Use of a plastic scintillator detector for patient-specific quality assurance of VMAT SRS.

PURPOSE: To evaluate a scintillator detector for patient-specific quality assurance of VMAT radiosurgery plans. METHODS: The detector was comprised of a 1 mm diameter, 1 mm high scintillator coupled to an acrylic optical fiber. Sixty VMAT SRS plans for treatment of single targets having sizes ranging from 3 mm to 30.2 mm equivalent diameter (median 16.3 mm) were selected. The plans were delivered to a 20 cm × 20 cm x 15 cm water equivalent plastic phantom having either the scintillator detector or radiochromic film at the center. Calibration films were obtained for each measurement session. The films were scanned and converted to dose using a 3-channel technique. RESULTS: The mean difference between scintillator and film was -0.45% (95% confidence interval -0.1% to 0.8%). For target equivalent diameter smaller than the median, the mean difference was 1.1% (95% confidence interval 0.5% to 1.7%). For targets larger than the median, the mean difference was -0.2% (95% confidence interval -0.7% to 0.1%). CONCLUSIONS: The scintillator detector response is independent of target size for targets as small as 3 mm and is well-suited for patient-specific quality assurance of VMAT SRS plans. Further work is needed to evaluate the accuracy for VMAT plans that treat multiple targets using a single isocenter.

Jaw Immobilization for Gamma Knife Surgery in Patients with Mandibular Lesions: A Newly, Innovative Approach.

BACKGROUND: The purpose of our report is to describe an innovative system used for mandibular immobilization during Gamma Knife surgery (GKS) procedures. It is based on an approach originally developed in Marseille in extracranial lesions, close to or involving the mandible, which may imply a certain degree of movement during the therapeutic image acquisitions and/or GKS treatment. METHODS: The maxillofacial surgeon applied bone titanium self-tapping monocortical screws (4; 2 mm diameter, 10 mm length) between roots of the teeth in the fixed gingiva (upper and lower maxillae) the day before GKS (local anesthesia, 5-10 min time). Two rubber bands were sufficient for the desired tension required to undergo GKS. We further proceeded with application of the Leksell stereotactic G frame and carried out the usual GKS procedure. RESULTS: The mean follow-up period was 2.3 years (range 0.6-3). Three patients have been treated with this approach: 2 cases with extracranial trigeminal schwannomas involving the mandibular branch, with decrease in tumor size on MR follow-up; 1 case with residual paracondylian mandibular arteriovenous malformation following partial embolization, completely obliterated at 7 months (digital subtraction angiography programmed 1 year after treatment). CONCLUSIONS: Jaw immobilization appears to be a quick, minimally invasive, safe and accurate adjunctive technique to enhance GKS targeting precision.

Pushing the limits of the Leksell stereotactic frame for spinal lesions up to C3: fixation at the maxilla.

BACKGROUND: Spinal radiosurgery is not considered in the domain of traditional Gamma Knife radiosurgery (GKRS) setup. The major obstacles in GKRS for upper cervical spine lesions remain in difficulty of frame fixation, avoiding collision and maintaining the integrity of the relative position of the lesion from image acquisition to treatment. METHODOLOGY: The supraorbital margin remains the standard lowest fixation point for Leksell stereotactic frame. We describe fixation at the maxilla to target and treat upper cervical spine lesions (up to C3 vertebra) with measures to ensure cervical immobilisation and precision of the GKRS treatment. RESULTS: We have treated two patients at the upper cervical spine up to C3 vertebra by fixing anterior pillars of the Leksell stereotactic frame at the maxilla. To ensure cervical immobilisation and precision of treatment, the neck was immobilised with a Philadelphia collar. The relative position between the head and sternum with the couch from image acquisition to the radiation delivery was kept constant. Docking angle was kept neutral (90 degrees) throughout the treatment (from image acquisition to actual treatment). CONCLUSIONS: The maxilla is a potential alternative for stereotactic frame fixation. Measures to ensure cervical immobilisation with lower-down frame position permits treatment of lesions as low as C3 vertebra.

Useful base plate to support the head during Leksell skull frame placement in gamma knife perfexion radiosurgery.

We developed an original base plate to support both the patient's head and a Leksell stereotactic skull frame during frame placement in the supine position. The base plate is made of transparent acrylic board with holes at the posterior posts for injection of local anesthetics and maneuver of fixation screws through them. A stable and comfortable position of the patient's head in a supine position is obtained and maintained on this base plate with an air-pressure cuff beneath the patient's head. The patient is able to keep a stable, relaxed and comfortable posture during the procedures of skull frame placement.

Importance of CBCT setup verification for optical-guided frameless radiosurgery.

The purpose of this study is to quantify the discrepancy between optical guidance platform (OGP) frameless localization system (Varian) and Trilogy on-board imaging (OBI) system (Varian) for setting up phantom and stereotactic radiosurgery (SRS) patient; and to determine whether cone-beam CT (CBCT) is necessary for OGP patient setup, and compare CBCT and orthogonal kV-kV in term of their verification capability. Three different phantoms were used in the study: a custom-made phantom, a Penta-Guide phantom, and a RANDO phantom. Five patients using both OGP and CBCT setup and 14 patients using CBCT setup alone were analyzed. One patient who had big couch shifts discrepancy between OGP and CBCT was selected for further investigation. Same patient's CBCT and planning CT were fused. A RANDO phantom simulation experiment was performed using OGP setup with both CBCT and orthogonal kV-kV verification. For all of three phantom experiments, the shifts performed by CBCT beam and orthogonal kV-kV were all within 1 mm. Among five SRS patients using OGP setup, three had 3D couch corrections more than 3 mm. The image fusion of CBCT and planning CT clearly illustrated a tilt of bite-block in a patient's mouth. For 14 SRS patients using CBCT-guided setup, overall 3D correction was 3.3 ± 1.5 mm. RANDO phantom experiment demonstrated how a tilted bite-block caused isocenter shift. CBCT-calculated shifts are the same as expected, but kV-kV results differed by 1-2 mm if the initial head position is tilted. The bite-block tilting in patient's mouth is a major reason for the cause of positioning error for OGP frameless SRS setup. CBCT verification is necessary. CBCT provides more accurate couch corrections than orthogonal kV-kV when head was tilted. OGP is useful for detecting patient movement, but it does not necessarily imply that the isocenter has moved.

[Dose impact of a carbon fiber couch for stereotactic body radiation therapy of lung tumors].

The aim of this study was to measure the dose attenuation caused by a carbon fiber radiation therapy table (Imaging Couch Top; ICT, BrainLab) and to evaluate the dosimetric impact of ICT during stereotactic body radiation therapy (SBRT) in lung tumors. The dose attenuation of ICT was measured using an ionization chamber and modeled by means of a treatment planning system (TPS). SBRT was planned with and without ICT in a lung tumor phantom and ten cases of clinical lung tumors. The results were analyzed from isocenter doses and a dose-volume histogram (DVH): D95, Dmean, V20, V5, homogeneity index (HI), and conformity index (CI). The dose attenuation of the ICT modeled with TPS agreed to within ±1% of the actually measured values. The isocenter doses, D95 and Dmean with and without ICT showed differences of 4.1-5% for posterior single field and three fields in the phantom study, and differences of 0.6-2.4% for five fields and rotation in the phantom study and six fields in ten clinical cases. The dose impact of ICT was not significant for five or more fields in SBRT. It is thus possible to reduce the dose effect of ICT by modifying the beam angle and beam weight in the treatment plan.

Dosimetric performance and array assessment of plastic scintillation detectors for stereotactic radiosurgery quality assurance.

PURPOSE: To compare the performance of plastic scintillation detectors (PSD) for quality assurance (QA) in stereotactic radiosurgery conditions to a microion-chamber (IC), Gafchromic EBT2 films, 60 008 shielded photon diode (SD) and unshielded diodes (UD), and assess a new 2D crosshair array prototype adapted to small field dosimetry. METHODS: The PSD consists of a 1 mm diameter by 1 mm long scintillating fiber (BCF-60, Saint-Gobain, Inc.) coupled to a polymethyl-methacrylate optical fiber (Eska premier, Mitsubishi Rayon Co., Ltd., Tokyo, Japan). Output factors (S(c,p)) for apertures used in radiosurgery ranging from 4 to 40 mm in diameter have been measured. The PSD crosshair array (PSDCA) is a water equivalent device made up of 49 PSDs contained in a 1.63 cm radius area. Dose profiles measurements were taken for radiosurgery fields using the PSDCA and were compared to other dosimeters. Moreover, a typical stereotactic radiosurgery treatment using four noncoplanar arcs was delivered on a spherical phantom in which UD, IC, or PSD was placed. Using the Xknife planning system (Integra Radionics Burlington, MA), 15 Gy was prescribed at the isocenter, where each detector was positioned. RESULTS: Output Factors measured by the PSD have a mean difference of 1.3% with Gafchromic EBT2 when normalized to a 10 × 10 cm(2) field, and 1.0% when compared with UD measurements normalized to the 35 mm diameter cone. Dose profiles taken with the PSD crosshair array agreed with other single detectors dose profiles in spite of the presence of the 49 PSDs. Gamma values comparing 1D dose profiles obtained with PSD crosshair array with Gafchromic EBT2 and UD measured profiles shows 98.3% and 100.0%, respectively, of detector passing the gamma acceptance criteria of 0.3 mm and 2%. The dose measured by the PSD for a complete stereotactic radiosurgery treatment is comparable to the planned dose corrected for its SD-based S(c,p) within 1.4% and 0.7% for 5 and 35 mm diameter cone, respectively. Furthermore, volume averaging of the IC can be observed for the 5 mm aperture where it differs by as much as 9.1% compared to the PSD measurement. The angular dependency of the UD is also observed, unveiled by an under-response around 2.5% of both 5 and 35 mm apertures. CONCLUSIONS: Output Factors and dose profiles measurements performed, respectively, with the PSD and the PSDCA were in agreement with those obtained with the UD and EBT2 films. For stereotactic radiosurgery treatment verification, the PSD gives accurate results compared to the planning system and the IC once the latter is corrected to compensate for the averaging effect of the IC. The PSD provides precise results when used as a single detector or in a dense array, resulting in a great potential for stereotactic radiosurgery QA measurements.

Development of a phantom to evaluate the positioning accuracy of patient immobilization systems using thermoplastic mask and polyurethane cradle.

PURPOSE: The purpose of this study was to develop a new phantom to evaluate the positioning accuracy of patient immobilization systems. METHODS: The phantom was made of papers formed into a human shape, paper clay, and filling rigid polyester. Acrylonitrile butadiene styrene (ABS) pipes were inserted at anterior-posterior (A-P) and right-left (R-L) directions in the phantom to give static load by pulling ropes through the pipes. First, the positioning precision of the phantom utilizing a target locating system (TLS) was evaluated by moving the phantom on a couch along inferior-superior (I-S), A-P, and R-L directions in a range from -5 mm to +5 mm. The phantom's positions detected with the TLS were compared with values measured by a vernier caliper. Second, the phantom movements in a tensile test were chosen from patient movements determined from 15 patients treated for intracranial lesions and immobilized with a thermoplastic mask and polyurethane cradle. The phantom movement was given by minimum or maximum values of patient movements in each direction. Finally, the relationship between phantom movements and the static load in the tensile test was characterized from measurements using the new phantom and the TLS. RESULTS: The differences in all positions between the vernier caliper measurement and the TLS detected values were within 0.2 mm with frequencies of 100%, 95%, and 90% in I-S, A-P, and R-L directions, respectively. The phantom movements according to patient movements in clinical application in I-S, A-P, and R-L directions were within 0.58 mm, 0.94 mm, and 0.93 mm from the mean value plus standard deviation, respectively. The regression lines between the phantom movements and static load were given by y = 0.359x, y = 0.241x, and y = 0.451x in I-S, A-P, and R-L directions, respectively, where x is the phantom movement (mm) and y is the static load (kgf). The relationship between the phantom movements and static load may represent the performance of inhibiting patient movements, so the accuracy of the immobilization system in the intracranial lesion will be estimated in advance by basic tensile test on the new phantom. CONCLUSIONS: The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.

Development and implementation of an EPID-based method for localizing isocenter.

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter to an accuracy of less than 1 mm using the EPID (Electronic Portal Imaging Device). The proposed solution uses a collimator setting of 10 × 10 cm2 to acquire EPID images of a new phantom constructed from LEGO blocks. Images from a number of gantry and collimator angles are analyzed by automated analysis software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180° collimator rotation to determine if the phantom is centered in the dimension being investigated. Repeated tests show that the system is reproducibly independent of the imaging session, and calculated offsets of the phantom from radiation isocenter are a function of phantom setup only. Accuracy of the algorithm's calculated offsets were verified by imaging the LEGO phantom before and after applying the calculated offset. These measurements show that the offsets are predicted with an accuracy of approximately 0.3 mm, which is on the order of the detector's pitch. Comparison with a star-shot analysis yielded agreement of isocenter location within 0.5 mm. Additionally, the phantom and software are completely independent of linac vendor, and this study presents results from two linac manufacturers. A Varian Optical Guidance Platform (OGP) calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment and OGP calibration on a monthly basis.

MAGAT gel and EBT2 film-based dosimetry for evaluating source plugging-based treatment plan in Gamma Knife stereotactic radiosurgery.

This work illustrates a procedure to assess the overall accuracy associated with Gamma Knife treatment planning using plugging. The main role of source plugging or blocking is to create dose falloff in the junction between a target and a critical structure. We report the use of MAGAT gel dosimeter for verification of an experimental treatment plan based on plugging. The polymer gel contained in a head-sized glass container simulated all major aspects of the treatment process of Gamma Knife radiosurgery. The 3D dose distribution recorded in the gel dosimeter was read using a 1.5T MRI scanner. Scanning protocol was: CPMG pulse sequence with 8 equidistant echoes, TR = 7 s, echo step = 14 ms, pixel size = 0.5mm × 0.5mm, and slice thickness of 2 mm. Using a calibration relationship between absorbed dose and spin-spin relaxation rate (R2), we converted R2 images to dose images. Volumetric dose comparison between treatment planning system (TPS) and gel measurement was accomplished using an in-house MATLAB-based program. The isodose overlay of the measured and computed dose distribution on axial planes was in close agreement. Gamma index analysis of 3D data showed more than 94% voxel pass rate for different tolerance criteria of 3%/2 mm, 3%/1 mm and 2%/2 mm. Film dosimetry with GAFCHROMIC EBT 2 film was also performed to compare the results with the calculated TPS dose. Gamma index analysis of film measurement for the same tolerance criteria used for gel measurement evaluation showed more than 95% voxel pass rate. Verification of gamma plan calculated dose on account of shield is not part of acceptance testing of Leksell Gamma Knife (LGK). Through this study we accomplished a volumetric comparison of dose distributions measured with a polymer gel dosimeter and Leksell GammaPlan (LGP) calculations for plans using plugging. We propose gel dosimeter as a quality assurance (QA) tool for verification of plug-based planning.

Six degrees of freedom CBCT-based positioning for intracranial targets treated with frameless stereotactic radiosurgery.

Frameless radiosurgery is an attractive alternative to the framed procedure if it can be performed with comparable precision in a reasonable time frame. Here, we present a positioning approach for frameless radiosurgery based on in-room volumetric imaging coupled with an advanced six-degrees-of-freedom (6 DOF) image registration technique which avoids use of a bite block. Patient motion is restricted with a custom thermoplastic mask. Accurate positioning is achieved by registering a cone-beam CT to the planning CT scan and applying all translational and rotational shifts using a custom couch mount. System accuracy was initially verified on an anthropomorphic phantom. Isocenters of delineated targets in the phantom were computed and aligned by our system with an average accuracy of 0.2 mm, 0.3 mm, and 0.4 mm in the lateral, vertical, and longitudinal directions, respectively. The accuracy in the rotational directions was 0.1°, 0.2°, and 0.1° in the pitch, roll, and yaw, respectively. An additional test was performed using the phantom in which known shifts were introduced. Misalignments up to 10 mm and 3° in all directions/rotations were introduced in our phantom and recovered to an ideal alignment within 0.2 mm, 0.3 mm, and 0.4 mm in the lateral, vertical, and longitudinal directions, respectively, and within 0.3° in any rotational axis. These values are less than couch motion precision. Our first 28 patients with 38 targets treated over 63 fractions are analyzed in the patient positioning phase of the study. Mean error in the shifts predicted by the system were less than 0.5 mm in any translational direction and less than 0.3° in any rotation, as assessed by a confirmation CBCT scan. We conclude that accurate and efficient frameless radiosurgery positioning is achievable without the need for a bite block by using our 6DOF registration method. This system is inexpensive compared to a couch-based 6 DOF system, improves patient comfort compared to systems that utilize a bite block, and is ideal for the treatment of pediatric patients with or without general anesthesia, as well as of patients with dental issues. From this study, it is clear that only adjusting for 4 DOF may, in some cases, lead to significant compromise in PTV coverage. Since performing the additional match with 6 DOF in our registration system only adds a relatively short amount of time to the overall process, we advocate making the precise match in all cases.

Motion monitoring for cranial frameless stereotactic radiosurgery using video-based three-dimensional optical surface imaging.

PURPOSE: To establish a new clinical procedure in frameless stereotactic radiosurgery (SRS) for patient setup verification at treatment couch angles as well as for head-motion monitoring during treatment using video-based optical surface imaging (OSI). METHODS: A video-based three-dimensional (3D) OSI system with three ceiling-mounted camera pods was employed to verify setup at treatment couch angles as well as to monitor head motion during treatment. A noninvasive head immobilization device was utilized, which includes an alpha head mold and a dental mouthpiece with vacuum suction; both were locked to the treatment couch. Cone beam computed tomography (CBCT) was used as the standard for image-guided setup. Orthogonal 2D-kV imaging was applied for setup verification before treatment, between couch rotations, and after treatment at zero couch angle. At various treatment couch angles, OSI setup verification was performed, relative to initial OSI setup verification at zero couch angle after CBCT setup through a coordinate transformation. For motion monitoring, the setup uncertainty was decoupled by taking an on-site surface image as new reference to detect motion-induced misalignment in near real-time (1-2 frames per second). Initial thermal instability baseline of the real-time monitoring was corrected. An anthropomorphous head phantom and a 1D positioning platform were used to assess the OSI accuracy in motion detection in longitudinal and lateral directions. Two hypofractionated (9 Gy x 3 and 6 Gy x 5) frameless stereotactic radiotherapy (SRT) patients as well as two single-fraction (21 and 18 Gy) frameless SRS patients were treated using this frameless procedure. For comparison, 11 conventional frame-based SRS patients were monitored using the OSI to serve as clinical standards. Multiple noncoplanar conformal beams were used for planning both frameless and frame-based SRS with a micromultileaf collimator. RESULTS: The accuracy of the OSI in 1D motion detection was found to be 0.1 mm with uncertainty of +/- 0.1 mm using the head phantom. The OSI registration against simulation computed tomography (CT) external contour was found to be dependent on the CT skin definition with -0.4 mm variation. For frame-based SRS patients, head-motion magnitude was detected to be <1.0 mm (0.3 +/- 0.2 mm) and <1.0 degree (0.2 degrees +/- 0.2 degrees) for 98% of treatment time, with exception of one patient with head rotation <1.5 degrees for 98% of the time. For frameless SRT/SRS patients, similar motion magnitudes were observed with an average of 0.3 +/- 0.2 mm and 0.2 degrees +/- 0.1 degree in ten treatments. For 98% of the time, the motion magnitude was <1.1 mm and 1.0 degree. Complex head-motion patterns within 1.0 mm were observed for frameless SRT/SRS patients. The OSI setup verification at treatment couch angles was found to be within 1.0 mm. CONCLUSIONS: The OSI system is capable of detecting 0.1 +/- 0.1 mm 1D spatial displacement of a phantom in near real time and useful in head-motion monitoring. This new frameless SRS procedure using the mask-less head-fixation system provides immobilization similar to that of conventional frame-based SRS. Head-motion monitoring using near-real-time surface imaging provides adequate accuracy and is necessary for frameless SRS in case of unexpected head motion that exceeds a set tolerance.

Assessment of variation in Elekta plastic spherical-calibration phantom and its impact on the Leksell Gamma Knife calibration.

PURPOSE: Traditionally, the dose-rate calibration (output) of the Leksell Gamma Knife (LGK) unit is performed using a 160 mm diameter plastic spherical phantom provided by the vendor of the LGK, Elekta Instrument AB. The purpose of this study was to evaluate variations in the Elekta spherical phantom and to assess its impact and use for the LGK calibration. METHODS: Altogether, 13 phantoms from six different centers were acquired, 10 of these phantoms were manufactured within the past 10 years and the last 3 approximately 15-20 years ago. To assess variation in phantoms, the diameter and mass densities were measured. To assess the impact on LGK calibration, the output of two models of LGK (LGK Perfexion and LGK 4C) were measured under identical irradiation conditions using all 13 phantoms for each LGK model. RESULTS: The mean measured deviation in diameter from expected nominal 160 mm for 13 phantoms was 0.51 mm (range of 0.09-1.51 mm). The mean measured phantom mass density for 13 phantoms was 1.066 +/- 0.019 g/cm3 (range of 1.046-1.102 g/cm3). The percentage deviation of output for individual phantom from mean of 13 phantom outputs ranged from -0.37% to 0.55% for LGK Perfexion. Similarly, the percentage deviation of output for individual phantom from mean of 13 phantom outputs ranged from -0.72% to 0.47% for LGK 4C. CONCLUSIONS: This study demonstrated that small variations in terms of phantom size and mass density of the phantom material do not have a significant impact on dose-rate measurements of the Leksell Gamma Knife. Also, date of manufacture of the phantom did not show up to be a significant factor in this study.

Estimation of dosimetric discrepancy due to use of Onyx™ embolic system in Stereotactic Radiosurgery/Radiotherapy (SRS/SRT) planning.

More often the embolic materials in the brain create artefacts in the planning CT images that could lead to a dose variation in planned and delivered dose. The aim of the study was to evaluate the dosimetric effect of artefacts generated by the Onyx™ embolization material during Stereotactic Radiosurgery/Radiotherapy (SRS/SRT) planning. An in-house made novel Polymethyl Methacrylate (PMMA) head phantom (specially designed for SRS/SRT plans) was used for this purpose. For the evaluation process, we have created concentric ring structures around the central Onyx materials on both the CT sets (with and without Onyx material). The verification plans were generated using different algorithms namely Analytical Anisotropic Algorithm (AAA), Acuros XB and Monaco based Monte Carlo on both CT sets. Mean integral dose over the region of interest were calculated in both CT sets. The dosimetric results shows, due to the presence of Onyx material, relative variation in mean integral dose to the proximal structure (Ring 1) were -4.02%, -2.98%, and -2.49% for Monte Carlo, Acuros XB, and AAA respectively. Observed variations are attributed to the presence of artefacts due to Onyx material. Artefacts influence the accuracy of dose calculation during the planning. All the calculation algorithms are not equally capable to account such variations. Special cares are to be taken while choosing the calculation algorithms as it impacts the results of treatment outcome.

Unintended attenuation in the Leksell Gamma Knife Perfexion calibration-phantom adaptor and its effect on dose calibration.

The calibration of Leksell Gamma Knife Perfexion (LGK PFX) is performed using a spherical polystyrene phantom 160 mm in diameter, which is provided by the manufacturer. This is the same phantom that has been used with LGK models U, B, C, and 4C. The polystyrene phantom is held in irradiation position by an aluminum adaptor, which has stainless steel side-fixation screws. The phantom adaptor partially attenuates the beams from sectors 3 and 7 by 3.2% and 4.6%, respectively. This unintended attenuation introduces a systematic error in dose calibration. The overall effect of phantom-adaptor attenuation on output calibration of the LGK PFX unit is to underestimate output by about 1.0%.

Quality assurance for Gamma Knife Perfexion using the Exradin W1 plastic scintillation detector and Lucy phantom.

The goal of this work is to validate the use of the Exradin W1 plastic scintillation detector (PSD) to measure profiles and output factors from Gamma Knife Perfexion collimators in a Lucy phantom. The Exradin W1 PSD has a small-volume, near-water-equivalent, energy-independent sensitive element. Output measurements were performed for all 3 collimators (4 mm, 8 mm, and 16 mm) of the Gamma Knife Perfexion system, and these measurements were compared to measurements made with an A16 ion chamber and an EBT3 film and to the nominal values. We showed that a configuration in which the focus or 'shot' moves while the detector remains fixed is essentially equivalent to a configuration in which the focus is fixed while the detector moves. A Lucy phantom containing a PSD was moved in small steps to acquire profiles in all three dimensions. EBT3 film was inserted in the Lucy phantom and exposed to a single shot for each collimator. The relative values for output factors measured with the PSD were 1.000, 0.892, and 0.795, for the 16 mm, 8 mm, and 4 mm collimators, respectively. The values measured with EBT3 film were 1.000, 0.881, and 0.793, and the values measured with the A16 ion chamber were 1.000, 0.883, and 0.727. The nominal output factors for the Gamma Knife Perfexion are 1.000, 0.900, and 0.814, respectively. There was excellent agreement between all profiles measured with the PSD and EBT3 as well as with the treatment planning system data provided by the vendor. In light of our results, the Exradin W1 PSD is well suited for beam quality assurance of a Gamma Knife Perfexion irradiator.

Positioning accuracy in stereotactic radiotherapy using a mask system with added vacuum mouth piece and stereoscopic X-ray positioning.

PURPOSE: For cranial patients receiving stereotactic radiotherapy, we use the Exactrac stereoscopic X-ray system to optimize patient positioning. Patients are immobilized with the BrainLAB Mask System (BrainLAB, Feldkirchen, Germany). We have developed an adapter to this system that accommodates a vacuum mouth piece (VMP). Measurements with the Exactrac system have been performed to study the positioning accuracy after corrections with this system and to evaluate the accuracy of the VMP vs. the standard available upper jaw support (UJS). METHODS AND MATERIALS: Positioning results were collected for 20 patients with the UJS and 20 patients with the VMP, before treatment (1,122 fractions) and after treatment (400 fractions). For all 6 degrees of freedom the average, the random error and systematic error were calculated. RESULTS: The average vector length before and after correction with the Exactrac system was 2.1 +/- 1.2 mm and 0.7 +/- 0.6 mm respectively for UJS and 1.7 +/- 0.7 mm and 0.4 +/- 0.4 mm for VMP. Interfraction positioning for translations was greatly improved after correction with the Exactrac system (p < 0.0005) and is better with VMP than with UJS (p = 0.005). Outliers were greatly reduced. Interfraction rotations were significantly smaller for VMP. Intrafraction errors for vertical and longitudinal translations and for rotations were smaller for the VMP. CONCLUSIONS: Positioning correction using the Exactrac X-ray system greatly improves accuracy. Adding the VMP results in even better patient fixation and smaller rotations, making it a useful addition to the Mask System. Combined, this is a convenient and accurate alternative to invasive fixation methods.

Development of a PMMA phantom as a practical alternative for quality control of gamma knife® dosimetry.

BACKGROUND: To measure the absorbed dose rate to water and penumbra of a Gamma Knife® (GK) using a polymethyl metacrylate (PMMA) phantom. METHODS: A multi-purpose PMMA phantom was developed to measure the absorbed dose rate to water and the dose distribution of a GK. The phantom consists of a hemispherical outer phantom, one exchangeable cylindrical chamber-hosting inner phantom, and two film-hosting inner phantoms. The radius of the phantom was determined considering the electron density of the PMMA such that it corresponds to 8 g/cm2 water depth, which is the reference depth of the absorbed dose measurement of GK. The absorbed dose rate to water was measured with a PTW TN31010 chamber, and the dose distributions were measured with radiochromic films at the calibration center of a patient positioning system of a GK Perfexion. A spherical water-filled phantom with the same water equivalent depth was constructed as a reference phantom. The dose rate to water and dose distributions at the center of a circular field delimited by a 16-mm collimator were measured with the PMMA phantom at six GK Perfexion sites. RESULTS: The radius of the PMMA phantom was determined to be 6.93 cm, corresponding to equivalent water depth of 8 g/cm2. The absorbed dose rate to water was measured with the PMMA phantom, the spherical water-filled phantom and a commercial solid water phantom. The measured dose rate with the PMMA phantom was 1.2% and 1.8% higher than those measured with the spherical water-filled phantom and the solid water phantom, respectively. These differences can be explained by the scattered photon contribution of PMMA off incoming 60Co gamma-rays to the dose rate. The average full width half maximum and penumbra values measured with the PMMA phantom showed reasonable agreement with two calculated values, one at the center of the PMMA phantom (LGP6.93) and other at the center of a water sphere with a radius of 8 cm (LGP8.0) given by Leksell Gamma Plan using the TMR10 algorithm. CONCLUSIONS: A PMMA phantom constructed in this study to measure the absorbed dose rates to water and dose distributions of a GK represents an acceptable and practical alternative for GK dosimetry considering its cost-effectiveness and ease of handling.

Technical Note: Evaluation of plastic scintillator detector for small field stereotactic patient-specific quality assurance.

PURPOSE: To evaluate the performance of a commercial plastic scintillator detector (PSD) for small-field stereotactic patient-specific quality assurance (QA) measurements using flattening-filter-free beam. METHODS: A total of 10 spherical targets [volume range: (0.03 cc-2 cc)] were planned with two techniques: (a) dynamic conformal arc (DCA-10 plans) and (b) volumetric modulated arc therapy (VMAT-10 plans). All plans were generated using Varian Eclipse treatment planning system, and AcurosXB v.13 algorithm in 1.0 mm grid size. Additionally, 14 previously treated cranial and spine SRS plans were evaluated [6 DCA, 8 VMAT, volume range: (0.04 cc-119.02 cc)]. Plan modulation was quantified via two metrics: MU per prescription dose (MU/Rx) and Average Leaf Pair Opening (ALPO). QA was performed on the Varian Edge linear accelerator equipped with HDMLC. Three detectors were used: (a) PinPoint ion chamber (PTW; active volume 0.015 cc), (b) Exradin W1 PSD (Standard Imaging; active volume 0.002 cc), and (c) Gafchromic EBT3 film (Ashland). PinPoint chamber and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging); films were placed horizontally capturing the coronal plane. RESULTS: PSD, film, and PinPoint chamber measured average differences of 1.00 ± 1.54%, 1.30 ± 1.69%, and -0.66 ± 2.36%, respectively, compared to AcurosXB dose calculation. As the target volume decreased, PinPoint chamber measured lower doses (maximum -5.07% at 0.07 cc target), while PSD and film measured higher doses (2.87% and 2.54% at 0.03 cc target) than AcurosXB. Film agreed with the benchmark detector PSD by an average difference of 0.31 ± 1.20%, but suffered from larger uncertainty; PinPoint chamber underestimated dose by more than 4% for targets smaller than 0.2 cc. Taking PSD as the measurement standard, DCA plans achieved good QA results across all volumes studied, with an average of -0.07 ± 0.89%; for VMAT plans, PSD measured consistently higher dose (1.95 ± 1.36%) than AcurosXB. Correlation study revealed that plan modulation quantified by both MU/Rx and ALPO correlated significantly with QA results. CONCLUSION: Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2 cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for nonmodulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation.