Multiple Gantry Research Articles

Dose Verification of Stereotactic Radiosurgery Treatment for Trigeminal Neuralgia with Presage 3D Dosimetry System

Achieving adequate verification and quality-assurance (QA) for radiosurgery treatment of trigeminal-neuralgia (TGN) is particularly challenging because of the combination of very small fields, very high doses, and complex irradiation geometries (multiple gantry and couch combinations). TGN treatments have extreme requirements for dosimetry tools and QA techniques, to ensure adequate verification. In this work we evaluate the potential of Presage/Optical-CT dosimetry system as a tool for the verification of TGN distributions in high-resolution and in 3D. A TGN treatment was planned and delivered to a Presage 3D dosimeter positioned inside the Radiological-Physics-Center (RPC) head and neck IMRT credentialing phantom. A 6-arc treatment plan was created using the iPlan system, and a maximum dose of 80Gy was delivered with a Varian Trilogy machine. The delivered dose to Presage was determined by optical-CT scanning using the Duke Large field-of-view Optical-CT Scanner (DLOS) in 3D, with isotropic resolution of 0.7mm3. DLOS scanning and reconstruction took about 20minutes. 3D dose comparisons were made with the planning system. Good agreement was observed between the planned and measured 3D dose distributions, and this work provides strong support for the viability of Presage/Optical-CT as a highly useful new approach for verification of this complex technique.

Tracking Accuracy of a Real-Time Fiducial Tracking System for Patient Positioning and Monitoring in Radiation Therapy

In radiation therapy there is a need to accurately know the location of the target in real time. A novel radioactive tracking technology has been developed to answer this need. The technology consists of a radioactive implanted fiducial marker designed to minimize migration and a linac mounted tracking device. This study measured the static and dynamic accuracy of the new tracking technology in a clinical radiation therapy environment. The tracking device was installed on the linac gantry. The radioactive marker was located in a tissue equivalent phantom. Marker location was measured simultaneously by the radioactive tracking system and by a Microscribe G2 coordinate measuring machine (certified spatial accuracy of 0.38 mm). Localization consistency throughout a volume and absolute accuracy in the Fixed coordinate system were measured at multiple gantry angles over volumes of at least 10 cm in diameter centered at isocenter. Dynamic accuracy was measured with the marker located inside a breathing phantom. The mean consistency for the static source was 0.58 mm throughout the tested region at all measured gantry angles. The mean absolute position error in the Fixed coordinate system for all gantry angles was 0.97 mm. The mean real-time tracking error for the dynamic source within the breathing phantom was less than 1 mm. This novel radioactive tracking technology has the potential to be useful in accurate target localization and real-time monitoring for radiation therapy.

Determining crane areas in intermodal transshipment yards: The yard partition problem

At rail–road transshipment yards, gantry cranes move containers from freight trains to trucks and vice versa. They constitute important entities in today’s intermodal transportation systems. Real-world yards are often partitioned into several disjunct crane areas, so that crane interferences during container transshipment are avoided. In practice, the lengths of such crane areas are typically determined by simple rules of thumb, i.e., each crane receives an equally sized area, which might result in an unleveled division of labor among cranes and, thus, prolong train processing times. This paper provides an exact solution procedure which determines disjunct yard areas of varying size for multiple gantry cranes in polynomial runtime, so that the workload for a given pulse of trains is equally distributed among cranes. Furthermore, we investigate the potential acceleration of train processing as compared to equally sized areas in a yard simulation.

SU‐GG‐J‐81: Geometric Accuracy of Imaging Systems on Trilogy MX Using an Automated Geometric Test Tool

Purpose: To characterize the geometric accuracy of the imaging systems of a newly designed accelerator (Trilogy MX, Varian Medical Systems, Palo Alto, CA) using a fully automated, geometric calibration tool. Methods: A geometric calibration tool ‐ consisting of a phantom containing 16 tungsten carbide ball‐bearings and a MV collimator insert with a central pin ‐ has been used to characterize the geometric coincidence of MV and kV imaging equipment with the radiation isocenter of Trilogy MX. Three sets of data are acquired: MV images at multiple MV collimator rotations, and both MV and kV images at multiple gantry angles. The MV collimator images are used to identify the central axis of the MV beam, and the MV images at different gantry angles are then used to identify the MV radiation isocenter. Calculations then determine the corrections needed to move both the MV and kV imagers so that their central pixels exactly align with the projection of the radiation isocenter. The Trilogy MX employs a new control system so corrections can be downloaded to the control system and used to physically correct the imager positions while the gantry is being rotated. The positions of the imagers have been verified using the geometric calibration tool and a Winston‐Lutz test. Results: Coincidence between the MV isocenter ‐ measured using the geometric tool ‐ and the central imager pixels of both imagers was less than ±0.1mm. The coincidence between the central pixels of both imagers and the projection of the Winston‐Lutz BB was better than ±0.3mm. Conclusions: An automated tool for calibrating the geometry of MV and kV imaging equipment has been developed and tested. The tool gives similar results to conventional Winston‐Lutz measurements. The geometric accuracy of the imaging systems of Trilogy MX is better than ±0.3mm.

Dose perturbations by two carbon fiber treatment couches and the ability of a commercial treatment planning system to predict these effects

To measure the effect of the treatment couch on dose distributions and to investigate the ability of a modern planning system to accurately model these effects. This work measured the dose perturbation at depth and in the dose buildup region when one of two treatment couches, CIVCO (formerly MED-TEC) or Medical Intelligence, was placed between a photon beam source (6, 10, and 18 MV) and the phantom. Beam attenuation was measured in the center of a cylindrical acrylic phantom with a Farmer type ion chamber at multiple gantry angles. Dose buildup was measured in Solid Water with plane parallel ion chambers (NACP-02 and PTW Markus) with the beam normal to both the phantom and couch surfaces. The effective point of measurement method as described [M. R. McEwen et al. "The effective point of measurement of ionization chambers and the build-up anomaly in MV x-ray beams," Med. Phys. 35(3), 950-958 (2008)] was employed to calculate dose in the buildup region. Both experiments were modeled in XiO. Images of the treatment couches were merged with images of the phantoms such that they were included as part of the "patient" image. Dose distributions calculated with superposition and fast superposition algorithms were compared to measurement. The two treatment couches have different radiological signatures and dissimilar water equivalent thicknesses (4.2 vs 6.3 mm.) Maximum attenuation was 7%. Both couches caused significant loss of skin sparing, the worst case showing an increase in surface dose from 17% (no couch) to 88% (with couch). The TPS accurately predicted the surface dose (+/-3%) and the attenuation at depth when the phantom was in contact with the couch. For the open beam the TPS was less successful in the buildup region. The treatment couch is not radio-transparent. Its presence between the patient and beam source significantly alters dose in the patient. For the most part, a modern treatment planning system can adequately predict the altered dose distribution.

Optimization of a Linac-based 4D Cone-beam CT Acquisition Protocol for Image-guided Radiation Therapy

Materials/Methods: We have developed a 4D CBCT acquisition technique to support IGRTfor free-breathing thoracic andabdominal cancer patients. The Varian OBI imager was displaced (half-fan acquisition geometry) extending the FOV to 450 mm. The projections were acquired in 360 gantry rotation arcs. The 8 to 12 minute scanning time was set to cover 120 breathing cycles (BC), with a sampling rate of 20 projections per BC. We used the Varian Real-time positioning and management system as an external surrogate to correlate the patient respiration with the projection acquisition. View aliasing artifacts for single and multiple gantry arc acquisitions were evaluated on patient datasets. Using a synthetic 4D CBCT dataset derived from a 10 minute scan, we evaluated the distribution of gantry angle increments as a function of number of gantry rotations assuming 1 minute rotation time and 10 frames per second. Actual patient respiratory traces were used to sort the projections into phase bins. IQ was assessed as function x-ray exposure (mAs) per projection.

MatriXX system in the study of the influence of respiratory motion on target dose distribution

Objective To investigate the influence of respiratory motion on target dose distribution in three-dimensional conformal radiation therapy(3DCRT) and intensity modulated radiation therapy(IM-RT). Methods A 2D air vented ionization chamber array MatriXX system,a platform which can mimic the clip motion of lung tumor, and a home-made solid water(RW3) phantom were used to measure the 2D dose distribution of static and moving targets irradiated by 3DCRT and IMRT beams using multiple gantry an-gles. The period of the moving platform was set to 3.5 s and the amplitude was ± 1 cm. Then the differences of dose distribution between the static phantom and moving phantom were compared and analyzed with the MatriXX system software. Results Compared with the static phantom in 3DCRT, the penumbra of dose distribution of the periodic moving phantom along the moving directions was increased by 6-9 mm, the high dose area was shrinked by about 5 ram,and the low dose area was extended by 5 mm,but the area of 50% i-sodose and the dose center area changed very little. When a single segment beam of IMRT was delivered and measured and the maximum dose of measuring plane was normalized to 100% ,the average difference of dose distribution between the static and dynamic phantoms was ±27% (from -56.4% to 56.1%) ;When all of the segment beams of IMRT were dehvered and the integrated dose distribution was measured ,the dose differ-ences were less than ±3% ,and the maximum difference of dose distribution was about ±15% which mainly appeared at the field margin and were similar to 3DCRT. Conclusions The dose distribution of most cen-ter areas of the periodic moving target using multi-fraction 3DCRT/IMRT beams is similar to that of the static target, while the high dose area of the former is shrinked and the low dose area is extended. Key words: Phantom/radiotherapy; Three-dimensional conformal; Intensity modulated; Ioni-zation chambers array; Dose verification

Potential Impact of Volumetric Modulated Arc Therapy on the Planning and Delivery of Radiation Therapy

Volumetric modulated arc therapy (VMAT) is a technique in which radiation therapy can be delivered in single or multiple gantry rotations with dynamic MLC motion and dose rate and gantry speed modulation of a linear accelerator. The aim of this work was to evaluate the implications of using VMAT in place of IMRT or conformal plans on plan quality, delivery efficiency, and dosimetric accuracy.

Volumetric Arc Therapy (VMAT) Reduces Treatment Time Compared to Conventional IMRT (cIMRT) While Maintaining Similar Plan Quality in Whole Pelvic Gynecologic Radiotherapy

VMAT is a novel extension of IMRT where an optimized 3D dose distribution may be delivered in single or multiple gantry rotations. This optimization algorithm is the predecessor to Varian's RapidArc. It has the potential to deliver monitor units (MU) more efficiently and may allow for reduced whole body integral dose, and overall treatment time, compared to conventional IMRT (cIMRT). Intensity modulated approaches are emerging in the pelvic radiotherapy of gynecologic malignancies in an effort to reduce acute and late bowel toxicities.

SU-GG-T-210: A Comparison of Dosimetry Devices

Purpose: With more and more departments moving to a filmless environment there is a need to find alternative methods for IMRT and routine QA. Method and Materials: This study compares results from 8 different devices. They include, MapCheck from Sun Nuclear, 729 chamber array from PTW, MatriXX chamber array from Scanditronix-Wellhofer, EBT gafchromic film from ISP, Varian EPID dosimetry, Kodak CR, Bang Gel from MGS Research, and Delta4 3D diode array from ScandiDos. The devices were compared using 3 IMRT fields at 6 and 18MV energies. The 3 fields consisted of a small prostate field, a large head and neck field, and a solid compensator field. In addition, a series of 10×10 open field exposures were made at various gantry angles for 6MV. The results are compared to traditional Kodak EDR2 film. Results: All devices reported doses to within about 3% of the calculated values for points inside the treatment area. Larger differences were found for the measured high gradient areas. Differences of up to 60% in cGy/mm were found. There were also large differences in the dose measured outside the treatment area. Only the Mapcheck and EBT film were within 4% of the calculated values in this area. Angular response was found to vary up to about 13% for some of the devices. Conclusion: All of the devices provide reasonable results for measuring doses in the treatment area for normal incidence of the beam. If it is desired to measure at multiple gantry angles or to accurately measure the dose outside the treatment area, then the choices are more limited.

WE-D-351-06: Automated Quality Assurance Test for MLC-Based Stereotactic Radiosurgery

Purpose: We present an automated method of Linac mechanical quality assurance (QA) for MLC‐based Stereotactic Radiosurgery. The test is designed to determine 3D deviation of the Linac beam central axis defined by the center of the collimator aperture defined by the MLC leaves from the machine isocenter defined by the small tungsten sphere target. Method and Materials: The automation is achieved using Portal Imager as a 2D detector. Based on the approach of Winston‐Lutz Alignment Test, we monitor the alignment of the isocentrically‐placed tungsten sphere target, mounted on a metal stem, with the center of the MLC‐defined small square field. Multiple projection images are acquired for a specified set of gantry angles. After acquisition, the EPIDimages are saved in DICOM format by the imager's software for subsequent automated analysis on a Windows‐based PC running MATLAB®. Results: The center of the sphere deviated by 0.75 ±0.17 mm (1SD) from the MLC defined field center. The lateral, longitudinal and vertical sphere displacements were [0.66±0.17, 0.20±0.18, 0.217±0.13] mm, respectively. The average extent of the MLC motion in the direction parallel to the leaf travel was within 0.2 mm maximum span, while the carriage motion in the perpendicular direction ranged from 0.2 mm for 5×5cm2 field to 1 mm motion for 2×2cm2 field. All results were automatically processed using GUI. Conclusion: Our test is capable of sub‐millimeter sensitivity to the MLC leaf position and carriage position, as well as gantry sag and carriage shift due to the effect gravity at multiple gantry angles. The system employs a graphic interface, which is intended to facilitate the frequent clinical use of the presented QA technique. Conflict of Interest: This work is partially supported by Varian Medical Systems (PaloAlto, CA).

SU‐GG‐J‐172: Static and Dynamic Tracking Accuracy of a Novel Radioactive Tracking Technology for Target Localization and Real Time Tracking in Radiation Therapy

Purpose: In radiation therapy there is a need to accurately know the location of the target in real time. A novel radioactive tracking technology is being developed to answer this need. The technology consists of a radioactive non‐migrating implanted fiducial marker and a linac mounted tracking device. This study measured the static and dynamic accuracy of the new tracking technology in a clinical radiation therapy environment. Method and Materials: The tracking device was installed on an Elekta Synergy® linac gantry. The radioactive marker was located in a tissue equivalent phantom. Actual marker location was measured using a Microscribe G2 coordinate measuring machine (CMM) arm (certified spatial accuracy of 0.38 mm). The marker was attached to the tip of the CMM arm and its location was measured simultaneously by the CMM and the tracking system. Static accuracy was measured at multiple locations covering a 12cm cube centered at the linac iso‐center. The measurements were repeated at multiple gantry angles. Dynamic accuracy was measured with the marker located inside a breathing phantom. Results: The mean localization error for the static source was less than 0.7mm throughout the tested region at all measured gantry angles. The mean real time tracking error for the dynamic source within the breathing phantom was less than 1mm. Conclusion: The novel radioactive tracking technology has the potential to be useful in accurate target localization and real time monitoring for radiation therapy. Conflict of Interest: Research sponsored by Navotek Medical Ltd.

Enhanced 4D cone‐beam CT with inter‐phase motion model

Four-dimensional (4D) cone-beam CT (CBCT) is commonly obtained by respiratory phase binning of the projections, followed by independent reconstructions of the rebinned data in each phase bin. Due to the significantly reduced number of projections per reconstruction, the quality of the 4DCBCT images is often degraded by view-aliasing artifacts easily seen in the axial view. Acquisitions using multiple gantry rotations or slow gantry rotation can increase the number of projections and substantially improve the 4D images. However, the extra cost of the scan time may set fundamental limits to their applications in clinics. Improving the trade-off between image quality and scan time is the key to making 4D onboard imaging practical and more useful. In this article, we present a novel technique toward high-quality 4DCBCT imaging without prolonging the acquisition time, referred to as the "enhanced 4DCBCT". The method correlates the data in different phase bins and integrates the internal motion into the 4DCBCT image formulation. Several strategies of the motion derivation are discussed, and the resultant images are assessed with numerical simulations as well as a clinical case.

SU‐FF‐I‐38: Reduction of In‐Plane Breathing Motion Artifacts Using Optimized‐Angle Sinogram Reconstruction

Purpose: Breathing motion mapping utilizes 4‐dimensional computed tomography (4DCT) which is typically acquired either in ciné or helical mode. Even with rapid CT scanners, tissues can move across a space of multiple pixels during image acquisition, causing motion artifacts. We are investigating a method to reduce the in‐plane motion artifacts using 180° reconstruction with CT gantry angles whose orientation is matched to the tissue motion vectors. Method and Materials: To model vessels and bronchial branches, CT scans were simulated with small cylindrical targets. The targets were moved at constant velocities during image acquisition and both 360° and 180° filtered back projection reconstructions were employed. The start angles of the 180° reconstructions were varied to examine the effect of the relative orientation between the start gantry angle and the motion vectors. Single and multiple targets were examined, including targets that moved orthogonally. Results: In all but static cases, 360° reconstruction provided the worst image quality. 180° reconstruction quality depended heavily on the relative orientation of the motion vector and the start CT gantry angle (α). When α was 0° (parallel), the image quality was good and when α was 90° (perpendicular), the image quality suffered. The image quality remained high when α was less than 45°. When multiple non‐parallel moving targets were studied, each was imaged well when its motion vector was parallel to the gantry start angle. A single good quality image could be pieced together from multiple image reconstructions created using multiple gantry start angles. Conclusion: Improving the quality of reconstruction for CT scans acquired during free breathing is possible using tuned gantry start angles even when using multiple patched image datasets. It is possible to compensate for potentially increased statistical noise due to the sub‐360° reconstruction by increasing the scanner's mA. This work supported in part by NIHR0196679.

TH-D-AUD-08: Automated Stereotactic Radiosurgery Quality Assurance Using a Portal Imager

Purpose: An automated portal vision based quality assurance method is implemented to replace the labor intensive manual used to ensure the collimator alignment accuracy prior to performing stereotactic radiosurgery. Method and Materials: We use Varian PortalVision™ EPID on a Varian Trilogy™ linear accelerator to acquire the projection images of an isocentrically-placed tungsten sphere produced during Winston-Lutz Alignment Test that includes multiple gantry and couch combinations. The analysis routine applied to each acquired image consists of 1) search for the sphere and collimator edges 2) fitting them to a circle and 3) calculation of distance between the collimator and sphere optimized center points. The method, based on the properties of pseudo-inverse matrices, is implemented to calculate the cumulative 3D shift of the sphere center relative to the isocenter of the Trilogy machine. The graphic interface was created using MatLab™ that permits the choice of the type of the QA procedure (daily or weekly), performs image analysis and allows visual examination, saving and printing of the results. Results: The in-house software was tested in 17 trials by comparing it to the conventional SRS QA technique based on film image acquisition with subsequent manual shift measurements. The calculated mean of the difference between film and portal vision — based methods was: 0.07±0.11mm vertical, −0.11±0.17mm lateral, 0.02±0.08 mm longitudinal shift components. The mean of 0.11±0.13 mm was shown for the absolute value of the vector shift differences in 3D. Conclusion: The automated QA procedure for SRS LINAC QA is created, tested and proven to be fast, precise and efficient replacement for the conventional film-based quality assurance technique. The use of the presented routine should allow more frequent SRS QA tests, which ensures high accuracy and quality of the patient treatments. Conflict of Interest: Partial support by Varian Medical Systems, Inc. acknowledged.

WE‐D‐AUD‐03: Characterization of a 2D Ionization Chamber Array for IMRT QA

Background and Purpose: Measurement‐based methods of quality assurance (QA) for Intensity Modulated Radiotherapy (IMRT) fields often include individual field verification at a neutral gantry angle using film or other 2D detector in addition to a composite ionization chamber check. Unfortunately, film is time‐consuming to use. Diode systems are currently available but these arrays are limited in size and spacing of the diodes. This work characterizes a new 2D ionization chamber array (MatriXX, Scanditronix‐Wellhofer) that can be used for treatment verification at neutral and planned gantry angles. Material and methods: The 2D array consists of 1020 ion chambers, arranged in a 32×32 cm2 grid. Each chamber is separated by 7.62 mm center to center and has a volume of 0.08 cm3. Properties such as linearity, reproducibility, pressure dependence, homogeneity, and accuracy of the 2D array have been evaluated for 6 and 16MV beams at neutral and planned gantry angles. Results: The 2D array response was found to be linear with dose from 0–800 cGy. The 2D array readings were reproducible to within a SD of ±0.14%. Examination of the array's pressure dependence, homogeneity, sensitivity as a function of ion chamber location, and the interpolated dose showed accuracies of better than 0.3%. Absolute dose measurements were done with the array and a calibrated ion chamber. Agreement was better than 0.6%. TPR measurements were made and found to agree with commissioning data in our planning system (UMplan). Measurements done using a 2 × 2 cm2 field placed randomly around the array show a standard deviation of 0.21%. Delivery of IMRT plans using multiple gantry angles showed excellent agreement with calculated results. Conclusions: The 2D array offers an excellent solution to simplifying QA and saving time, particularly for IMRT pre‐treatment QA. The device can be used for both relative and absolute dose measurements.

SU‐FF‐T‐175: A New Linac QA Procedure for the Characterization of Gantry Radiation Isocenter

Purpose: A new linac QA method is developed to characterize the size and shape of gantry radiation isocenter. The method accurately measures gantry sag as a function of gantry angles and the size of gantry radiation isocenter in the gantry rotation plane. Method and Materials: In order to determine both sag and radiation isocenter size in the gantry rotation plane we wrap a film (or a phosphor imaging plate) on a cylinder of circumference equal to the film length. We designed a static MLC “dashed line” pattern consisting of regions of radiation along a line. The pattern is designed such that the center of the irradiation line can be easily determined. The cylinder is positioned such that its axis is the same as the gantry rotation axis. The film is exposed to the MLC line pattern for multiple gantry angles. Both entry and exit patterns are seen on the film. A special MLC pattern was designed to accurately characterize the gap between the two ends of the film. An automated analysis program determines the MLC line angles and centers on the film, characterizes the cylinder position and generates the desired gantry radiation isocenter information. Results: The gantry sag is observed to have a smooth dependence on gantry angle with extreme values for 0° and 180°. The smallest circle crossing or tangent to all rays in the gantry rotation plane determines the gantry radiation isocenter size in this plane. The inherent accuracy of the method is estimated at ±/− 0.1 mm. The total irradiation time is about 15 min. The data analysis is less than 5 min. Conclusion: The designed method characterizes both gantry sag and wobble in one simple and accurate test.

SU‐FF‐T‐183: Application of a Gantry‐Mounted Diode Array System for QA Dosimetry of High‐Energy Photon Beams

Purpose: To investigate the utility of a commercial diode‐array system mounted on the gantry of linear accelerators to perform relative QA dosimetry of photon beams at different gantry angles. Method and Materials: The diode‐array Profiler (Sun Nuclear Corp.) was mounted on the head of the linac. Beam profiles were measured at multiple gantry angles for 6 and 10 MV photon beams. Flatness, asymmetry, and central‐axis relative output were determined. Dynamically‐wedged (EDW) profiles were measured at various gantry positions for different wedge angles. For a given EDW field, the speed of the Y‐jaw motion was varied by changing the number of MUs for studying the effect of jaw speed on the EDW profile. The potential effects of gravity and leaf speed on the dynamic delivery of MLC‐based IMRT beams were also investigated. The IMRT fields were designed such that the MLC leaves move against or along gravity when the linac gantry is at horizontal position. The MLC leaf speed was varied by changing the total MU. Results: (1) The gantry angle has no influence on the flatness, asymmetry, and the central‐axis output of the clinical beams. (2) Our data validated the delivery of EDW treatments at any gantry position. (3) For IMRT fields, the beam profiles are accurately reproduced at any gantry angle for all leaf speeds. No gravity effect on dMLC delivery was observed. However, the diode‐array system was able to detect a difference of about 3% in beam profiles produced by leafs traveling in opposite directions at a high leaf speed of 2.5 cm/sec, attributable to the communication time lag between MLC controller and the Linac beam control but not to gravity. Conclusion: The gantry‐mounted linear diode array Profiler is an efficient tool for performing QA of basic beam characteristics, EDWs, and dMLC, especially at different gantry angles.

MO-D-T-6E-01: A Computational Analysis of Lung Damage From 3D, IMRT, and Helical Tomotherapy

Purpose: It is well known that the lungs are extremely sensitive to radiation damage. Based on low‐dose tolerance models, the argument has been made that delivering small doses from multiple gantry angles may actually cause more lung damage than delivering large doses from fewer gantry angles. The purpose of this work is to calculate the percent reduction in lung function for several existing lung models and treatment delivery techniques. Method and Materials: Conformal APPA, non‐coplanar 3D conformal, and helical tomotherapy plans were created for 15 lung patients. All plans used the same beam energy, the PTV, and prescribed dose. The percent reduction in lung function was calculated by multiplying the DVH doses by a dose response function. Three published models were used: 1.) Lung function reduces linearly at 1% per Gy (Linear), 2. Lung function reduces to 0% at the threshold dose of 13 Gy (Delta13), and 3.) Lung function decreases to 0% at the threshold dose of 36 Gy (Delta36). Results: The linear model yielded the least difference in lung function reduction between the three delivery techniques, with a mean difference of 3 percent (range 1–7%). The delta13 model clearly favors APPA treatment delivery, with the exception of targets smaller than 50cc. If the threshold for lung damage is greater than 20 Gy, then the conformal techniques overtake APPA in providing the least lung damage. A delta36 model greatly favors highly conformal rotation delivery techniques, such as tomotherapy. Conclusion: Although the model for lung damage is unknown, several conclusions can be draw on the appropriateness of delivery techniques. If the threshold for damage is at very low doses, then techniques that spread the dose are less advantageous. However, if the threshold is greater than 20 Gy, or is linear, then conformal techniques provide the best lung sparing.

Isocenter characteristics of an external ring proton gantry

Determine the shape, size, geometric center, and virtual center of the isocenter for a proton gantry and compare to electron/X-ray accelerator gantries. The majority of commercial electron/X-ray accelerator gantries consist of a rotating treatment head mounted to a stationary stand through a slewing ring bearing. The world's first proton gantry uses two rotating external rings, to which is mounted a fixed treatment nozzle with a movable snout that extends close to the center of rotation. The radial aspect of the isocenter for two similar proton gantries and two different electron/X-ray gantries were measured in the gantry frame of reference with a front pointer and a theodolite. These results were then transformed into room coordinates. The axial aspect of the isocenter was measured with a dial indicator. The radial aspect of the isocenter for slewing ring gantries has the shape of two concentric circles. The radial aspect of the isocenter for external ring gantries is shaped like a butterfly. The size of the mechanical isocenter is independent of the gantry style. The locations of the geometric and virtual centers can be determined to within 0.2 mm. Multiple gantry angle treatments can be delivered with a single setup allowing 2 mm for gantry and nozzle deflections. Precision treatments can be delivered allowing only 0.5 mm if the measured isocenter path is applied.