Collimator Size Research Articles

Gamma knife irradiation - induced histopathological changes in the trigeminal nerves of Rhesus monkeys

Objective The authors sought to observe histopathological changes in the trigeminal nerve(TN)after Gamma knife treatment in rhesus monkeys and to investigate the radiobiological mechanism of Gamma knife treatment for primary trigeminal neuralgia. The nerve length dose effect of irradiation was discussed. Method One of five rhesus monkeys randomly served as control and the other four monkeys were randomly administered a target dose of 60,70,80, and 100 Gy respectively. The size of the collimator was 4 mm and the target point was the TN root. In each experimental monkey, one side was exposed to single - target point irradiation and the contralateral side to two - target point irradiation. After 6 months, the TN root was examined by light microscopy, transmission electron microscopy, and immunohistochemistry.Results At the same dose of radiation, the damage to the nervous tissue by single - target point irradiation was identical to that caused by two - target point irradiation. In the trigeminal nerve tissues of the monkeys irradiated with 60 and 70 Gy,there was scant nerve demyelination and degeneration, fragmentation, or loss of axons. In the trigeminal nerve tissue of the monkey irradiated with 80 Gy, the nervous tissue showed a disordered structure. In the trigeminal nerve tissue of the monkey irradiated with 100 Gy, there were severe derangement in the structure of the nervous tissue and extensive demyelination, fragmentation, and loss of axons. Conclusions The target doses of 60 and 70 Gy have very little impact on the structure of the TN. Irradiation at 80 Gy can cause partial degeneration and loss of axons and demyelination. A 100 Gy dose can cause some necrosis of neurons. Comparing the single - target irradiation with the two - target point irradiation,the extent of damage to the nervous tissue is identical and no difference in the nerve length dose effect was found. Key words: Gamma knife; Macaque; Trigeminal neuralgia; Pathology; Stereotactic radiosurgery

Poster — Thur Eve — 57: Use of a Micro Liquid Ionization Chamber for Commissioning of Radiosurgery Beams

A commercially available liquid ionization chamber (PTW, Freiburg, Germany) was used in the commissioning of stereotactic radiosurgery beams for a Novalis Tx linear accelerator. The chamber has a small collecting volume (0.002cm3) yet providing a relatively high signal for its size. The chamber was attached to a solitary electrometer for output measurements, and to a water scanning system for PDD and beam profile measurements. The commissioning set consisted of output factors, percentage depth doses and off‐axis beam profiles for six collimator sizes ranging from 4mm to 15mm in diameter. The measured data was compared to both Gafchromic film dosimetry measurements and to data from the literature. Output factors measured with the liquid ion chamber are in good agreement with film measurements (<2% difference, except for 3.8% difference for the 4mm collimator). Similar overall agreement was found with a recent publication by Fan et al.,(2009). Beam profile measurements were found to be in very good agreement with Gafchromic film measurements. Our PDD data was equally in reasonable agreement with data from a recent publication by Chang et al., (2008) with discrepancies of 2–3% observed. A comparison of TMRs calculated from our PDD data and from direct measurement revealed larger differences, increasing with depth, that have yet to be resolved. The liquid ion chamber was easy to use required only a brief pre‐irradiation to provide a stable signal. Our preliminary results suggest this high spatial resolution chamber has usefulness in small field measurements.

SU‐GG‐T‐519: Improving Gamma Knife Plans Using Independent Sector Weighting

Purpose: The Gamma Knife Perfexion uses 192 Co‐60 sources divided into eight sectors. For each treatment isocenter, each sector can be either blocked or collimated using one of three collimator sizes, but the beam‐on time is identical for all sectors. We postulated that independently variable sector weighting might confer dosimetric advantages with regard to critical structure sparing. In this work, we investigate the potential advantages of independently varying the weighting of individual source sectors to create sector modulated Gamma Knife (SMoG) plans. Materials and Methods: We simulated independent sector weighting for a clinical Gamma Knife plan for a trigeminal neuralgia case. We first obtained dose matrices corresponding to the contribution of each sector. The sector weighting was varied in eight discrete levels and an optimum combination determined using an optimization algorithm. The resulting sector modulated Gamma Knife plan was then entered into the Gamma Knife treatment planning system by decomposing the weighted sectors into multiple shots at the same location with different weights and blocking. The process was repeated using four levels of sector weighting. Results: Our results showed that the optimized plan reduced the maximum dose to the organ at risk (the brainstem) from 18.6 Gy to 11.8 Gy (with eight levels) compared to the clinical plan. However, the beam‐on time for delivery increased from 32 minutes to 84 minutes. The SMoG plan with four sector weighting levels reduced the maximum brainstem dose to 12.6 Gy and only increased treatment beam‐on time to 42 minutes. Conclusions: We have demonstrated that independent weighting of the individual sectors in the Gamma Knife Perfexion can improve the quality of treatment plans, although at the cost of longer delivery times.

TH‐C‐204B‐02: Monte Carlo Feasibility Study of a Compact Microbeam Small Animal Irradiator

Purpose: Microbeam radiation therapy (MRT) is a promising but poorly understood experimental treatment modality. As an alternative to synchrotron‐based facilities we propose a compact carbon nanotube‐based MRT device for small animal research. We report on a Monte Carlo‐based feasibility study of the proposed design. Method and Materials: Monte Carlo calculations were performed using EGSnrc‐based codes. The proposed device design includes a carbon nanotube cathode shaped to match the collimator aperture a reflection anode with filter and a microbeam collimator. The collimator is sized to deliver a beam width ranging from 30–200 microns at 15 cm SAD. Design parameters studied with Monte Carlo include electron energy anode angle filtration and collimator design. Results: Increasing the energy from 100 kVp to 150 kVp increased the photon fluence through the collimator by a factor of 1.7. Both energies produced a largely uniform fluence along the microbeam with 5% decreases in intensity near the edges of the long dimension. The isocentric dose rate for 150 kVp was calculated to be 1200 Gy/min/A in the center of a 3 cm diameter target. Expected cathode currents of 1 A project an isocentric dose rate of 80 Gy/sec for a four‐beam system. Scatter contributions resulting from collimator size were found to produce only small (<7%) changes in the dose rate for field widths greater than 50 microns. Dose vs. depth was weakly dependant on filtration material. The peak‐to‐valley ratio outside the beam was less than 0.5% indicating the valley dose between adjacent microbeams will be no more than 1% of the peak dose. Conclusion: Monte Carlo simulations demonstrate that the proposed microbeam device will be capable of delivering a sufficient dose rate and peak‐to‐valley‐ratio to be useful for small‐animal MRT studies.Conflict of interest: The work is supported by NIH grant 1RC2CA148487‐01

SU-GG-T-143: MLC-Based CyberKnife Radiotherapy for Prostate Cancer

Purpose: The Cyberknife system allows for real‐time organ motion correction and it enables delivery of a large number of non‐isocentric, non‐coplanar beams. A CyberKnife using multi‐leaf collimators(MLC) may achieve an improved dose advantage while maintaining a short delivery time. The purpose of this study is to evaluate the plan quality of MLC‐based prostate roboticradiotherapy.Method and Materials:Treatment plans were compared between regular IMRT, CyberKnife using the IRIS collimator (CK IRIS) and CyberKnife using a MLC (CK MLC). The IMRT plan and the CK MLC plan were generated using the Eclipse inverse planning system. Various numbers of non‐coplanar fields have been tried for CK MLC plans. With the CK IRIS plan, all 12 possible collimator sizes were included in the optimization process to achieve the best performance. Results: Isodoses for the IMRT plan, the CK MLC plan and the CK IRIS plan are compared in the same mid‐axial plane. All plans provide very conformai and complete converge of the target. However, the 50% isodose clearly demonstrates that the CK MLC plan provides much sharper dose fall off than the IMRT and the CK IRIS plans. The CK IRIS plan shows the worst rectal sparing with the 50% isodose line covering more than half of the rectum. A further comparison of DVHs shows significantly improved sparing of rectum and bladder with the CK MLC plan, when compared to the IMRT plan and the CK IRIS plan. Conclusions: The CyberKnife with MLC can produce superior dose distributions for sparing rectum and bladder and excellent DVHs for the target compared with IMRT and CyberKnife IRIS plans, and produces similar dose heterogeneities as IMRT plans. With significantly less beams, it can deliver a 2Gy/fx treatment in 10 minutes.

SU-GG-T-398: Monte Carlo Commissioning of BrainLab SRS Cones on a NovalisTX

Purpose: To commission a Monte Carlo model of the high dose rate 6MV photon beam of the Varian NovalisTX linear accelerator for patient dose calculations. Methods and Materials: The high dose rate 6 MV beam on the Varian NovalisTX accelerator is utilized for stereotactic radiosurgery (SRS) treatment. The SRS beams are confined by cone collimators and the available collimator sizes range from 5 and 10 to 30 mm, in every 2 mm increment. Furthermore, the linear accelerator is equipped with multileaf collimator with MLC leaf width of 2.5mm. It is necessary to investigate an accurate and detector independent method for the narrow beam output factor measurement. In this study, the beam data was collected using a microLion chamber (vol. 0.002cc) and used to commission the linac model and all the SRS cones. Film measurement were used to measure the MLC transmission and leakage. Results: Monte Carlo results are found to be in excellent agreement with the measurements for the open, and cone beam fields. Measured output factors were in agreement with Monte Carlo results but there was a discrepancy against the the VARIAN recommended output factors measured directly by Wellhöfer CC01 chamber and Scanditronix photon field diode which are consistently higher for all the cones. Conclusions: A Monte Carlo model of the NovalisTX high dose rate SRS beam has been created that is able to simulate the beam with high accuracy. Beam parameters that are otherwise difficult or detector dependent can be determined using the Monte Carlo method.

SU-GG-T-531: A Unique Method for End-To-End Quality Assurance Test for Trigeminal Neuralgia Treatments in CyberKnife

Purpose: To develop an End‐to‐End QA test for Trigeminal‐Neuralgia in CyberKnife. Method and Materials: Functional treatments like Trigeminal‐Neuralgia are treated with small collimators(5mm–10mm) at high doses.Currently all End‐to‐End tests performed in CyberKnife utilize larger collimators(20mm–30mm).There is no specific QA for trigeminal‐path using small collimators.It is critical to have specific End‐to‐End test for this path as it lacks correction for rotational shifts during treatments.A styrofoam insert was designed to retrofit the mini‐ball cube replacing the regular‐ball cube inside the stereotactic head phantom.The insert was designed to maintain precise geometry similar to original ball cube.The head phantom was CT‐scanned with precision cut films positioned inside the mini ball cube in anterior‐superior and anterior‐left planes.A spherical target of 5mm contoured in center of mini ball cube simulates the volume of Trigeminal Nerve.A isocentric treatment plan was developed with 7.5mm collimator with trigeminal‐path.A maximum dose of 3000cGy to target is planned using RPC protocol,with 80% isodose line covering entire target volume.The plan is then delivered on Cyberknife machine with rotational limits reduced to 0.5mm during treatment delivery.The films are then scanned using Epson Flat bed scanner and analyzed. Results: The total targeting error for Trigeminal End‐to‐End test was found to be 0.3±0.1mm.An analysis of films using RPC protocol for stereotactic QA shows that the 80% isodose widths measured in the sagittal‐axial,axial‐coronal,sagittal‐coronal plane and treated volume are in close agreement with the plan.A comparison and analysis of separate films without center cut is also presented. Conclusion: This specific End‐to‐End test with small collimators provides a unique method for proper verification of trigeminal‐path.It is recommended that this test can be implemented with other End‐to‐End tests.Because of small collimator size,the passing criteria recommended for targeting error is < 0.5 mm.End‐to End test with separate films without center cut,enhances the accuracy and precision of the test.

WE-C-BRA-07: Multi-Leaf Collimators for Robotic Radiotherapy: Feasibility Study

Purpose: The robotic mobility of the CyberKnife System enables the delivery of a large number of non‐coplanar beams, which enhances dose conformality and creates very steep dose gradients, thus reducing doses to surrounding critical structures. A typical CyberKnife treatment plan for stereotactic radiotherapy utilizes approximately 150–200 beams to deliver conformal dose coverage to the target. By replacing the current cone‐shaped collimator on the CyberKnife machine with multi‐leaf collimators(MLC), we anticipate further improvement of dose conformality while reducing beam numbers and monitor units, and thus, treatment time. The purpose of this study is to evaluate the plan quality of MLC based roboticradiotherapy.Method and Materials:Treatment plans were generated for 5 cases with varied tumor sites using the MLC (with 5 mm leaf width defined at 80 cm SAD) and the Iris Collimator. Each plan was generated based on clinical cases that had previously been treated with the CyberKnife System. With the Iris Collimator, all 12 potential collimator sizes were included in the optimization process. For the MLCtreatment plan, 23 fields were used for planning. The beam angles were selected manually based on the frequency distribution of node usage in Iris Collimator plans. The Eclipse planning system was used in the MLC plan optimization. Results: The results of the 5 cases showed that a reduction in total MU of 70–80% and a reduction of treatment time of 80–90% are generally achievable for the MLC plan compared to the IRIS plan. Both plans provide very conformal target dose coverage. However, the MLC plan delivers a much lower treatmentdose to critical structures and surrounding tissues with a sharper dose fall off. Conclusions: The reduction of total MU and treatment time by the multiple leaf collimators offers the potential to greatly increase the clinical application for roboticradiotherapy.

SU-GG-T-604: GPU-Accelerated KV/MV Dose Computation

Purpose: To develop a comprehensive GPU‐accelerated, superposition/convolution based dose calculation engine supporting both a modern dual‐source MV model and an analytical kV source model Method and Materials: An analytical kV source model was developed for use with the Small Animal Research Radiation Project (SARRP), which analytically calculated the fluence to every voxel. The MV source model contains a traditional primary and an enhanced extra‐focal algorithm that efficiently accounts for leaf‐height effects. The spectral and attenuation effects of static beam modifiers were integrated into each source's spectral function. The total energy released per unit mass (TERMA) computation used back‐projection with optional exact multi‐spectral attenuation. Superposition/convolution was implemented using the inverse cumulative‐cumulative kernel and exact radiological path ray‐tracing with optional kernel tilting. Two superposition variants were implemented and benchmarked. Multi‐resolution superposition approximates true, solid angle ray‐tracing. Arc superposition increases the relative temporal TERMA sampling. Results: The kV source model performance was integrated into the TERMA computation, increasing performance in a volume resolution and collimator size dependent manner. MV source model performance was <9ms (data dependent) for a high resolution (4002) field using an NVIDIA GeForce GTX 280. Our TERMA GPU implementation was improved, doubling performance. Our GPU Superposition implementation was improved by ∼18% to 0.058 s and 1.186 s for 643 and 1283 water phantoms; a speed‐up of 80–142x over the highly optimized Pinnacle3 (Philips‐Madison, WI) implementation. Pinnacle3 times were 8.3 s and 94 s, respectively, on an AMD Opteron 254 (2 cores, 2.8GHz). Extensive accuracy benchmarks indicated that Arc Superposition increases dose computation efficiency and that Multi‐Resolution Superposition slightly under estimates MV intra‐patient scatter. Conclusions: We have completed a comprehensive, GPU‐accelerated dose engine in order to provide a substantial performance gain over CPU based implementations.

Influence of beam efficiency through the patient‐specific collimator on secondary neutron dose equivalent in double scattering and uniform scanning modes of proton therapy

Conventional proton therapy facilities use double scattering nozzles, which are optimized for delivery of a few fixed field sizes. Similarly, uniform scanning nozzles are commissioned for a limited number of field sizes. However, cases invariably occur where the treatment field is significantly different from these fixed field sizes. The purpose of this work was to determine the impact of the radiation field conformity to the patient-specific collimator on the secondary neutron dose equivalent. Using a WENDI-II neutron detector, the authors experimentally investigated how the neutron dose equivalent at a particular point of interest varied with different collimator sizes, while the beam spreading was kept constant. The measurements were performed for different modes of dose delivery in proton therapy, all of which are available at the Midwest Proton Radiotherapy Institute (MPRI): Double scattering, uniform scanning delivering rectangular fields, and uniform scanning delivering circular fields. The authors also studied how the neutron dose equivalent changes when one changes the amplitudes of the scanned field for a fixed collimator size. The secondary neutron dose equivalent was found to decrease linearly with the collimator area for all methods of dose delivery. The relative values of the neutron dose equivalent for a collimator with a 5 cm diameter opening using 88 MeV protons were 1.0 for the double scattering field, 0.76 for rectangular uniform field, and 0.6 for the circular uniform field. Furthermore, when a single circle wobbling was optimized for delivery of a uniform field 5 cm in diameter, the secondary neutron dose equivalent was reduced by a factor of 6 compared to the double scattering nozzle. Additionally, when the collimator size was kept constant, the neutron dose equivalent at the given point of interest increased linearly with the area of the scanned proton beam. The results of these experiments suggest that the patient-specific collimator is a significant contributor to the secondary neutron dose equivalent to a distant organ at risk. Improving conformity of the radiation field to the patient-specific collimator can significantly reduce secondary neutron dose equivalent to the patient. Therefore, it is important to increase the number of available generic field sizes in double scattering systems as well as in uniform scanning nozzles.

X-ray diffraction studies of Pompeian wall paintings using synchrotron radiation and dedicated laboratory made systems

The full identification of artwork materials requires not only elemental analysis but also structural information of the compounds as provided by X-ray diffraction (XRD). This is easily done when taking samples (or micro-samples) from artworks. However, there is an increasing interest in performing non-destructive studies that require adapted XRD systems. Comparative study of synchrotron high-resolution X-ray powder diffraction (SR-HRPD) and laboratory non-destructive systems (portable XRD and micro-XRD) is the main objective of this work. There are no qualitative differences among the three systems as for detected phases in the Pompeian wall paintings that were studied, except in the case of minority phases which only were detected by SR-HRPD. The identified pigments were goethite, hematite, cinnabar, glauconite, Pompeian blue, together with calcite, dolomite and aragonite. Synchrotron XRD diagrams show better resolution than the others. In general, the peak widths in the diagrams obtained with the portable XRD system are similar to those obtained by micro-diffraction equipment. Factors such as residual divergence of X-ray sources, incidence angle and slit or collimator size are discussed in relation with the quality of XRD diagrams.

Direct tumorin vivodosimetry in highly-conformal radiotherapy: A feasibility study of implantable MOSFETs for hypofractionated extracranial treatments using the Cyberknife®system

In highly-conformal radiotherapy, due to the complexity of both beam configurations and dose distributions, traditional in vivo dosimetry is unpractical or even impossible. The ideal dosimeter would be implanted inside the planning treatment volume so that it can directly measure the total delivered dose during each fraction with no additional uncertainty due to calculation models. The aim of this work is to verify if implantable metal oxide semiconductors field effect transistors (MOSFETs) can achieve a sufficient degree of dosimetric accuracy when used inside extracranial targets undergoing radiotherapy treatments using the Cyberknife system. Based on the preliminary findings of this study, new prototypes for high dose fractionations were developed to reduce the time dependence for long treatment delivery times. These dosimeters were recently cleared and are marketed as DVS-HFT. Multiple measurements were performed using both Virtual Water and water phantoms to characterize implantable MOSFETs under the Cyberknife beams, and included the reference-dosimetry consistency, the dependence of the response on the collimator size, on the daily delivered dose, and the time irradiation modality. Finally a Cyberknife prostate treatment simulation using a body phantom was conducted, and both MOSFET and ionization readings were compared to Monte Carlo calculations. The feasibility analysis was conducted based on the ratios of the absorbed dose divided by the dose reading, named as "further calibration factor" (FCF). The average FCFs resulted to be 0.98 for the collimator dependence test, and about 1.00 for the reference-dosimetry test, the dose-dependence test, and the time-dependence test. The average FCF of the prostate treatment simulation test was 0.99. The obtained results are well within DVS specifications, that is, the factory calibration is still valid for such kind of treatments using the Cyberknife system, with no need of further calibration factors to be applied. The final accuracy of implantable MOSFETs when used for such kind of treatments was estimated to be within +/- 4%. Additional investigations using dose/fraction higher than 12 Gy, different beam configurations, and tracking systems could extend the present findings to other kind of treatments. MOSFET technology was proven to have high versatility in fast adaptation of existing detectors to new applications. It is plausible to expect a general feasibility of implantable MOSFET technology for in vivo dosimetry of the extracranial-targets treatments using the Cyberknife, provided each particular application will be validated by suitable both physical and clinical studies.

Verification of source and collimator configuration for Gamma Knife® Perfexion™ using panoramic imaging

The new model of stereotactic radiosurgery system, Gamma Knife Perfexion, allows automatic selection of built-in collimation, eliminating the need for the time consuming manual collimator installation required with previous models. However, the configuration of sources and collimators inside the system does not permit easy access for the verification of the selected collimation. While the conventional method of exposing a film at the isocenter is useful for obtaining composite dose information, it is difficult to interpret the data in terms of the integrity of each individual source and corresponding collimation. The primary aim of this study was to develop a method of verifying the geometric configuration of the sources and collimator modules of the Gamma Knife Perfexion. In addition, the method was extended to make dose measurements and verify the accuracy of dose distributions calculated by the mathematical formalism used in the treatment planning system, Leksell Gamma Plan. A panoramic view of all of 192 cobalt sources was simultaneously acquired by exposing a radiochromic film wrapped around the surface of a cylindrical phantom. The center of the phantom was mounted at the isocenter with its axis aligned along the longitudinal axis of the couch. The sizes and shapes of the source images projected on the phantom surface were compared to those calculated based on the manufacturer's design specifications. The measured dose at various points on the film was also compared to calculations using the algorithm of the planning system. The panoramic images allowed clear identification of each of the 192 sources, verifying source integrity and selected collimator sizes. Dose on the film surface is due to the primary beam as well as phantom scatter and leakage contributions. Therefore, the dose at a point away from the isocenter cannot be determined simply based on the proportionality of collimator output factors; the use of a dose computation algorithm is required. Scatter and leakage dose contributions from neighboring sources were calculated and found to be 6.3% (ranging from 4.5% to 7.4%), 16.7% (12.5%-19.3%), and 66.6% (38%-78%) for the 4, 8, and 16 mm collimators, respectively, at the centers of the source images. The measured average dose on films with 16 mm collimators agrees with the dose model of the treatment planning system to within 1.0%. The average doses on the film were 24.0, 60.8, and 186.2 cGy for 4, 8, and 16 mm diameter collimators, respectively, when the machine was set to deliver a reference dose of 100 Gy to the center of an 80 mm radius spherical dosimetry phantom. A method of simultaneously capturing and analyzing the panoramic images of 192 cobalt sources has been developed to verify the source and collimator configuration of GK systems. The method was extended to verify the dose calculation model of the treatment planning system by comparing the measured doses on the panoramic film images and the corresponding calculated doses. The method presented can play a significant role in comprehensive commissioning and routine quality assurance testing of the Gamma Knife systems.

Target and peripheral dose during patient repositioning with the Gamma Knife automatic positioning system (APS) device

The GammaPlan treatment planning system does not account for the leakage and scatter dose during APS repositioning. In this study, the dose delivered to the target site and its periphery from the defocus stage and intershot couch transit (couch motion from the focus to defocus position and back) associated with APS repositioning are measured for the Gamma Knife model 4C. A stereotactic head‐frame was attached to a Leksell 16 cm diameter spherical phantom with a calibrated ion chamber at its center. Using a fiducial box, CT images of the phantom were acquired and registered in the GammaPlan treatment planning system to determine the coordinates of the target (center of the phantom). An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 8, 14 and 18 mm collimator helmets to determine the relationship of measured dose to the number of repositions of the APS system and to the helmet size. The target coordinate was identical throughout entire study and there was no movement of the APS between various shots. This allowed for measurement of intershot transit dose at the target site and its periphery. The couch was paused in the defocus position, allowing defocus dose measurements at the intracranial target and periphery. Measured dose increases with frequency of repositioning and with helmet collimator size. During couch transit, the target receives more dose than peripheral regions; however, in the defocus position, the greatest dose is superior to the target site. The automatic positioning system for the Leksell Gamma Knife model 4C results in an additional dose of up to 3.87±0.07%,4.97±0.04%, and 5.71±0.07% to the target site; its periphery receives additional dose that varies depending on its position relative to the target. There is also dose contribution to the patient in the defocus position, where the APS repositions the patient from one treatment coordinate to another. This may be important for treatment areas around critical structures within the brain. Further characterization of the defocus and transit exposures and development of a dose calculation algorithm to account for these doses would improve the accuracy of the delivered plan.PACS numbers: 87.53.‐j, 87.53.Bn, 87.53.Dq, 87.53.Ly

Comparison of Planned Dose Distributions Calculated by Monte Carlo and Ray-Trace Algorithms for the Treatment of Lung Tumors With CyberKnife: A Preliminary Study in 33 Patients

To compare dose distributions calculated using the Monte Carlo algorithm (MC) and Ray-Trace algorithm (effective path length method, EPL) for CyberKnife treatments of lung tumors. An acceptable treatment plan is created using Multiplan 2.1 and MC dose calculation. Dose is prescribed to the isodose line encompassing 95% of the planning target volume (PTV) and this is the plan clinically delivered. For comparison, the Ray-Trace algorithm with heterogeneity correction (EPL) is used to recalculate the dose distribution for this plan using the same beams, beam directions, and monitor units (MUs). The maximum doses calculated by the EPL to target PTV are uniformly larger than the MC plans by up to a factor of 1.63. Up to a factor of four larger maximum dose differences are observed for the critical structures in the chest. More beams traversing larger distances through low density lung are associated with larger differences, consistent with the fact that the EPL overestimates doses in low-density structures and this effect is more pronounced as collimator size decreases. We establish that changing the treatment plan calculation algorithm from EPL to MC can produce large differences in target and critical organs' dose coverage. The observed discrepancies are larger for plans using smaller collimator sizes and have strong dependency on the anatomical relationship of target-critical structures.

Determination of output factors for stereotactic radiosurgery beams

Accurate dosimetry of the narrow beam tends to be difficult to perform due to the absence of lateral electronic equilibrium and the steep dose gradient, as well as the finite size of detectors. Thus, although the high dose rate 6 MV beam on the VARIAN Trilogy accelerator is increasingly utilized for stereotactic radiosurgery (SRS) treatment, there is no general agreement in the SRS beam output factor values among the Trilogy user community. Trilogy SRS beams are confined by cone collimators and the available collimator sizes range from 5 and 10 to 30 mm, in every 2 mm increment. A range of the relative output factors are in clinic use. This variation may impair observations of dose response and optimizations of the prescribed dose. It is necessary to investigate an accurate, easily performable, and detector independent method for the narrow beam output factor measurement. In this study, a scanning beam/scanning chamber method was proposed to overcome the limitation/difficulty of using a relatively large detector in narrow beam output factor measurement. Specifically, for the scanning beam method, multiple narrow beams are used for the dose measurement using a finite size chamber. These multiple scanning beams form an equivalent large uniform field which provides lateral electron equilibrium condition. After the measurement, the contributions from neighboring beams are deconvolved and the value is used for output factor determinations. For a Linac that cannot move a beam laterally, the scanning chamber method can be used to achieve the same result. The output factors determined in such a method were compared to chambers (a 0.015 cc PTW PinPoint ion chamber and a 0.125 cc PTW ion chamber) and film measurement, as well as with Monte Carlo simulation. Film and Monte Carlo results are found to be in excellent agreement with the measurement using the scan beam method. However, the VARIAN recommended output factors measured directly by Wellhöfer CC01 chamber and Scanditronix photon field diode are consistently higher for all the cones. Especially for the 5 mm cone, the difference is more than 10%. Overall, the results suggested that the new method can help overcoming the detector volume averaging effect and the positioning uncertainties, which constitute the major challenge in small radiosurgical beam output factor measurement, and provide reliable output factors.

Gamma-ray attenuation method as an efficient tool to investigate soil bulk density spatial variability

Abstract The spatial variability of soil bulk density ( ρ b ) was measured by using the volumetric ring method (VRM) and the gamma-ray attenuation method (GAM). Collimated radiation from 3.7 GBq of 241 Am was used to evaluate the soil mass attenuation coefficient and its bulk density. Circular lead collimators were adjusted and aligned between source ( D = 1, 2 and 3 mm) and detector ( D = 4.5 mm). Results of GAM for average ρ b provided good agreement with the corresponding values obtained gravimetrically. Variations in bulk density for different collimator dimensions can be attributed to multiple scattering after photons interaction with soil, mainly for 3 mm collimator size. The best result of ρ b by the nuclear technique was obtained when ρ b represents an average of the measurements for collimators of 1 and 2 mm. Another cause for the differences in ρ b by GAM and VRM is the heterogeneity of soil when the collimated beam can interact with stones or large air-filled holes or channels present in the sample. Therefore, the pattern of spatial variability obtained by VRM was confirmed by GAM for all collimator sizes. This result is a good indication that GAM can be used with success to analyze soil spatial variability.

SU-FF-T-585: Dose Gradient Analysis with Beam Concentricity in Conformal Planning

Purpose: The CyberKnife can perform both isocentric and nonisocentric plans for stereotactic radiosurgery. Isocentric plans typically have higher dose gradients while non-isocentric plans offer higher conformality. The objective of this work was to combine the best of both planning techniques as appropriate for the clinical situation. Method: A parameter, Target Boundary Distance (TBD), was introduced for the nonisocentric planning in cyberknife planning system. TBD defines as the distance between the target boundary and the pencil beam boundary. If TBD is 0 (the default value), the pencil beam is pre-defined to be tangent to the inner target boundary. The beams will shoot further inside the target, and geometrically closer to the centroid of the target, as TBD decreases. Dose gradient analysis was performed for a total of 10 clinical cases using a variety of treatment plan parameters. Results: Analyzed plans included different collimator size and TBD values along with isocentric plans. Compared with the TBD=0 plan, the plans with an appropriate negative TBD had higher dose gradients, lower total MU, a smaller number of beams, and similar tumor coverage - at the cost of a slight decrease in the conformality and homogeneity indexes. The tissue toxicity can be reduced up to 20% with this technique in all these 10 cases. When TBD is smaller than [— (PTV Diameter — Collimator) / 2], the conformality and homogeneity indexes may deteriorate significantly. The small collimator plan had higher homogeneity, a larger number of beams and higher total MU, but not necessarily result in higher dose gradients. Conclusion: Our analysis shows that the TBD can be adopted to adjust the dose gradient. We found that the inclusion of an appropriate negative TBD into non-isocentric treatment planning can be an effective tool to increase the dose gradient and reduce surrounding normal tissue toxicity.

SU-FF-T-539: Independent Treatment Time Verification for GammaPlan PFX

Purpose: The purpose of this study is to develop an independent calculation method for treatment time verification of Leksell Gamma Knife GammaPlan radiosurgery plans for the Perfexion treatment unit. Method: An independent dose calculation method is performed to verify the result of the treatment planning system for each patient prior to treatment for most radiotherapy procedures. Due to the complex patient and source geometry such a check is not commercially available for Gamma Knife stereotactic radiosurgery. The main goal of this study is to develop a program that verifies the irradiation time as calculated by the Leksell Gamma Knife GammaPlan PFX. The Leksell Gamma Knife Perfexion unit delivers ionizing radiation from 192 cobalt‐60 sources arranged in eight sectors and embedded within the shielded radiation unit. The beams of gamma radiation that arise from the cobaltsources coincide at a fixed focal point within the radiation unit. The dose is shaped to the precise contour of the target by combination of shots using different collimator sizes and blocking of sectors. Our program is implemented in MATLAB. To obtain the irradiation time for each shot in the treatment plan, we first determine the depth that each of the individual 192 beams penetrates the patient's skull to the isocenter. Once the depth of each beam has been determined, the Tissue Maximum Ratio (TMR) is obtained and the dose rate at isocenter for each individual shot is calculated. The individual and total treatment times are calculated and compared to that obtained from GammaPlan PFX. Results and Conclusions: We have developed a MATLAB program which could be used for independent verification of the treatment planning irradiation times prior to Gamma Knife radiosurgery.Treatment times calculated from our program agree with GammaPlan PFX within 3%.

SU-FF-T-533: Target and Peripheral Dose During Patient Repositioning with the Gamma Knife APS Device

Purpose: Measure unaccounted dose delivered to the target site and its periphery from the defocused and transit beam during automatic positioning system (APS) repositioning for Gamma Knife Radiosurgery.Methods and Materials: A stereotactic head‐frame was attached to a 16cm diameter spherical phantom with a calibrated ion‐chamber at its center. Using a fiducial‐box to determine the coordinates of the target, a CT scan with 1mm slice thickness was taken of the phantom and registered in the GammaPlan TPS. 10Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 18mm, 14mm and 8mm helmets with varying number repositions for each plan to determine the relationship of measured dose with number of repositions of the APS system and helmet size. The shot isocenter was identical in the entire study and there was no movement of APS between various shots; this allows for measurement of transit dose (couch moves from the focus to defocused position and back) and the least defocused dose (at defocused couch position). The couch was suspended in the defocused position allowing intracranial defocused dose measurements. Results: Dose increases with frequency of repositioning and collimator size. Overdose of up to 5.713±0.066% at target position can result from couch transfer. Dose rate of 8.807±0.411cGy/min (18mm helmet) and 5.891±0.509cGy/min (8mm helmet) where measured. During couch transit, the target receives more dose than peripheral regions; in the defocused position, the greatest dose is superior on the phantom where dose rate is 4.910±0.005cGy/min. Conclusion: APS repositioning results in additional dose to the target site and its periphery for multi‐shot runs. Doses in superior regions should be monitored due to epilation. Consideration of conformity index is suggested, especially around critical structures (e.g. optic nerve), when generating a treatment plan as risk of toxicity is a concern.