator setting. As the gap increases from 0.1 to 1.0 cm the dynamic collimator scatter factors converge from a maximum difference of about 30% toward the static field values. At the same time, there is no measurable difference between dynamic field phantom scatter factors and those conventionally obtained for static fields. Second, we evaluated the two planning systems as to how well they account for collimator scatter by attempting to mimic the dynamic apertures used above by planning and measuring dose distributions to several small, cylindrical targets for a similar range of fixed collimator settings. We found that the ratio of measured-to-planned doses as a function of target size were similar to the measured, dynamic S(c) data for the Varian Eclipse planning system, indicating underestimation of dose for targets smaller than 1 cm diameter, but were close to unity for the Philips Pinnacle system, suggestive of the underlying differences in the dose calculation algorithms. We discuss the measurements and results and potential impact on the dosimetry of small clinical targets.

Radiation treatment decreases bone cancer pain, osteolysis and tumor size.

Radiotherapy is the cornerstone of palliative treatment for primary bone cancer in animals and metastatic bone cancer in humans. However, the mechanism(s) responsible for pain relief after irradiation is unknown. To identify the mechanism through which radiation treatment decreases bone cancer pain, the effect of radiation on mice with painful bone cancer was studied. Analysis of the effects of a 20-Gy treatment on localized sites of painful bone cancers was performed through assessments of animal behavior, radiographs and histological analysis. The findings indicated that radiation treatment reduced bone pain and supported reduced cancer burden and reduced osteolysis as mechanisms through which radiation reduces bone cancer pain.

Lung dose calculations at kilovoltage x-ray energies using a model-based treatment planning system.

The determination of the dose to organs from diagnostic x rays has become important because of reports of radiation injury to patients from fluoroscopically guided interventional procedures. We have modified a convolution/superposition-based treatment planning system to compute the dose distribution for kilovoltage beams. We computed lung doses using this system and compared them to those calculated using the CDI3 organ dose calculation program. We also computed average lung doses from a simulated radiofrequency ablation procedure and compared our results to published doses for a similar procedure. Doses calculated using this system were an average of 20% lower for AP beams and 7% higher for PA beams than those obtained using CDI3. The ratio of the average dose to the lungs to the skin dose from the simulated ablation procedure ranged from 25% higher to 15% lower than that determined by other authors. Our results show that a treatment planning system designed for use in the megavoltage energy range can be used for calculating organ doses in the diagnostic energy range. Our doses compare well with those previously reported. Differences are partly due to variations in experimental techniques. Using a three-dimensional (3-D) treatment planning system to calculate dose also allows us to generate dose volume histograms (DVH) and compute normal tissue complication probabilities (NTCP) for diagnostic procedures.

Evaluation of a model-based treatment planning system for dose computations in the kilovoltage energy range.

The ability to determine dose distribution and calculate organ doses from diagnostic x rays has become increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedures. In an attempt to determine the dose from both diagnostic and orthovoltage x rays, we have used a commercial treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kilovoltage x rays. The planning system's capabilities for dose computation have been extended to lower energies by the addition of energy deposition kernels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp beams using the system. We compared the dose calculated by the system with that measured using thermoluminescent dosimeters (TLDs) placed in various positions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogeneities, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment plan was generated using CT scans of each of these phantoms and point doses at the positions of TLD chips were calculated. Comparisons of measured and computed values show an average difference of less than 2% within materials of atomic number less than and equal to that of water. The algorithm, however, does not produce accurate results in and around bone inhomogeneities and underestimates attenuation of x rays by bone by an average of 145%. A modification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyond bone.

Generation and use of photon energy deposition kernels for diagnostic quality x rays.

Accurately determining the dose from low energy x rays is becoming increasingly important. This is especially so because of high doses in interventional radiology procedures and also because of the desire to model accurately the dose around low energy brachytherapy sources. Various methods to estimate the dose from specific procedures are available but they only give a general idea of the true dose to various organs. The use of sophisticated three-dimensional (3D) dose deposition algorithms designed originally for radiation therapy treatment planning can be extended to lower photon energy regions. The majority of modern 3D treatment planning systems use a variation of the convolution algorithm to calculate dose distributions. This could be extended into the diagnostic energy range with the availability of lower energy deposition kernels ( < 100 keV). We have used version four of the Electron Gamma Shower (EGS4) system of Monte Carlo codes to generate photon energy deposition kernels in the energy range of 20-110 keV and have implemented them in a commercial 3D treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA). The kernels were generated using the "SCASPH" EGS4 user code by selecting the appropriate transport parameters suitable for the relative low energy of the incident photons. The planning system was subsequently used to model diagnostic quality beams and to calculate depth dose and cross profile curves. Comparisons of the calculated curves have been made with measurements performed in a homogeneous water phantom.

In vivo diode dosimetry for routine quality assurance in IMRT.

Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within +/- 10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved +/- 5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process.

Measurement of the dose deposition characteristics of x-ray fluoroscopy beams in water.

The purpose of this study was to characterize the x-ray dose distribution of fluoroscopy beams by measuring their percent depth dose curves and lateral dose profiles in a water phantom. Percent depth dose curves were measured near the surface with an Attix parallel plate chamber and deep within the water phantom with a Farmer-type cylindrical chamber. Percent depth dose curves were compared to published data where applicable. Lateral dose profiles were measured at depths of 2, 5, 10, and 15 cm in phantom with a Farmer chamber. Pulsed, 50 mA x-ray beams with peak tube potentials of 60, 80, 100, and 120 kV and half value layers of 1.89, 2.52, 3.20, and 4.09 mm Al, respectively, were investigated.

SU-E-T-512: Evaluation of Treatment Planning Dose Calculation Accuracy at the Interface of Prosthetic Devices.

PURPOSE: To evaluate the ability of treatment planning algorithm to accurately predict dose delivered at the interface of high density implanted devices. METHODS: A high density (7.6 g/cc) Cobalt-Chromium-Molybdenum hip prosthesis was molded into an epoxy-based cylindrical leg phantom. The phantom was designed to be separated in half to access the prosthesis and to place the TLDs. Using MVCT to image the apparatus, a simple treatment plan was developed using the Philips Pinnacle treatment planning system. Wires were placed in the molded epoxy to allow for accurate definition of measurement sites (TLD positions) along the surface of the prosthesis. Micro-cube TLDs (1 mm3 ) were placed at six measurement locations for which the dose had been calculated by the treatment planning system. An Elekta Synergy linear accelerator was used to deliver a 400 cGy plan to the phantom with 6 MV photons in a single fraction. A total of four 10 cm × 21 cm fields were used at 0, 90, 180, and 270 degree gantry rotations. RESULTS: Initial results indicate that the measured dose is 7-17% lower than the dose calculated by the treatment planning system. Further study using high energy beams are also in progress. CONCLUSIONS: Initial results indicate that the treatment planning system does predict the dose near a high density prosthetic device within 10-15% but underestimates the dose. The results of this study could help in designing treatment plans which would reduce the uncertainty of the dose delivered in the vicinity of prosthetic hip implants and similar devices.

SU-E-T-272: Commissioning an Orthovoltage Unit Used for Radiobiology Research.

PURPOSE: Orthovoltage X-ray units are used to irradiate cell cultures or small animals in research. Beam characteristics of these units are often not well understood and only nominal dose specified is used by researchers in their studies. This work describes commissioning of an orthovoltage unit similar to that of a linear accelerator. METHODS: The X-Rad 320 Orthovoltage unit is a self-contained x-ray system which can be operatedwithin a wide range of kVp and mAs settings. This work characterizes the beam produced by this system. Various beam data, including depth dose, cross profiles, collimator and total scatter factors, have been measured. Collimator and total scatter measurements were done with cylindrical farmer chambers (0.6 cc and 0.057 cc volumes) for field sizes ranging from 2×2 to 20×20 cm2 . Sc measurements were done in air at 50 cm SCD and Sc,p measurements were done in 2 cm depth of phantom at 50 cm SSD.The depth dose curves were generated for three different field sizes for depths up to 15 cm. A parallel plate chamber was used for surface and near surface dose measurements while the cylindrical chamber was used for other depths. RESULTS: Measurements of Sc and Sc,p indicate minimal variation of these factors with field size, except for very small fields. The results of depth dose measurements produce results comparable with published data for similar beams and indicate a very small dose buildup at shallow depths. CONCLUSIONS: Proper characterization of an x-ray beam and accurate dosimetry is of importance in radiobiology investigations which may lead to advent of new therapies for humans. This work investigates the beam characteristics of an orthovoltage unit to ensure accurate irradiation of samples. The data collected here could further be used for simulation of dose distribution within irradiated volume.

SU-E-J-15: Calculating the Dose from KV Cone Beam CT Within and Outside the Treatment Volume Using a Treatment Planning System.

PURPOSE: To assess the dose from kilovoltage cone beam CT from two image acquisition protocols, pelvis and head and neck, by addition of the dose to the patient treatment plans. METHODS: A total of 20 patients (10 pelvis and 10 head and neck) undergoing radiation therapy were selected and the dose from kV CBCT was calculated using a treatment planning system previously commissioned for this purpose. The imaging dose was added to the CT images used for treatment planning. Daily shifts as a Result of imaging were recorded and applied to imaging beam whenever the sum of the shifts exceeded 0.5 cm. The kV CBCT dose can also be computed prior to planning, in case of IMRT treatments, and used during optimization. RESULTS: The additional dose as a Result of daily CBCT is in the order of few cGy for head and neck and up to 90 cGy for the pelvis cases using the standard head and neck and pelvis protocols. The pelvic dose is especially dependent on patient size, being higher for smaller patients. Due to the low energy of the kV CBCT beam, the maximum energy deposition is at or near the surface with the highest dose being on the patient's left side for the head and neck and on the posterior for the pelvic cases. CONCLUSIONS: Dose from daily kilovoltage CBCT can be added to patient treatment plans using previously commissioned kV CBCT beams in a treatment planning system. In the case of IMRT planning, optimization can be done accounting for kV CBCT dose.

SU-E-I-17: Characterization of Rotating Source MicroCT for Evaluating in Vivo Murine Trabecular Bone.

PURPOSE: The objective of this study is to determine the optimal physical parameters of the Inveon Multi-modality microCT to assess the trabecular bone of mice. METHODS: The x-ray Source-to-Axis Distance is increased from 100mm to 183mm to 263mm. Similarly, combining local pixels, or binning, is examined from no binning to 2 to 4. Energy is varied from 40kVp to 80kVp in 10kVp increments and filter thickness is changed from no filtration to 1.5mmAl in 0.5mmAl increments. A lucite phantom with six different density-equivalent rods is used to measure changes in Hounsfield Units (HU) and calibrate Bone Mineral Density estimation. Mice are scanned at four different magnification and binning combinations to evaluate dose and microstructure changes of high to low resolution images. RESULTS: An increase in magnification and decrease in binning results in an effective pixel size ranging from 95µm to 9µm. This decreases the signal to noise ratio from 19.2 to 1.7HU and density estimation from 1585 to 1383mg/cc for 1250mg/cc equivalent material. Increasing the average energy of the radiation beam also decreases HU estimation from 1466HU to 1132HU. Higher resolution scans extend the scan time and absorbed dose from 5.1 to 13.4min and 3.9 to 125cGy respectively. An 18 micron pixel provides distinguishable trabecular bone surface from cortices with a 4.2HU signal to noise ratio. CONCLUSIONS: A high magnification, binning of 2, 80kVp beam with a 0.5mmAl filter are the optimal parameters to evaluate the trabecular bone of mice for the Inveon MM microCT unit. This work was supported by the National Institute of Health grants (1R01CA154491- 01, 1R03AR055333-01A1 and 1K12-HD055887-01). This work was also supported by PHS Cancer Center Support Grant P30 CA77398. This work was also supported by PHS Cancer Center Support Grant P30 CA77398. Susanta K Hui is a scholar of the BIRCWH (Building Interdisciplinary Careers in Women's Health) program. Luke Arentsen is supported by the Grant in Aid funding from the Universit.

Inhibition of protein prenylation down-regulates signalling by inflammatory mediators in human keratinocytes.

Several inflammatory mediators have been shown to activate phospholipase C in human keratinocytes via GTP-binding protein-coupled receptors. Since GTP-binding proteins are prenylated proteins, we have examined the role of prenylation in signal transduction in HaCaT keratinocytes. Indirect inhibition of prenylation with the HMG CoA reductase inhibitors fluvastatin or compactin decreased bradykinin-stimulated inositol 1,4,5-triphosphate generation. This effect was abolished by mevalonic acid but not by serum, indicating a requirement for a non-sterol metabolite for signal generation. The BK response was also inhibited by zaragozic acids B and C, known inhibitors of prenyl protein transferases. These results suggest that protein prenylation may be a novel therapeutic target in dermatological conditions where an up-regulation of the inositol lipid pathway has been demonstrated.