Collimator Size Research Articles

SU‐FF‐J‐157: A Monte Carlo Model for Small Animal Stereotactic Irradiation

Purpose: To develop a Monte Carlo model of the treatment beam of a small animal stereotactic irradiator. Method and Materials: A Monte Carlo source model has been created for a custom small animal stereotactic irradiation device. X‐ray generation is performed by an XRAD‐320 x‐ray tube (Precision X‐Ray Inc., North Branford, CT) operated at a potential of 250 kVp. Determination of the un‐collimated energy spectrum from the XRAD‐320 at 250 kVp was calculated using the “SpekCalc” GUI program by Poludniowski and Evans. These calculation results were used to form the photon source probability distribution in MCNPX (Los Alamos National Laboratories, Los Alamos, NM). A custom, in‐house field collimation system is used in conjunction with the x‐ray source to collimate the source into 1.0, 3.5, 5.0, 7.5, and 10.0 mm stereotactic beams. This collimation system has been incorporated into the MCNPX beam model. Percent Depth Dose (PDD) curves from the irradiator were measured using radiochromic film (Gafchromic EBT, International Specialty Products, Wayne, NJ) with all collimator apertures and two treatment SSDs of approximately 20 and 25 cm. Results: PDD curves created with film measurements and Monte Carlo simulation results were compared for agreement. Simulated PDD curves were found to agree with measured data for all collimator sizes and source‐to‐surface distances (SSDs) for 2%/2 mm normalization criteria. Conclusion: The agreement between simulated data and repeatedly measured PDD data suggests accuracy in this application of the SpekCalc GUI program. An accurate beam model of this small animal irradiator in conjunction with ongoing micro CT‐based “patient” modeling will provide a system for Monte Carlo‐based clinical and investigational research of small animal stereotactic radiotherapy.

SU-FF-T-561: Measuring Extracranial Dose with the Leksell Gamma Knife Model 4C Using Gafchromic EBT Film

Purpose: Measure scatter and leakage dose in critical, extracranial organs for patients undergoing Leksell Gamma Knife Radiosurgery.Method and Materials: A stereotactic frame was attached to the head of an anthropomorphic Rando phantom. Using a fiducial box to determine the coordinates of the target (center of the Rando head), a CT scan was taken and registered in the GammaPlan treatment planning system where a dose prescription of 25Gy to the 50% isodose line was applied to the target site. The plan was generated for the 18 mm collimator size helmet with a single shot run using the automatic positioning system (APS). Multiple (2 to 5) 2″×2″ Gafchromic EBT films were placed between each slice of the Rando body phantom, from the neck to pelvic region (phantom slice 8 to 31), and analysis was done using a Vidar VXR‐16 scanner with the RIT113 Version5.1 analysis software. An HD dose will also decrease with increasing distance from the focal point. The doses are low but may be considered for younger patients with longer life expectancy.

SU-FF-T-640: Evaluation of Monte Carlo Treatment Planning for CyberKnife Stereotactic Radiosurgery/radiotherapy Treatments of Head and Body Lesions

Purpose: To evaluate Monte Carlo calculated plans for Cyberknife Stereotactic Radiosurgery/Radiotherapy treatments of head and body lesions. Method and Materials: The Multiplan is the Cyberknife treatment planning system which recently implemented a Monte Carlo(MC) in addition to the original pencil‐beam based Raytrace (RT) algorithm for dose calculations. This study compares DVHs and maximum doses (Dmax) to the lesions and selected critical organs obtained by the two algorithms. Planning is done on CT fused with either MRI or PET image sets as appropriate. A treatment plan is created on the Multiplan 2.1 using the RT with equivalent pathlength inhomogemeity corrections. When an acceptable distribution is obtained, the plan is recalculated using MC with 1% relative mean standard deviation. The number of beams, beam directions, and monitor units are kept the same. The dose distributions are then compared with respect to maximum doses to target and critical organs, as well as target coverage. Results: For lung lesions PTV Dmax calculated by the RT are on average larger then the MCdoses by up to a factor of 1.63 while Dmax to critical structures were up to 4 times greater. For cranial and abdominal targets the differences in maximum doses and tumor coverage were generally less than 2% between the two plans. Larger differences of ∼5% are noted for planning for lesions in the nasopharynx in the vicinity of air cavities. Conclusion: In lungMC can produce large differences in target and critical organs'dose coverage relative to RT. RT tends to underdose target structures and the effect is more pronounced as the collimator size decreases. Smaller differences are observed in plans for sites where beams traverse mostly homogeneous water‐equivalent tissue. Therefore the Monte Carlo algorithm should be consistently used for planning for lesions in lung and near air cavities.

TU‐E‐213A‐01: Quality Assurance for the Leksell Gamma Knife Perfexion™

The Leksell Gamma Knife_ Perfexion_ is the latest gamma stereotactic radiosurgery (GSR) unit manufactured by Elekta (Elekta Instrument, AB, Sweden). Introduced in 2006, it is substantially different in design and operation from previous Gamma Knife (GK) models. The major differences are in the collimation system, the configuration of the radioactive 60Co sources, and in the patient positioning system. The Perfexion_ has one fixed tungsten collimator containing 576 different collimating channels, whereas older GK models have one primary collimator, located inside the unit, and 4 interchangeable secondary collimators that are mounted manually. In the Perfexion_, 192 60Co sources are situated on 8 moveable sectors (24 sources per sector), that slide over the collimator placing the sources over the planned collimating channels to deliver the desired radiation shot size. In older GK models, by contrast, 201 60Co sources are fixed in space and arranged in a pseudo‐hemispherical configuration. Finally, for the Perfexion™ the treatment table, which moves with sub‐millimeter accuracy in three orthogonal directions, is the patient positioning system. For older model GK units, patient positioning is accomplished by attaching the Leksell stereotactic frame to the secondary collimator via specialized hardware called trunnions or via the automatic‐patient‐positioner (APS).The specific goals of quality assurance (QA) for gamma stereotactic radiosurgery include determining the dose rate for the largest available collimator, measuring the relative output factors and beam profiles for each collimator size and comparing the results to those used in or calculated by the treatment‐planning program, and verifying that the radiation focal point coincides with the center of the patient positioning system. Although these QA tasks for GSR units have remained essentially unchanged over the years, new tools and protocols designed for linear accelerator QA have become available that can be applied to QA for GSR devices. With this in mind, a new AAPM Task Group (TG‐178) has recently been formed and charged with updating QA procedures for older GSR devices that have static sources, creating new QA protocols for GSR devices that are characterized by moving sources, and suggesting a protocol for ionization chamber calibration specific to GSR devices.This lecture will describe the operation of the Leksell Gamma Knife_ Perfexion_ and the unique features of QA for GSR units, with emphasis on the Perfexion_. Some of the questions faced by TG‐178 will be discussed, old QA procedures will be reviewed and possible new QA tests will be suggested.Learning Objectives:1. Review QA requirements for GSR units2. Understand the design differences between older GK models and the Perfexion_ and the implications of these differences on routine QA3. Become familiar with tools provided by the manufacturer to facilitate GK QA4. Understand the difficulties associated with dosimetric calibration of GSR units5. Review the use of radiochromic film for GK QA

Investigation of intracranial peripheral dose arising from the treatment of large lesions with Leksell GammaKnife®Perfexion™

This investigation involves quantifying the extent of intracranial peripheral dose arising from simulated targets situated in the skull-base or upper-spine region using the Leksell GammaKnife Perfexion treatment unit. For each of three spherical target volumes--denoted as Vs (4 cm3), VM (18 cm3), and VL (60 cm3)--three treatment plans were manually generated, one for each of the three collimator sizes--4, 8, and 16 mm. Each of the plans was delivered to a spherical dosimetry phantom with an insert containing EBT Gafchromic film. The total dose at 70 mm from the targets' edges, %D(70 mm), was measured as a function of elevation angle and expressed as a percentage of the prescription dose. The film insert was placed centered in the median sagittal plane (Leksell X = 100) and %D(70 mm) was measured for the angular range from 0 degree (superior/along Z axis) to 90 degrees (anterior/along Y axis). For a given collimator i, the irradiation time ti to treat a spherical target of volume V using the 50% isodose line was observed to follow a power-law relationship of the form ti = Ai(V/ Vi)n where Ai was the maximum dose divided by collimator dose rate and Vi was the volume encompassed by the 50% isodose line for a single shot. The mean value of n was 0.61 (range: 0.61-0.62). Along the superior (Z) direction (angle=0 degree) and up to angles of around 30 degrees, the %D(70 mm) was always highest for the 4 mm plans, followed by the 8 mm, followed by the 16 mm. In this angular range, the maximum measured %D(70 mm) was 1.7% of the prescription dose. The intracranial peripheral dose along the superior direction (combined scatter and leakage dose) resulting from irradiation of upper-spine or base-of-skull lesions is measured to be less than 2% of the prescription dose, even for very large (60 cm3) targets. The results of this study indicate that, for a given target volume, treatment plans consisting of only 4 mm shots yield larger peripheral dose in the superior direction than 8 mm shot only plans, which in turn yield larger peripheral dose than 16 mm shot only plans.

The effect of x‐ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging

This study has the following objectives: To measure the geometric efficiency (GE) of a multidetector CT (MDCT) system and investigate its dependence on several exposure parameters and to correlate GE with radiation dose burden of patients undergoing MDCT imaging. Dose profiles for all available beam collimations, tube voltage values, focal spot sizes, and modes of operation were determined for a modern MDCT scanner using an array of thermoluminescent chip dosimeters positioned side by side. Dose profiles measured free-in-air and at the center and periphery of the standard polymethyl methacrylate (PMMA) phantoms were used to estimate GE. The standard free-in-air and a new measure of GE determined using the standard PMMA phantoms were correlated with volume computed tomography dose index normalized to the z-axis coverage per rotation which is directly related to patient radiation dose burden. GE was found to be from 30% to 88% as beam collimation was changed from 1.2 to 24 mm, with thin beam collimations corresponding to higher "wasted" dose. For the same tube voltage and beam collimation, the use of small focal spot was associated with higher GE compared to the large focal spot. Besides, beam quality was found to have a much weaker effect on the GE value. In comparison to free-in-air, a weighted GE determined using the standard PMMA phantoms was found to have significantly better correlation with patient radiation burden (p < 0.05). Overbeaming strongly depends on the beam collimation and focal spot size, while the impact of beam quality on GE is less pronounced. Thin beam collimations are associated with a GE of as low as 30%.

RADIOSURGERY WITH THE WORLD'S FIRST FULLY ROBOTIZED LEKSELL GAMMA KNIFE PERFEXION IN CLINICAL USE

The world's first Leksell Gamma Knife PerfeXion (Elekta Instrument AB, Stockholm, Sweden) for radiosurgery of the head and neck became operational at Timone University Hospital in Marseille on July 10, 2006. To allow strict evaluation of the capabilities, advantages, disadvantages, and limitations of this new technology, patients were enrolled in a prospective, randomized trial. In 66 working days, between July 10 and December 20, 2006, 363 patients were treated by gamma knife surgery at Timone University Hospital, Marseille. Of these patients, 200 were eligible for the comparative prospective study (inclusion criteria were informed consent obtained, tumor or vascular indication, and no previous radiosurgery or radiotherapy). In accordance with the blinded randomization process, 100 patients were treated with the Leksell Gamma Knife 4C (Elekta Instrument AB) and Gamma Knife 100 (Elekta Instrument AB) with the Leksell Gamma Knife PerfeXion. Dose planning parameters, dosimetry measurements on the patient's body, workflow, patient comfort, quality assurance procedure, and a series of other treatment-related parameters were systematically and prospectively evaluated in both arms of the trial. No technical failure of the treatment procedure was encountered. The new dose-planning system led to the use of composite shots in 39.4% of the patients. The median number of different collimator sizes used was larger with the PerfeXion than with the 4C (2 and 1, respectively). The mean number of isocenters used was lower (10.67 and 13.08, respectively). The median total treatment time was significantly shorter with the PerfeXion (40 and 60 minutes, respectively), but there was no significant difference in the median radiation time (34.02 and 33.40 minutes, respectively). The procedure was performed using only a single run in 98.99% of the PerfeXion cases and in 42% of the 4C cases. Collision risk on the 4C forced us to change the frame gamma angle for at least 1 shot in 24% of the patients and led to treatment in manual mode for at least 1 shot in 21% of the patients. Collision risk requiring technical adaptation did not occur with the PerfeXion. In 1 patient treated with the PerfeXion, the system required a direct collision check. In terms of dose to structures outside the target area, the PerfeXion delivers 8.2 times less to the vertex, 10 times less to the thyroid, 12.9 times less to the sternum, and 15 times less to the gonads. Our prospective study indicates that procedures with the PerfeXion were collision-free, even with very eccentric lesions (e.g., multiple metastases). The duration of the surgical procedure, the amount of time required for nurse, physicist, and physician intervention on the machine, and the duration of the quality assurance procedure were all shown to be dramatically reduced with the PerfeXion gamma knife. Patient protection is greatly improved with the PerfeXion. In our experience, the technological advances of the Leksell Gamma Knife PerfeXion will make a very significant contribution to future progress in head and neck radiosurgery.

An Interlaced IMRT Technique for Elongated Tumor Volumes

Treatment of large target volumes with intensity modulated radiotherapy (IMRT) can be restricted by the maximum field size of the multileaf collimator (MLC). In this work, a straightforward technique for MLC-based IMRT is presented, which is generally applicable and does not depend on the capabilities of the linear accelerator's IMRT delivery system. A dual isocenter technique was developed that maximizes beam overlap. The beams at the first isocenter are arranged such that they interlace with the beams at the second isocenter. All beams contribute to the overlap region, whereas only some contribute to the superior and some to the inferior part of the target. The interlaced technique (9 beams) was compared with an alternative more complex approach (14 beams) for a head-and-neck case with simultaneous integrated boost and 3 different dose levels. The plans were compared in terms of complexity, dosimetry, and the effect of inaccurate translation between the isocenters. The interlaced and the more complex IMRT technique resulted in nearly identical dose distributions without clinically relevant differences. The total number of monitor units (MUs) was comparable with more MUs per segment for the interlaced technique. For the interlaced technique, the number of segments ≤5 MU was reduced by 43%. Simulation of isocenter setup errors of ±1, ±2, and ±3 mm revealed maximum dose point errors of 1.8%, 3.8%, and 5.4% in the target volume for the interlaced technique. The interlaced IMRT technique resulted in an equivalent plan to the more complex technique without compromising the dose distribution. The technique is less complex and is robust against inaccurate isocenter translations of up to ±1 mm. Due to the versatility of the technique, it can easily be applied to other anatomical regions and is well suited for clinical routine usage.

Dosimetric comparison of the Leksell Gamma Knife Perfexion and 4C

The recently introduced Leksell Gamma Knife (LGK) Perfexion is an entirely new system with a different beam geometry compared with the LGK 4C. The new Perfexion system has 192 cobalt-60 sources that are fixed on 8 sectors (each sector has 24 sources). Each sector can be moved independently of the others and can be set to 1 of 5 different positions: 3 positions defining collimator sizes of 4, 8, and 16 mm; an off position (sources are blocked); and a home position. The purpose of this study is to compare the dosimetric characteristics of the GK 4C and the Perfexion models. This comparison is important especially for the treatment of functional disorders when only a single shot with the 4- or 8-mm collimator is used. A 160-mm-diameter spherical polystyrene phantom was used for all measurements and calculations. The irradiation geometry consisted of the placement of a single shot at the center of this phantom. Comparisons were made among different dosimetric parameters obtained from calculations performed using Leksell GammaPlan v. 8.0 and measurements performed using film dosimetry. The dosimetric parameters investigated were dose profiles for all collimators in all 3 stereotactic planes (x, y, and z) including the full width at half maximum and the penumbra for each profile, cumulative dose-volume histograms, the volume encompassed by the 50% isodose surface, the mean doses delivered to a defined matrix volume, and relative output factors for all collimator sizes. There was excellent agreement between the dosimetric parameters of GK 4C and Perfexion for the 4- and 8-mm collimators. The results of this study suggest that consistent treatments of functional disorders will be delivered using either GK 4C or Perfexion.

Abdomen: Angiography with 16-Detector CT—Comparison of Image Quality and Radiation Dose between Studies with 0.625-mm and those with 1.25-mm Collimation1

To prospectively compare image quality and volume computed tomographic (CT) dose index (CTDI(vol)) of 16-detector CT angiograms of the abdomen acquired with 0.625-mm collimation with those of images acquired with 1.25-mm collimation. This study had institutional review board approval, and informed consent was obtained from all patients. Dual-phase contrast material-enhanced CT was performed in 78 patients (48 men and 30 women; age, 34-91 years; mean age, 64.8 years) by using a 16-detector CT scanner. Patients were prospectively randomized into two equal-sized groups: those who underwent CT with 0.625-mm collimation and nonoverlapped reconstruction and those who underwent CT with 1.25-mm collimation and 50% overlapped reconstruction. Scan acquisition time was 7.5 seconds in both groups. CTDI(vol) was recorded. Arterial phase volume-rendered, arterial phase multiplanar reformatted, and portal venous phase multiplanar reformatted CT angiograms were generated. Qualitative assessment was performed for image quality and for depiction of splanchnic, intercostal, and lumbar arteries and veins. The unpaired t test was used for statistical comparison. On the arterial phase CT angiograms, there was no difference between the two collimation groups for the depiction of proximal splanchnic arteries, while the dorsal pancreatic, intercostal, and lumbar arteries and some peripheral splanchnic arterial branches were better delineated on CT scans obtained with 0.625-mm collimation than on scans obtained with 1.25-mm collimation (P < .05). Regarding the portal venous phase CT angiograms, no difference between the two groups was found in most veins, except the right adrenal vein (P = .003). Image quality was superior for 1.25-mm collimation (P < .001). CTDI(vol) values were positively correlated with patient body weight (r = 0.34, P < .001) but had no correlation with collimation size (P = .24). Scanning with 1.25-mm collimation seems adequate for a routine CT angiography examination of most arteries and veins at 16-detector CT, while scanning with 0.625-mm collimation facilitates improved delineation of fine vessels. CTDI(vol) values correlate positively with body weight but have no correlation with collimation size.

Application of a glass rod detector for the output factor measurement in the CyberKnife

A radiophotoluminescent glass rod detector has recently become commercially available. We evaluated the feasibility of the commercial glass rod as a new detector for measuring output factors in the CyberKnife. The glass rod detector was irradiated in a water phantom using a holder stand, which was specially designed for this study. The holder was composed of a PMMA tube with an attached vertical bar for the glass rod detector. The measured output factors obtained with the glass rod detector were compared with measurements made with a pinpoint ionization chamber, a diode, and a radiochromic film. The measured relative output factors obtained with the glass rod detector agreed with other detectors within 1.0% for collimator sizes larger than 20 mm. However, it was observed that the differences between the output factors measured with the glass rod detector and those obtained with the pinpoint chamber increased rapidly as the collimator size decreased. The relative output factors measured with the diode were consistently higher than those obtained using other detectors for the collimators sizes less than 10 mm in diameter. The glass rod detector results were in good agreement with those obtained from the radiochromic EBT film over the entire range of collimator sizes.

WE-E-AUD B-02: Validation Tests for CyberKnife® Monte Carlo Dose Calculations Using Heterogeneous Phantoms

Purpose: To validate the Monte Carlo dose calculation algorithm for the CyberKnife® Robotic Radiosurgery System (Accuray Inc.) in the MultiPlan® Treatment Planning System. Method and Materials: Dose was measured in heterogeneous phantoms for clinically relevant dosimetry situations and compared with Monte Carlo dose calculations. The validation tests involve dose measurements in slab phantoms for single beams as well as multiple beams delivered on phantoms such as a modified Accuray ball cube and the RPC thorax-lung phantom. Depth doses as well as dose distributions at the inhomogeneity boundaries were measured. Single beam tests were performed for four collimator sizes (5, 10, 30, and 60 mm). Measurements were performed primarily with EBT films and complemented with MOSFETs at inhomogeneity interfaces. Results: 1) EBT film and MOSFET measurements at the tissue inhomogeneities show excellent agreements in all phantoms and for all collimators in both orthogonal and oblique incidences. 2) The Monte Carlo depth dose calculations in heterogeneous phantoms show excellent agreements with measurements. For example, the depth doses in water-lung-water phantom show a drop in the lung region that was predicted very well by the Monte Carlo calculation. 3) The treatment plans delivered to the heterogeneous ball cube and RPC thorax-lung phantom show excellent agreements between radiochromic film measurements and Monte Carlo calculations. Conclusion: The tests and phantoms collectively cover a wide range of tissue types (including air and lung, water, and bone-equivalent materials), angles of incidence of beams to tissue interfaces, collimator sizes, and single and multiple beam situations. A total of 91 tests were performed and in 83 (91%) of these tests, 90% or more pixels pass γ(2% dose difference, 2 mm distance-to-agreement) condition.

WE-D-351-04: Dosimetric Considerations of Treating Larger Target Volumes with Perfexion

Purpose: To quantify the background absorbed dose to the uninvolved brain as a function of target volume for varying isocenter spacing and collimator sizes using Perfexion™. Method and Materials: Multiple treatment plans were generated for a series of spherical targets, using a sphere-packing algorithm to automatically determine the isocenter coordinates. Target volumes investigated ranged from 2.1 cm3 (diameter=16 mm) to 33.5 cm3 (diameter=40 mm), with isocenter spacing for each volume ranging between 3.2 mm and 8 mm. The dose was determined for each scenario at fixed distances from 3 cm to 7 cm from the edge of the target volume and expressed as a percentage of the dose to the target edge. Dose was calculated using Gamma Plan (v8.0) and measured using EBT Gafchromic film inserted in a plastic spherical phantom (8 cm radius). Results: No statistically significant difference in background dose was found between isocenter spacing levels. Relative dose at each point from the edge of the target increased with increasing target volume. Using only 4-mm collimated isocenters, the background dose at 7 cm from the target edge was 0.5% (23 isocenters) and 4.6% (226 isocenters) for 2.1 cm3 and 33.5 cm3 targets, respectively. For a given volume size, background dose was higher when using 16-mm compared to 4-mm collimated isocenters. Using only 16-mm collimated isocenters, the average dose rate at 7 cm from the target edge was 2.3% (1 isocenter) and 6.8% (9 isocenters) for 2.1 cm3 and 33.5 cm3 targets, respectively. Conclusion: Treating larger target volumes with Perfexion™ results in larger background dose to uninvolved brain, irrespective of what spacing is used between isocenters. The use of 4-mm collimated isocenters yields lower background dose levels than 16-mm collimated isocenters for equivalent target volumes. Conflict of Interest: Research sponsored by Elekta Instruments AB.

SU‐EE‐A1‐04: Dual Focus Collimator Design Studies For Temporally Precise IMRT Delivery

Purpose: Increasing the temporal precision of IMRT delivery is hampered by current mechanical limitations. We are studying the feasibility of a novel system that would electronically modulate a radiation beam and deliver a time dependent IMRT beam by incorporating pencil beam scanning with a dual focus collimator. This design requires a sharply focused photon pencil beam (FWHM ∼ 1 cm at 100 cm SSD), comparable to beams achievable with current LINAC technology. Collimator specifics were investigated using Monte Carlo (MC) techniques to evaluate the capability of producing photon pencil beams. Method and Materials: The BEAMnrc code was used to develop a geometry incorporating a novel 200 hexagon channel tungsten collimator. Simulations were performed varying collimator thickness, size, and spacing of the channels. Dose in water phantoms was calculated at depth of 10 cm using DOSXYZ. Similar MLC generated pencil beams were measured at equivalent depth using Computed Radiography (CR). Script files were written in MATLAB® to calculate peak output, FWHM, and off‐axis dose/dmax ratios. Results: Pencil beam FWHM and output decreased with increasing collimator thickness (2 cm collimator: FWHM = 1.5 cm, 9 cm collimator: FWHM = 0.6 cm), keeping all other factors constant, and compared favorably to the MLC pencil beam FWHM of 0.9 cm. Off‐axis dose leakage ratios decreased 20% between 2 and 9 cm thickness. These ratios also decreased 20% with increasing hexagon spacing (0.25 – 0.75 mm). An 8‐cm collimated (0.5 mm channel spacing, 0.43 mm radius) pencil beam (FWHM = 1.1 cm) matched closely the MLC pencil beam. Conclusion: A dual focus collimator was designed using Monte Carlo modeling to produce photon pencil beams comparing favorably with current LINAC technology. Future work will focus on comparing theoretical isodose distributions to those of our clinical IMRT system. Conflict of Interest: Research sponsored in part by Varian.

SU‐EE‐A2‐02: Verification of Source and Collimator Configuration for Gamma‐Knife® Perfexion TM using Panormaric Imaging

Purpose: To develop a method of verifying the source and collimator configuration of Leksell Gamma Knife® Perfexion™. Method and Materials: The new model of stereotactic radiosurgery machine, Perfexion™, with modified source configuration allows extended reach of targets located in the cranial, neck and cervical regions. The control system allows automatic selection of appropriate built‐in collimator modules eliminating the need of time consuming manual installation of collimator helmets as in the older model of Gamma Knife system. However, the geometric configuration of collimator modules cannot be easily verified. The conventional method of exposing a film at the isocenter plane provides only a composited dose image, which is difficult to interpret in terms of the integrity of each individual source and corresponding collimator system. A method has been developed to capture a panoramic view of 192 Cobalt sources and corresponding images are utilized to verify the integrity and configuration of 192 sources. The images were acquired by exposing Gafchromic films wrapped around the surface of a specially designed 16 cm diameter cylindrical phantom. The phantom was mounted at the isocenter, with its axis aligned along the longitudinal axis of the couch. Depending upon the azimuthal angle of the source location, the shape and size of the source images were calculated and compared with the acquired images. Results: The images allowed clear identification of each of the 192 sources, verifying their integrity and selected collimator sizes. In this presentation the results of the source image alignment with respect to the expected collimator geometry will be described. In addition, the feasibility of relative dosimetric evaluation of the individual sources by the panoramic images will be presented. Conclusion: This method of source/collimator verification by panoramic imaging can provide an enhancement of commissioning and routine quality assurance of the Gamma Knife systems.

Prototyping a large field size IORT applicator for a mobile linear accelerator

The treatment of large tumors such as sarcomas with intra-operative radiotherapy using a Mobetron® is often complicated because of the limited field size of the primary collimator and the available applicators (max Ø100 mm). To circumvent this limitation a prototype rectangular applicator of 80 × 150 mm2 was designed and built featuring an additional scattering foil located at the top of the applicator. Because of its proven accuracy in modeling linear accelerator components the design was based on the EGSnrc Monte Carlo simulation code BEAMnrc. First, the Mobetron® treatment head was simulated both without an applicator and with a standard 100 mm applicator. Next, this model was used to design an applicator foil consisting of a rectangular Al base plate covering the whole beam and a pyramid of four stacked cylindrical slabs of different diameters centered on top of it. This foil was mounted on top of a plain rectangular Al tube. A prototype was built and tested with diode dosimetry in a water tank. Here, the prototype showed clinically acceptable 80 × 150 mm2 dose distributions for 4 MeV, 6 MeV and 9 MeV, obviating the use of complicated multiple irradiations with abutting field techniques. In addition, the measurements agreed well with the MC simulations, typically within 2%/1 mm.

Analytical description of dose profile behaviour in Gamma Knife radiosurgery

Stereotactic Gamma Knife radiosurgery utilizes ionizing beams from 60Co sources and relies on a combination of collimator sizes, weighting, etc to generate a high-dose region that is conformal with a designated target volume. Dose computation is typically performed by computer, but in this paper, single collimator dose profile behaviour is modelled analytically and then extended to accommodate multiple collimators of different weights with co-located isocentres. The dose profile from a single helmet is derived from a top-hat beam profile approximation and an idealized symmetric distribution of sources is used to represent the 201 sources within a collimating helmet. The results from the analysis are validated by an independent numerical model and also compared with those obtained by other groups using numerical and experimental methods. With respect to multiple collimators, the relationship between the size (full width half maximum) of the irradiated volume and relative collimator weighting is also examined using the simple analytical model. The simplicity of the mathematics clarifies the relationship between beam profile, dose profile and multiple collimator behaviour, and provides data that compare favourably with published literature.

5510 POSTER Heparanase expression in the differentiation of follicular thyroid lesions: from laboratory to clinical practice

A radiophotoluminescent glass rod detector has recently become commercially available. We evaluated the feasibility of the commercial glass rod as a new detector for measuring output factors in the CyberKnife. The glass rod detector was irradiated in a water phantom using a holder stand, which was specially designed for this study. The holder was composed of a PMMA tube with an attached vertical bar for the glass rod detector. The measured output factors obtained with the glass rod detector were compared with measurements made with a pinpoint ionization chamber, a diode, and a radiochromic film. The measured relative output factors obtained with the glass rod detector agreed with other detectors within 1.0% for collimator sizes larger than 20 mm. However, it was observed that the differences between the output factors measured with the glass rod detector and those obtained with the pinpoint chamber increased rapidly as the collimator size decreased. The relative output factors measured with the diode were consistently higher than those obtained using other detectors for the collimators sizes less than 10 mm in diameter. The glass rod detector results were in good agreement with those obtained from the radiochromic EBT film over the entire range of collimator sizes.

Comparative study of treatment dose plans after the refinement of Leksell Gamma Knife® single‐beam dose profiles

We investigated the amplification of discrepancy when using multiple shots of the same collimator size helmet, by comparing dose plans in the Leksell GammaPlan employing the default single-beam dose profiles and the Monte Carlo generated single-beam profiles. Four collimator helmets were studied. The results show that the largest amplification of discrepancy with multiple shots was found with the 8 mm collimator because of the largest discrepancy of its single-beam dose profile. The amplification of discrepancy is significant when tumor volumes increase but insignificant when the tumor volumes are in an elongated shape. Using close shot overlapping strategy (i.e., more shots close packed together) shows no observable increase in the amplification of discrepancy. For the best quality of Leksell Gamma Knife radiosurgery, it is suggested that the single-beam dose profiles should be refined, especially the 8 mm collimator, to prevent error amplification when using multiple collimator shots.

SU‐FF‐T‐404: The Effect of Detector Size On Measurement of Output Factor of 4 Mm Collimator in Gamma Knife

Purpose: We investigated the effect of detector size for output factor measurement of small field in Gamma knife. Measured and corrected output factors using deconvolution to remove the effect of detector size for the 4 mm collimator were compared. Method and Materials: The output factors were measured with a pinpoint ionization chamber, a glass rod detector (GRD) from the 14, 8 and 4 mm helmets relative to the 18 mm collimator. The measured output factors were corrected for spatial averaging effects by measuring dose profiles for the 4 mm collimator with the same detectors and deconvolving their response from the measured profiles. A Gaussian kernel was used to describe the detector response. The measuring volume of the pinpoint chamber was 0.015 cm3 and the size of the GRD was 1.5 mm in diameter and 8.5 mm in length. All measurements were performed in spherical polystyrene head phantom having a diameter of 160 mm. Results: The 4 mm collimator output factor measured by the pinpoint chamber was 0.699±0.009 and the GRD measurement was 0.861±0.015. The corrected output factor by deconvolving for the 4 mm collimator was 0.853 for the pinpoint chamber and 0.874 for the GRD. Conclusion: The output factors obtained with a GRD are within 1.1 % in good agreement with the values recommended by the manufacture. The difference for measured and corrected value of output factor based on GRD detector is within 1.5%. The GRD may be a useful detector for small field dosimetry. However, the measured value of the 4 mm collimator output factor from the pinpoint ion chamber is significantly lower than the results obtained using the GRD. Due to the large volume of pinpoint chamber compared to other detectors used, a significant deconvoltuion correction is noted for the for the smaller collimator size.