Melissa M. Blough

X-ray spectra estimation using attenuation measurements from 25 kVp to 18 MV

Attenuation measurements for primary x-ray spectra from 25 kVp to 18 MV were made using aluminum filters for all energies except for orthovoltage where copper filters were used. An iterative perturbation method, which utilized these measurements, was employed to derive the apparent x-ray spectrum. An initial spectrum or pre-spectrum was used to start the process. Each energy value of the pre-spectrum was perturbed positively and negatively, and an attenuation curve was calculated using the perturbed values. The value of x-rays in the given energy bin was chosen to minimize the difference between the measured and calculated transmission curves. The goal was to derive the minimum difference between the measured transmission curve and the calculated transmission curve using the derived x-ray spectrum. The method was found to yield useful information concerning the lower photon energy and the actual operating potential versus the nominal potential. Mammographic, diagnostic, orthovoltage, and megavoltage x-ray spectra up to 18 MV nominal were derived using this method. The method was validated using attenuation curves from published literature. The method was also validated using attenuation curves calculated from published spectra. The attenuation curves were then used to derive the x-ray spectra.

Calculated mammographic spectra confirmed with attenuation curves for molybdenum, rhodium, and tungsten targets

A model for calculating mammographic spectra independent of measured data and fitting parameters is presented. This model is based on first principles. Spectra were calculated using various target and filter combinations such as molybdenum/molybdenum, molybdenum/rhodium, rhodium/rhodium, and tungsten/aluminum. Once the spectra were calculated, attenuation curves were calculated and compared to measured attenuation curves. The attenuation curves were calculated and measured using aluminum alloy 1100 or high purity aluminum filtration. Percent differences were computed between the measured and calculated attenuation curves resulting in an average of 5.21% difference for tungsten/aluminum, 2.26% for molybdenum/molybdenum, 3.35% for rhodium/rhodium, and 3.18% for molybdenum/rhodium. Calculated spectra were also compared to measured spectra from the Food and Drug Administration [Fewell and Shuping, Handbook of Mammographic X-ray Spectra (U.S. Government Printing Office, Washington, D.C., 1979)] and a comparison will also be presented.

Monte Carlo characterization of target doses in stereotactic body radiation therapy (SBRT)

To compare finite-size pencil beam/equivalent path-length (FSPB/EPL) and Monte Carlo (MC) SBRT dose computations for serial tomotherapy and to quantitatively assess dose differences between the dose calculation methods. Based on 72 SBRT plans for pulmonary targets, FSPB/EPL, considering the inhomogeneous lung environment, and MC calculations were performed to establish differences between FSPB/EPL predicted dose and MC derived doses. Compared with MC, FSPB/EPL consistently overestimated minimum doses to the clinical target volume and planning target volumes by an average of 18.1±7.15% (range 4 to 33.4%), and 21.9±10.4% (range 1.2 to 45.5%), respectively. The respective mean target dose differences were 15.5±7.4% (2.8–36.4%) and 19.2±7.6% (3.6–40.1%). Deviations from MC doses were lesion size and location dependent, with smaller lesions completely embedded into lung parenchyma being most susceptible. Larger lesion in contact with mediastinum and chest wall showed lesser differences. In comparison with MC dose calculation, FSPB/EPL overestimates doses delivered to pulmonary SBRT targets. The observed dose differences may have impact on local tumor control rates, and may deserve consideration when using fast, but less accurate dose calculation methods.

Half-value layer and intensity variations as a function of position in the radiation field for film-screen mammography.

Differences in half-value layer (HVL) and radiation intensity are investigated as a function of position in the mammographic radiation field. Sources of systematic variation include the heel effect, the inverse square law, and differential photon path lengths through thicknesses of inherent and added filtration. The combination of these effects can increase the HVL by as much as 9% and reduce intensity by as much as 40% along the cathode-anode axis. To the left and right of the x-ray field central axis, reductions in radiation intensity of up to 9% and minor increases in HVL are noted as well. Optical density variations as a function of position in the field correlate well with the measured radiation intensity changes.

Dosimetric effects of air pocket sizes in MammoSite® treatment as accelerated partial breast irradiation for early breast cancer

MammoSite brachytherapy system has been used as one of the accelerated partial breast irradiation (APBI) techniques since 2002. The clinical results from several clinical institutions had shown comparable treatment efficacy, cosmesis, and toxicity to other APBI techniques. During MammoSite treatment, air cavities had been one of the primary issues causing treatment cancellation or delay. With the tolerance of the air volume less than 10% of the total Planning Target Volume (PTV) set, there is still no data available to show the actual dose delivered to the breast tissue with the existence of the air pocket. In this paper, Monte Carlo N‐Particle version 5 (MCNP5) was used to model a hypothesis MammoSite phantom with different sizes of air pockets, and compared to the calculation results from the treatment planning system (TPS) without heterogeneous corrections. It was found that without heterogeneous corrections, the difference between the TPS and MCNP5 calculations in the air cavity surface doses and PTV point doses can be up to 2.02% and 3.61%, respectively, using the balloon and air pocket size combinations calculated in this paper. Based on the distance from the point of interest to the balloon surface, an approximate dose can be calculated using the linear relationship found in this study. These equations provide a quick and simple way to predict the actual dose delivered to the breast soft tissue located within the PTV. With the equation applied to the dose from the TPS, the dose error caused by the air pocket during MammoSite treatment can be reduced to a minimum. PACS number: 87.53.Jw

SU‐FF‐T‐171: Dose Differences Due to Air Pockets in Mammosite Treatments for Partial Breast Irradiation

Purpose: To evaluate the effect of air pockets on the delivered dose for MammoSite® treatments using Monte Carlo (MC) calculations, conventional treatment planning system (TPS), and TLD measurements. Method and Materials: A solid water phantom was designed and fabricated to simulate a MammoSite® treatment with an air pocket outside the balloon. Dose measurements were performed using TLD‐100s with dimensions of 3 mm × 3 mm × 0.9 mm and sensitivity of ±3%. The phantom was composed of twelve slabs of 30 cm × 30 cm Plastic Water with varying thicknesses to provide different measurement distances from the balloon surface. The balloon volume was 34 cc, with a diameter of 4 cm. No contrast medium was added to the balloon to avoid possible dose effects due to the contrast. Hemisphere‐shaped air pockets with different radii were milled on top of the balloon surface. An MC algorithm with geometric modeling based on the phantom design was created for dose comparison. The dose discrepancies from the TPS, TLD measurements, and MC calculations were compared. Results: For a 25 mm air pocket, measured doses from TLDs at various locations of the customized phantom and MC results agreed with each other. The maximal discrepancy between TLDs and MC at different measured points was 3.68%. Dose differences between TPS and MC calculation at the air pocket was 6.37%, and less than 5% at locations other than air pockets. Doses will also be shown for a larger air pocket (5 mm radius). Conclusion: The air pocket located outside of MammoSite® balloon does affect the dose to surrounding tissue and this dose can be represented using MC calculations.

SU-FF-T-120: Characterisation of Dose in SBRT Lung Via Monte Carlo

Purpose: To accurately characterize the doses received by static lung lesions, as well as doses to the critical structures for the serial tomotherapeutic IMRT delivery method used for SBRT in our clinic. Method: 77 SBRTlung patients previously treated with doses calculated using the effective path length/Finite size pencil beam (EPL/ FSPB were retrieved. The critical structures (ipsi‐lateral lung, contra‐lateral lung, major airways, spinal cord and esophagus) were redelineated in order to standardize the contouring. All plans were run with Monte Carlo(MC), EPL/FSPB and No inhomogeneity correction (NI/FSPB). The intensity maps and MUs were the same for all three plans. The minimum, maximum and mean target doses were compared with MC calculation used as the benchmark. The normalized total dose, NTD; minimum, mean and maximum doses for critical structures were also compared. Results: The mean CTV volume of the 90 lesions presented here is 35.6 cm3 (range: 0.3–370.2 cm3). The minimum dose to both CTV and PTV were overestimated by the EPL/FSPB algorithm by an average of 17.3 ± 7.8% and 20.6 ± 10.8% of prescribed dose respectively. The absolute mean deviation in the minimum CTV and PTV doses were 5.7% (0.2–20.1) and 10.6% (0.03–27.3) respectively with NI. The magnitude of deviation depends on target location (embedded dense soft tissue, surrounded by lung and its proximity to a more dense interface) and dimensions. The minimum dose, mean dose and NTD for the lungs were in good agreement with MC. Larger, localized discrepancies exist for maximum dose. Doses to the other critical structures were generally in good agreement with those predicted by MC.Conclusion:MC dose calculation may prove valuable in accurately assessing the delivered dose in SBRT and may, thus, contribute to a more informed decision on the optimal dose and fractionation scheme.

SU‐FF‐T‐07: The Application of Sr‐90/Y‐90 for the Prevention of Abdominal Adhesions

Recently, a resurgence in the use of beta particles from 90Sr/90Y has occurred, primarily due to its use in intracoronary brachytherapy. 90Sr/90Y has also been employed in the ophthalmologic community for postoperative irradiation of pterygia. Due to these successes and other advantageous results of irradiating benign tumors and diseases, a new use for the 90Sr/90Y ophthalmologic applicator has been hypothesized: the use of beta radiation for the prevention of abdominal adhesions. To characterize the source for this use, preliminary measurements were made in a polystyrene phantom using GafChromic film and TLDs. Our results compared favorably with measured values for clinically relevant depths. Upon completion of the phantom measurements, experiments commenced to test the hypothesis in an animal model. Two potential adhesion sites were created in the abdomen of rats via denudation of the serosa of the small intestine. Irradiation of one site with the 90Sr/90Y beta applicator occurred; the other site was used as a positive control (no radiation). A 10-day recovery followed, allowing adhesion formation if it occurred; the animals were then euthanized and the injured areas analyzed for efficacy of treatment. Nine Sprague-Dawley rats were irradiated with varying doses (to determine a dose-response relationship) to a prescribed depth of 1mm. This choice was based on the Novoste clinical trials, clinical treatment depths for the 90Sr/90Y applicator, and experimental research on 90Sr/90Y beta particles (Buckley et al., 2001). The animals were sacrificed and gross and microscopic pathology was performed. Results show that radiation is effective in preventing adhesion formation. Eight of nine irradiated sections showed no formation of adhesions, while the ninth developed a single adhesion, whereas twelve of thirteen unirradiated sections formed adhesions. The Mann-Whitney U test yielded a p-value of 0.022 confirming the effectiveness of adhesion prevention by the addition of small amounts of beta radiation.

Ct Guided Prostate Brachytherapy Using Pinpoint Stereotactic Arm: P-36

CT guided implants are advantageous due to the imaging ability of CT and the ease of visualization of the prostate compared to ultrasound. In order to optimize this, Marconi developed a stereotactic arm (Pinpoint) which is correlated to CT data. This allows the user to see the path of a virtual needle through the patient in real time. An actual needle is then inserted along this path. This technology has been successfully applied to prostate implants. To determine a treatment technique, a CT scan is performed. The patient is positioned prone with the legs spread apart to allow access to the prostate. The physician determines the treatment volume, the source type and the amount of radiation to be given. The pre-planning involves determining a dose distribution as is typically done with TRUS prostate implants; however, the needle trajectories are determined by using the Pinpoint stereotactic arm instead of a template. This allows more freedom in seed placement. For the actual treatment, the patient is repositioned, and a second CT is performed to verify setup. A sterile field is set covering the treatment area. The arm is moved into position for the first needle trajectory, and the interventional radiologist guides the needle into place. The needle position is verified with CT. This needle is used to anchor the prostate while more needles are inserted. The position of these needles is verified, and the radiation oncologist places radioactive seeds at the planned coordinates based on the preplan. The seed positions can be verified with CT or with FACTS (fluoroassisted computed tomagraphy). Two patients have been implanted using this technology under spinal block anesthesia. Both patients tolerated the procedure well, and post-implantation dosimetric calculations were performed. Long term followup will yield further clinical data. This method is effective for performing prostate implants. It allows for more precise visualization of the prostate, and more accurate seed placement due to the ability to view the seeds in three-dimensions during the implant.

Measured water transmission curves and calculated zero field size tumor maximum ratios for 4, 6, and 15 MV x-rays

A multihole diverging Cerrobend plug for megavoltage energies was used to measure water transmission values at different locations in a 20 x 20 cm field at 100 cm source-to-axis distance (SAD) for 4, 6, and 15 MV therapy photon beams. The transmission curves in water were measured at 25 locations across the 20 x 20 field, and each location was separated by 5 cm at the isocenter. Each transmission value was made using a 0.3175 cm diameter (0.079 cm2 area) hole of 20 cm length at the central axis (CAX). The small field measured transmission curve in water was used to derive the zero field size tumor maximum ratio (TMR) and the primary photon exposure spectrum as a function of energy at depth. The exposure spectrum was used to find an effective photon energy and linear attenuation coefficient at depth and at different locations in the field. These values were found to vary with location in the field.