Assessing Dosimetric Accuracy with Lucy 3D QA Phantom Victoria Honetschlager, Department of Physics, Minnesota State University Moorhead, Moorhead, MN 56563 ABSTRACT Stereotactic radiosurgery (SRS) is a radiation therapy method which treats a very small and well defined volume with a high dose of radiation. New technologies such as the Novalis Tx allow this treatment on a wide-scale clinical level. In 2011, Roger Maris Cancer Center acquired the Novalis Tx and the clinic is now able to treat far more patients than before. With this new linear accelerator comes a need to quantify the accuracy of it during a SRS treatment. This study utilizes a Lucite medical imaging phantom, LUCY 3D QA, and iplan software to image, plan, and treat a pseudo patient and quantify machine accuracy in absorbed dose delivered (dosimetric accuracy). INTRODUCTION More than 180,000 brain tumors are diagnosed each year [U.S News]. While this number sounds staggering, it is only about 2-3% of diagnosed adult cancer cases in the United States. Of these diagnoses, the large majority are metastatic 1. In fact, nearly one in four patients diagnosed with cancer will develop a metastatic brain tumor. Current clinical treatment techniques include: Surgery to remove or, in some cases, debulk 2 the tumor Chemotherapy, though it is largely inefficient at providing a comprehensive treatment for cranial cancers Radiation therapy, including whole brain radiation and stereotactic radiosurgery Many of these cases include a combination of treatments and the patient s interventional care is specifically tailored by a team of health care professionals such as radiation oncologists, 1 Secondary tumors spread from an initial cancer such as lung cancer or breast cancer 2 To remove a large portion of the tumor s mass to reduce its sixe medical physicists, dosimetrists, and radiation therapists. Radiation therapy is a broad category designation for a treatment that uses radiation to damage the DNA of cells in order to halt the reproduction of invasive cells. The tumor will shrink over time, often taking six months to several years to fully clear. Unlike surgery, radiation therapy does not physically remove the tumor. Within radiation therapy are many specific treatments, summarized in Appendix A. Unfortunately, radiation can also damage healthy cells surrounding the tumor, which can lead to side effects. Thus, much research has been dedicated to treatment methods that minimize damage to surrounding structures. WHY STEREOTACTIC RADIOSURGERY? Unlike whole brain radiation therapy- in which a large volume of the brain is given radiation to treat both the visible lumps of a tumor and a preemptive dose to developing tumors not seen on an MRI scan- stereotactic radiosurgery (SRS) utilizes a high dose of radiation to a very narrow target. SRS differs from more conventional radiation therapy in several aspects: Single large dose of radiation is used rather than multiple daily fractions [Loeffler et al.] Sharp dose gradients are produced so radiation outside the target volume in minimized [Suh] SRS allows for accuracy of treatment within a few millimeters of the target [Loeffler et al.] With this in mind, SRS can be very useful in cases where the tumor is hard to reach, such as intracranial growths. For tumors outside the brain, SBRT 3 can be of huge benefit for tumors located close to vital organs, or those subject to movement within the body. Table 1 summarizes 3 Clinically, SRS is used when discussing cranial lesions; the same technique used on locations other than the head is referred to as SBRT, or stereotactic body radiotherapy.
characteristics that make a patient ideal for SRS or SBRT treatment. 1. Well defined borders on either MRI and/or CT 2. Spherical in shape (most metastic cancer satisfies this) 3. Small (usually under 4 cm in diameter) 4. Located near critical structures such as spinal cord 5. Surgery not an option due to tumor location or patient health Table 1: Consideration factors for SRS treatment Since no surgery is actually performed, risks such as infection, hemorrhaging and spinal fluid leakage are greatly reduced. While patients may be sedated during treatment, general anesthesia is not required 4, meaning they are able to communicate throughout the entirety of the procedure. Since SRS is non-invasive, there is no scarring, unlike open skull surgery. Unfortunately, stereotactic radiosurgery treatment is that it is far costlier that whole-brain radiation therapy. Estimated Medicare costs for SRS are $10,000 to $27,000 per procedure, compared to $2,300 to $7,650 for whole brain radiation therapy. Most insurance plans now cover SRS treatment in cases where patients satisfy several of the criteria described in Table 1. MATERIALS AND METHODS This study takes a quality assurance phantom through the basic treatment process that patients follow, with some modifications. Here, however, rather than seeing through to a final radiation treatment, the process is halted at a final quality assurance check as there is no human patient to treat. Data collected while treating the phantom was analyzed to assess the accuracy of the plan and machine in delivering therapy. It is beneficial to familiarize one with the materials used, such as the linear accelerator, phantom and ion chamber, as well as gain an understanding of the treatment and quality assurance process before discussing this study s examination of the cranial SRS phantom. 4 This discussion is limited to adult patients. Often in pediatric cases, children and toddlers must be under general anesthesia, and require more intensive treatment and care. NOVALIS TX In 2007, Varian Medical Systems partnered with BrainLAB AG to launch the Novalis Tx. This clinical linear accelerator has the ability to be used as a stereotactic radiosurgery platform, allowing local cancer centers the ability to more cost effectively provide SRS treatment options to patients [Chang 2007]. All linear accelerators, including the Novalis Tx, accelerate electrons and then allow then to collide with a target made of heavy metal. From these collisions, high energy x-rays are produced. The x-rays are shaped with wedges 5, cones 6, or multileaf collimators 7 (MLC). The patient lies on a couch, while the gantry, which houses the beam, rotates around the patient. By rotating both the gantry and the couch, the ability to treat at any angle is maintained. With all linear accelerators operating on the same basis, one may be tempted to ask what sets the Novalis Tx apart or makes it worth the $4 million dollar price tag. New technologies such as dual energy modes allow deeper penetration, beneficial for SBRT applications. 120 leaves reside in the collimator, allowing for better beam shaping. Imaging technologies such as ExacTrac and OnBoard Imager (OBI) allow for precise patient alignment and respiratory gating 8, and the maximum dose rate is 1,000 monitor units per second- higher than many older and stand-alone linacs (Agazaryan & Schultz, 2011). Figure 1 shows an illustrated view of the image guided techniques featured on the Novalis Tx. For more specific feature information, please see Appendix B. 5 Collimator insert used to tailor the beam attenuation 6 Fitting over the end of the collimator that narrows the beam or field size; when conformal arcs are used with a cone beam, a circular or ovular shape is created 7 Small metal pieces that move one-dimensionally to create shapes in the head of the collimator. This allows a dosimetrist to tailor the shape of the radiation beam to the tumor shape. 8 Used when treating tumors susceptible to respiratory induced breathing, such as lung cancer; measures patient breathing to treat at optimal time intervals
Figure 1(right): ExacTrac x-ray configuration. X-ray tubes are mounted in the floor and flat panel imagers are mounted to the ceiling In summer of 2011, installation of the Novalis Tx began at Roger Maris Cancer Center in Fargo, North Dakota. The machine was commissioned in late fall, and patient treatment began in winter 2012. As such, very few SRS treatments have been performed at this location, and questions such as the machine accuracy were important to ascertain. This is the motivation on which this study relies. QUALITY ASSURANCE Due to both the high radiation dose and dose rate, quality assurance is of utmost importance for medical professionals when dealing with SRS treatment. The large majority of the quality assurance falls under the duties of the medical physicist. The American Association of Physicists in Medicine (AAPM) has published a series of guidelines derived from task groups. These standards are widely accepted in the field and generally dictate quality assurance protocol on the clinical level. Every morning, radiation therapists follow a machine warm-up protocol which ensures that the machine is working as expected. Daily, monthly and annual checks are performed by the medical physicists. These checks assure the quality of the linear accelerator itself, rather than the patient plan. Table 2 shows in more detail the guidelines AAPM has established in task group 142[Klein et al.]. Patient specific quality assurance is integrated within the treatment process. Figure 2 shows a visual representation of the treatment process based off a similar proposed model by Dr. Fang Yin at the Duke University Medical Center. Daily Checks Audiovisual monitoring Door interlock (door should not open when machine beam is on) Check that OBI and ExacTrac work properly Weekly Checks Quantitative checks on imaging systems Monthly Checks Energy check for photons Respiratory gating system check Annual Checks Subset of tests executed during machine commissioning Table 2: Summary of TG-142 quality assurance policies for medical accelerators This visual representation highlights the importance of quality assurance within the entirety of treatment. While planning software such as iplan do give dose rates expected within the treatment volume, it is important that medical physicists take a quantitative measurement using a phantom- or acrylic form similar to the density of tissue that is used to assess and analyze the delivery of treatment plans- for a cross-reference. In doing so, irregularities with the machine or patient alignment may be detected. This study relies on a slightly modified patient model, as seen in Figure 3. This assumes that a patient has come into the hospital and received a diagnostic MRI scan and an oncologist has already prescribed a stereotactic radiosurgery treatment. From there, the phantom is imaged, planned for and treated. One benefit to representing this process visually is that it becomes easy to recognize that an error in an early-stage of planning or imaging may be piggy-backed or carried along until treatment. For this reason the phantom is used from the earliest stages throughout the entirety of the treatment process.
Case Selection Initial diagnostic imaging (MRI) Patient consultation with oncologist SRS Treatment determination 3-D Simulation CT scan for treatment planning Scans sent to dosimetry along with oncologist prescription Assessment Patient returns for imaging to determine success of treatment Consult with oncologist to determine if further treatment is necessary Retreatment Treatment Radiation therapists deliver treatment to patient Correction needed to position? NO Quality Assurance Check #3 Image patient using OBI and ExacTrac Treatment Planning Dosimetrist develops treatment plan based on oncologists specifications Brain LAB iplan software NO YES Patient Set-Up Immobilize patient on table, align them with a fusion of CT and MRI images Quality Assurance Check Medical physicist checks dosimetrists plan, submits it to oncologist for approval Planning software calculates monitor units (MU) to be given Approved? YES Quality Assurance Check #2 Exact patient plan is executed on a Lucite phantom for quantitative analysis Figure 2: Treatment process from diagnostic imaging to success assessment. The process highlights three key aspects of treatment execution: planning (pink), quality assurance (blue), and treatment delivery (green).
Case Selection Assumed Initial diagnostic imaging (MRI) assumed to have occurred Patient consultation with oncologist SRS Treatment determination 3-D Simulation on Phantom CT scan phantom for treatment planning Scans sent to dosimetry, a trivial dose of radiation is assigned to later compare quantitatively Treatment Planning for Phantom Medial physicist develops treatment plan based on oncologists specifications Brain LAB iplan software Planning software generates TaPos to be used to align phantom NO Assessment Ion chamber sends information to electrometer Readings are recorded and analyzed Total dose measured at isocenter by phantom is compared to expected dose at isocenter from planning software YES Treatment Medical physicist delivers treatment to phantom Correction needed to position? NO Quality Assurance Check #3 Image phantom using OBI and ExacTrac Phantom Set-Up Immobilize phantom with invasive head ring on table, align them with TaPos and CT Image Quality Assurance Check Medical physicist checks plan Planning software calculates monitor units (MU) to be given Approved? YES Quality Assurance Check #2 Exact patient plan is executed on a Lucite phantom for quantitative analysis Figure 3: Treatment process from diagnostic imaging to data analysis for this study. This process is similar to the one highlighted in Figure 2.
LUCY 3D QA PHANTOM This study utilizes the LUCY 3D QA Phantom. The phantom is produced by Standard Imaging, Inc. for specific use in perform[ing] comprehensive quality assurance tests for an entire stereotactic radiosurgery procedure (Standard Imaging, Inc.) and costs approximately $33,000 when purchased with all accessory packages. Lucy is a highly precise phantom with tolerances of 0.1 mm (Standard Imaging). The phantom can be mounted onto commercial SRS frames, such as those produced by Varian Medical Systems and BrainLAB AG for use with the Novalis Tx linac, as shown in Figure 4. This unique ability to interface without respect to branding allows hospitals and clinics to image and treat the phantom in the exact manner and position as the patient on the table, assuring a valid and accurate quality assurance protocol. This dose verification study uses the Exradin A16 Microchamber Ion Chamber. Figure 5 also shows the A16 Ion Chamber inserted into the Lucy Phantom. Different inserts must be used with different ion chambers, as each is milled slightly different. The A16 functions in the same manner as all ion chambers. It consists of a gas filled enclosure between two conducting electrodes. When the gas between the electrodes is ionized by photons generated by the linear accelerator, a current is created and measured by an electrometer. The charge is reported in nanocouloumbs and a correction factor is applied to convert the charge into an absorbed dose. EXRADIN A-16 ION CHAMBER Drilled into the Lucy phantom is a dosimetry insert compatible with several different ion chambers. The insert is used to verify a prescribed dose at the isocenter, or volumetric center. The insert is milled so that the center of the Lucy phantom. Figure 5 shows this ion chamber quite clearly on the right side of the photograph. Figure 4(left): Spherical Lucy phantom fitted into an invasive BrainLAB frame for SRS treatment and fixed onto Novalis Tx treatment couch. One can clearly see the ion chamber insert on the right side of the Lucy phantom. Figure 5 (above): Exradin A16 Mirochamber Ion Chamber fitted into Lucy phantom. Readings taken by the ion chamber appear on an electrometer and allow the medical physicist to calculate a total dosage received by the phantom
SRS SPECIFIC QUALITY ASSURANCE Having discussed the overall quality assurance process and method for assessing accuracy in SRS plans, this section describes in greater detail the exact methods used to quantify this end-to-end test. CT SCANS FOR PLANNING CT Scans for treatment planning were performed on a Philips scanner at Roger Maris Cancer Center in Fargo, North Dakota. Two scans were taken with the Lucy phantom and A16 ion chamber in a head frame localizer. The localizer box contains metal rods that appear during imaging. These markings allow alignment of the phantom during treatment. Figure 6 shows a series of CT images from several orientations. After scans are taken, they are imported into the treatment planning software, in this case, BrainLAB AG s iplan software. TREATMENT PLANNING With the images imported into iplan, a treatment plan was developed. The plan uses conformal circular arcs, a method whereby the dose is delivered by several arcs that intersect around the isocenter. Each arc is called a field when assigned specific amount of radiation to deliver. Together they create a spherical dose distribution which is very effective for most cranial (especially metastatic) tumors. To do this, circular cones ranging in diameter for 0.5 mm to 30 mm are fit into the linear accelerator collimator. Cones give the best dose uniformity for circular lesions, but irregular shapes can be treated by overlaying several spherical dose distributions. In an effort to simplify the procedure, it was assumed that the phantom had a single, spherical lesion located at the isocenter. For visual comparison, Figure 7 shows the collimator head with and without a cone attachment. Figure 6: Series of CT images of Lucy phantom with A16 ion chamber. Illustrates axial, sagittal and coronal images. The lower right corner shows the orientation of the phantom during scan in a clear, easy to comprehend manner.
Figure 7: Collimator without any cone attachment (left). 12.5 mm cone attachment (right). Specifically, this plan uses a 12.5mm cone to deliver treatment. As typical with most plans delivered at Roger Maris Cancer Center, the linear accelerator energy was planned at 6 MV at set for a dose rate of 1,000 monitor units/minute. This six-field 9 plan has an assigned, or prescribed, dose of 28.00 Gray 10 [Gy];after planning, however, the dose expected to be detected at the isocenter is 28.21 Gy. This difference is 0.75% which is acceptable and within tolerances, as defined by AAPM. Because the gantry travels dynamically through a starting and ending angle, collisions between the treatment couch and linac may occur. In order to reduce the likelihood of such 9 Common dosimetry planning uses either six or nine fields. 10 It may be a bit confusing that monitor units and Gray both seem to be getting tossed around. Unfortunately, the linac is not equipped to handle delivering dose in Gray and so the machine uses its own unit, a monitor unit, which is measure of the energy expended, while the medical physicist finds their calculations far easier when using units of Gray. Linacs are calibrated to deliver 1 cgy/mu for a 10cm x 10 cm field at 10 cm depth. an event and treat as closely to the phantom as possible, the plan features couch kicks, which is simply a vertical axis rotation of the treatment couch. By rotating the couch, a more even dose distribution can be given and hot spots that could occur from beam divergence are minimized. Figure 8 shows delivery parameters generated by iplan, including gantry angle rotation arcs for each field, treatment couch angles, and monitor units to be delivered by the linac for each field. The delivery parameters are in effect a plan summary showing the major details of the final plan. For more detailed plan parameters, please see Appendix C. iplan also created a Target Positioner Overlay (TaPo) which affixes to the head localizer box and allows for positioning based off the guided in-room lasers.
Figure 8: Delivery Parameters for SRS quality assurance plan with Lucy phantom. 6-field, 6 MV plan with 12.5 mm cone beam conformal circular arcs. HEAD FRAME LOCALIZATION With a finalized treatment plan in place, the next part of the process was to localize and position the phantom. During this phase, it is critical that the phantom isocenter matches as closely as possible with the machine isocenter so that treatment is spatially accurate. In order to do so, the Lucy phantom was placed in the invasive head ring, and mounted to the treatment couch with a specialized mount from Varian Medical Systems, Inc. and BrainLAB. Figure 9 (at left) shows the phantom mounted to the table. Knobs on the mount help correct the alignment of the phantom with the in-room lasers. In order to know what to align the phantom to, Lucy phantom is placed in the same head localizer that was used during the CT scan. The TaPo printouts are then attached to the Figure 9: Varian Medical Systems, Inc. and BrainLAB couch attachment for Lucy phantom in invasive head ring. Knobs that adjust the alignment (pitch, roll, and yaw) are boxed in yellow.
localizer frame. Figure 10 shows positioning of the Lucy phantom with the head localizer frame and TaPo overlay. Once proper positioning is achieved with the TaPo, the localizer frame can be removed and the ExacTrac system generates a kv x-ray image. This image is overlaid from the initial CT scan; it then enables the user to drag the two images together to align them as closely as possible. Once alignment is completed, then a suggested shift to table parameters is given based on the amount of realignment needed is made. Figure 11 shows the image fusion of the original CT image (red) and the OBI (blue). The images then get overlapped as shown in the set of images on the right and a suggested couch shift is given by the computer based on the amount of image adjustment. Suggested couch shifts based on x ray localization were 1mm. Figure 10: Positioning Lucy phantom for treatment using localizer frame (left) and TaPo overlay (right). In room lasers are used to align the target coordinate shown above. In order to this, alignment knobs may be adjusted as well as the couch position.
Figure 11: On-Board Imaging portal allows medical physicists the ability to image phantom on the couch and shift the table so current placement aligns with pre-treatment CT scans. Top image shows an initial in-room x-ray taken prior to any couch movement. From this, the two images were matched up to align and the couch was moved. Bottom image shows in-room x-ray post couch movement.
TREATMENT AND DATA COLLECTION With final positioning made and a properly positioned phantom, the machined delivered treatment through the treatment plan previously explained. Electrometer readings were recorded and later analyzed to compare the expected absorbed dose with the experimentally measured absorbed dose. RESULTS AND DISCUSSION ELECTROMETER READINGS Table 3 shows all six of the electrometer readings after the completion of each field s treatment. The chamber was calibrated by comparison with a PTW Farmer brand chamber model, a well regarded model in the field. The electrometer used was a Standard Imaging, Inc. SI Max 4000 model. Both the chamber and the electrometer are sent for calibration to an Accredited Dosimetry Calibration Laboratory (ADCL) approved by the American Association of Physicists in Medicine (AAPM). These labs run the equipment through a series of tests and then provide the clinic with a set of calibration factors to be used when taking measurements. Field Cumulative reading [pc] 1 955.2 ± 5 2 1918.6± 5 3 2895.9 ± 5 4 3859.8 ± 5 5 4827.0 ± 5 6 5779.9 ± 5 Table 3: Electrometer readings for each field. Final electrometer reading for the entirety of treatment was 5779.9 pc. The raw electrometer readings do not provide a helpful reference for analysis. Thus, the charge readings must be converted into an absorbed dose 11 reading. Theoretically, the absorbed dose can be calculated by: (Eqn. 1) where D is the absorbed dose to water, M is the fully corrected chamber reading and N is the calibration coefficient. The calibration coefficient can be put in terms of clinically measured variables: (Eqn. 2) where: Ctp: temperature and pressure correction factor CF: overall correction factor based on ion chamber and electrometer correction factors measured at an ACDL ROF: relative output factor, which is measured on each linac for a particular field size; in this case pre-determined as 0.947 The following quantities may be found from easily measured and recorded values: (Eqn. 3) (Eqn. 4) where CF chamber is the calibration factor of the chamber as determined by an AAPM ACDL and CF elect is the calibration factor of the electrometer, also determined by an AAPM ACDL. Table 4 shows these calibration factors. Chamber Electrometer A16 Calibration Factors 4.372 10 11 cgy/c 9.990 10-10 C/Rdg CF (CF chamber CF elect ) 436.76 cgy/rdg Table 4: Calibration factors for PTW Farmer chamber and SI Max 4000 electrometer. Calibrated at an ACDL. Then the final absorbed dose can be found as: Q tot : final electrometer charge reading in nc (Eqn.5) 11 Absorbed dose is measured in Gray [Gy], which is the commonly used radiation measurement for medical physicists. iplan gives expected dose in Gy.
Figure 12: Microsoft Excel document used for data analysis. The data was then placed in a tabulated spreadsheet. Figure 12 shows a portion of the Microsoft Excel document which was used. Appendix D contains the entire spreadsheet. The expected dose at isocenter was 27.99 Gy, or 2799 cgy. The absorbed dose as measured by the electrometer was 2753.5 cgy at isocenter, or 1.6 ± 0.1% difference. This falls within tolerances as defined by TG-142 and the AAPM 12. Overall, these results show excellent agreement between iplan dose calculation and measurement with the Lucy phantom. This demonstrates that an end-to-end test is appropriate in determining an end-to-end measurement of dosimetric accuracy for stereotactic radiosurgery processes. Further study of the Lucy 3D QA Phantom could include utilizing radiographic film during treatment, which is beneficial for small conformal fields such as those used in an SRS procedure. Film dosimetry allows spatial accuracy to be assessed [Robar 1999], which is to be pursued at a later date. 12 AAPM TG-40 published in 1994 recommended that dose delivered to patient be within ±5% of the prescribed dose. This is still the current recommendation, although many clinics such as Sanford run off smaller tolerances. Currently, Sanford Health aims for ±2% in most of their radiotherapy treatments
Appendix A: Breakdown of radiation therapy treatments Radiation Therapy Goal: irradiate and damage DNA of tumor cells to inhibit reproduction Internal (Brachytherapy) Radiation source placed inside or next to area requiring treatment External Beam Radiation delivered externally to the tumor location Permanent (Low dose rate) Radioactive source permanently implanted inside patient Temporary (Low or high dose rate) Radioactive source placed next to area; different sources allow for high or low dose rates Proton Therapy Cyclotron produces high energy protons, which do little harm to non-tumor tissue; only 9 centers in the U.S. IMRT Intensity Modulated Radiation Therapy (IMRT) uses a linear accelerator to vary the angle, shape and intensity of the radiation beams SRS Stereotactic Radiosurgery (SRS) uses precise, intense radiation dose to a target area; can treat when surgery is not possible. SRS traditionally refers t the head and sometimes neck SBRT Stereotactic Body Radiosurgery (SBRT) uses the same techniques as SRS, but on locations other than the head and neck. IGRT Image Guided Radiation Therapy (IGRT) uses medical imaging techniques while delivering treatment for accurate spatial placement
Appendix B: Novalis Tx Comprehensive Image Guided Radiosurgery (IGRS) ExacTrac X-Ray 6D system uses x-ray tubes and ceiling-mounted panels On-Board Imager (OBI) for soft-tissue targeting iplan Net BrainLAB AG s powerful planning software 13 Frameless SRS No longer must attach a frame to patient s skull Rapid Arc Fast and precise MLC controls mean dose delivery can occur up to 1,000 MU/second, shortening treatment times greatly Adaptive Gating Treat targets subject to respiration related movement, such as lung cancer patients 6-D Robotic Couch Easy patient positioning with both translational and rotational set-up 2.5-mmHD 120 MLC New micro-mlc leaves mean tighter beam shaping Dual Energy With both 6 and 20 MV energy, the ability to penetrate deeper is given Chart information is based off of product suite information provided by Varian. For more information about the Novalis Tx, please visit the Novalis Tx homepage at: http://www.varian.com/us/oncology/radiosurgery/novalis_tx.html 13 For product specific information, please visit: http://www.brainlab.com/art/2846/4/rt-treatment-planning-software/
Appendix C: iplan Treatment Planning Parameters
Appendix D: Electrometer Reading Analysis
Works Cited "Brain Tumors." U.S. News. 10 09 2009: n. page. Web. 16 Feb. 2012. <http://health.usnews.com/health-conditions/cancer/brain-tumor>. Chang, B.K., Timmerman, R.D.. "Stereotactic body radiation therapy: A comprehensive review." ONS Connect. 6. (2007): 637-644. Print. Klein, Eric, Joseph Hanley, et al. "Task Group 142 report: Quality assurance of medical accelerators." Medical Physics. 36.9 (2009): 4197-4212. Print. Loeffler, Jay S., Eugene Rossitch, Robert Siddon, et al. "Role of Sterotactic Radiosurgery With a Linear Accelerator in Treatment of Intracranial Arteriovenous Malformations and Tumors in Children." Journal of Pediatrics. 85.5 (1990): 774-782. Print. Robar, J.L., Clark, B.G.. "The use f radiographic film for linear accelerator stereotactic radiosurgical dosimetry." Medical Physics. 26.10 (1999): 2144-2150. Print. Suh, John. "Stereotactic Radiosurgery for the Management of Brain Metases." New England Journal of Medicine. 362.12 (2010): 1119-1127. Print. Yin, Fang. "Treatment quality assurance for linac based SRS/SBRT." SEAPPM Annual Meeting. Myrtle Beach. 2011. Keynote.