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Stereotactic Radiosurgery (SRS) 王怡振(Yizhen Wang), Medical Physicist Mississauga Halton/Central West Regional Cancer Program Trillium Health Partners Definition of Stereotactic Radiosurgery (SRS) “A single high dose of radiation, stereotactically directed to an intra-cranial region of interest. May be from X-ray, gamma ray, protons or heavy particles.” -Lars Leksell, 1951 High dose of “ablative” radiation delivered to a target localized in 3-dimensions with overall end to end precision on the order of 1-2 mm delivered over 1-5 treatments - Present The Evolution of SRS Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy The invention of Stereotactic technique Stereotaxis: Method of locating points in brain using an external 3D frame. -Concept originated with Robert Clarke (engineer, physiologist and surgeon) in 1895. -First device built by Clarke and Victor Horsley (neurosurgeon) in 1905. -First experiment in 1906. Fig a. original frame by Clarke and Horsley Fig c. Leksell frame Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy The invention of stereotactic radiosurgery (SRS) • 1947, Ernest Speigel and Henry Wycis performed first successful human cranial stereotactic surgery. (Temple university, Philladelphia) • 1949, Lars Leksell built his first stereotactic frame. • 1951, Leksell would use his frame to target narrow radiation beams. • 1955, Leksell and Bjore Larsson treated SRS patients using 280 KV X-ray. (Uppsala, Sweden) • Late 1950s and 1960s, Uppsala group, Berkeley group (CA) and Cambridge group (MA) used proton facilities for physics research to treat SRS patients. • 1967, Leksell, Larsson, Lidén and Walstam built “Gamma Knife I”. • 1980, Leksell and Jernberg developed CT compatible frame Gamma Knife Development Gamma Knife I: U Gamma knife: - 179 60Co sources -rectangle collimator - 201 60 Co sources - Cone collimator Model B: Model 4C: - Simpler source change -Allow 3D planning - More efficient helmet change Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy Gamma Knife Development Leksell Gamma Knife Perfexion • 8 independent sectors. 24 60Co sources/each sector • 4, 8, and 16 mm collimators can be combined within each sector. Automatic collimator adjustment. Linac Based SRS Techniques- Early Development 1982, Derechinsky and Betti. Buenos Aris, Argentina Linac Based SRS Techniques- Early Development Linac Based SRS Techniques- Early Development 1986, McGill SRS system. Montreal, QC Modern Dedicated SRS Machines - Cyberknife 110 beam angles Modern Dedicated SRS Machines - Novalis Modern Dedicated SRS Machines - Varian EDGE Flattening Filter Free (FFF) mode up to 2400 MU/min HD-MLC PerfectpitchTM couch VisionRT for patient setup and real time tracking Comparison of Various Techniques • Gamma Knife: - high accuracy (submillimeter) - for cranial disease only - cone collimation better for circular target - source change & radiation safety concern • Cyberknife: - high accuracy (submillimeter) - SRS/SBRT - cone collimation (MLC under development) - long treatment time • Linac: - Deliver all types of radiation treatment - capable of conforming to all target shapes (cone & MLC) - less cost - accuracy at a few millimeters. Clinical Aspects of SRS What SRS Treats • Malignant tumor: Brain metastases, Glioma, Glioblastoma (GBM), Astrocytoma, Oligodendroglioma • Benign tumor: meningioma, schwannoma, pituitary adenoma, acoustic neuroma, • Functional disorder or other benign conditions: trigeminal neuralgia, arteriovenous malformations (AVM), Epilepsy, Parkinson’s Disease Brain Metastases • Most commonly grey/white junction • 80% in cerebrum, 15% in cerebellum, 5% in brain stem • Multi sites more often. The Sources (Primary Cancers) of Brain Metastases Lymphoma GI cancers Neuroblastoma Head and neck cancer Melanoma Osteosarcoma GU cancers Breast cancer Lung cancer 0% 10% 20% 30% Chart created based on Memorial Sloan-Kettering study 40% 50% 60% Clinical Indication for Brain Mets • RTOG 90-05, as an example: - ≤ 2.0 cm, 24 Gy - 2.1-3.0 cm, 18 Gy - 3.1- 4.0 cm, 15 Gy • Less dose -> higher local recurrence Higher dose -> more toxicities (nausea, vomiting, dizziness, seizure, headaches) Outcome of SRS Studies from RTOG (95-08), EORTC (22952-26001) and MD Anderson reveal: • Radiation can reduce the risk of recurrence • SRS produce similar results as surgery+WBRT in terms of local control • WBRT provide better control on distant intracranial recurrence compared to SRS • SRS has much better neurocognitive outcome compared to WBRT • SRS/WBRT has little impact on total survival rate Technique Requirements for SRS 1. Reliable patient immobilization Headrings Relocatable frame based masks 2. Accurate target localization CT localizer: Z = ? 2. Accurate target localization CT: Good Geometry integrity, e- density information, but limited soft tissue contrast. Good for treatment planning MRI: Geometry distortion, no e- density information, but excellent soft tissue contrast. Good for target delineation 2. Accurate target localization Target positioner 3. Dose conformal to target. 3. Dose conformal to target. Micro multileaf collimators (Micro MLC): -Attached to or built-in a Linac - While projecting at isocenter, 14 leaf pairs of 3 mm width, 6 leaf pairs of 4.5 mm width, and 6 leaf pairs of 5.5 mm width. -Can conform to any shape 3. Dose conformal to target. Conical cone collimator: -Better for spherical or oval targets -Sharp penumbra Safety events involving radiosurgery Courtesy of Dr. Kelly Younge • SRS involves extremely small fields and very high doses • Very little opportunity to correct mistakes • Some incidents have been highly publicized: • Incorrect calibration of accelerator output – 77 patients in Florida, 145 patients in Toulouse, France, 152 patients in Springfield, MO – 25-100% overdoses in these patients • Lesson: use the right detector, and use more than one • Backup jaws set incorrectly – A Pinpoint Beam Strays Invisibly, Harming Instead of Healing, W. Bogdanich and K. Rebelo, NYT, Dec. 28, 2010 • These patients received an SRS dose to a significant area of normal brain • Multiple people were treated this way before the mistake was realized Commissioning of SRS • Dosimetry (small field dosimetry very challenging) • Mechanicals • Imaging • IGRT • Margin design • Secondary dosimetry check system • End-2-end tests • External audit Commissioning: Dosimetry PDDs, profiles, output factors Commissioning: Dosimetry • Multiple detectors to ensure accuracy: Micro ionization chamber Film Stereotactic Diode or Electron Diode Edge Diode Diamond dosimeter Liquid ion chamber Commissioning: Dosimetry Detector selection: •Need to select dosimeters with high resolution •Use at least two types of detectors to verify with each other •Ion chambers: volume ≤ ~0.01 cc (cc01, A16) •Diodes: unshielded diodes (SRS diodes, electron diodes) •Radiochromic film: Handling process dependent, better for verification only. Commissioning: Dosimetry PDD measurement: •Follow TPS’ requirement (e.g. depth > 30cm, F.S. covers clinical need), verify 10x10 PDD •Can use micro IC at vertical orientation (chamber parallel to beam) •Diodes Commissioning: Dosimetry Profile measurement: • Air-filled ion chamber too large • Use diodes • Verify with Film Commissioning: Dosimetry Commissioning: Dosimetry IAEA/AAPM Working Group’s Recommendation (and upcoming TG-155*): Reference dosimetry: Corrects for differences between the conditions of field size, geometry, phantom material, and beam quality Output factor: Accounts for detector response difference Alfonso, et al. "A new formalism for reference dosimetry of small and non-standard fields,"Med Phys 35, 51795186 (2008) *TG-155: Small fields and non-equilibrium condition photon beam dosimetry Commissioning: Dosimetry Commissioning: Dosimetry Commissioning: Dosimetry Absolute dose machine calibration: • Use standard ion chamber • Calibrated at reference field size (not always 10x10). Commissioning: Dosimetry Dosimetry verification of TPS configured • Deliver a set of fields and/or plans and perform measurements. Measurement and calculation should agree. Commissioning: Mechanicals • Linac mechanical/radiation isocenter: ≤1mm • Couch position accuracy: ≤1mm • Collimator position accuracy: - cone alignment - MLC position accuracy (fence test) • Laser alignment: Winston Lutz (WL) test (<1mm) Commissioning: Mechanicals Winston Lutz (WL) test (cone or MLC) Commissioning: Imaging • CT imaging: - good geometry integrity - for reference image, simulation, planning, - prefer thin slice(<), axial scan, with contrast • MRI imaging: - good soft tissue contrast - use for target delineation - need T1, T2, Axial, Sagittal, Coronal, 3D T1 with contrast most useful • Angiogram: - orthogonal X-rays, for Arteriovenous Malformation (AVM, 动静脉畸形) Commissioning: Imaging CT imaging Commissioning: Imaging MRI imaging Commissioning: Imaging MRI imaging: T1 weighted, before and after contrast Commissioning: Imaging MRI imaging: T2 weighted, Commissioning: Imaging CT/MRI image fusion Commissioning: Imaging Angiogram Angiogram/CT registration Commissioning: IGRT • Dedicated IGRT system • Cone beam CT (CBCT) • On board orthobonal X-rays Need to determine the accuracy of the imaging system Commissioning: IGRT Dedicated IGRT system • Mounted on floor and ceiling • Submillimeter accuracy • Realtime imaging at any couch angle ExacTrac® Commissioning: IGRT CBCT/OBI • Accuracy up to 1 mm • Only at couch = 0 Commissioning: Margin Design Need to include: • • • • • Size of mechanical and radiation isocenters CT/MRI slice thickness Image registration Contouring accuracy Patient setup accuracy (IGRT accuracy if imaging used as primary target positioning) Commissioning: Secondary dosimetry check system • Commercial monitor unit (MU) calculation software • Hand MU calculation table or spreadsheet (not for IMRT or VMAT) • Patient specific QA measurement (IMRT or Vmat) Commissioning: End-to-end tests • Localization/positioning end-to-end test: hidden target Curtesy: Dr. Kelly Younge Commissioning: End-to-end tests Dosimetry end-to-end test: • follow clinical procedure using clinical mode and R&V system • measuring with film (plus ion chamber if possible) Curtesy: Dr. Kelly Younge Commissioning: External Audit Perform external audit and/or invite external physicist with experience to review the program, if possible. SRS phantom from IROC, MD Anderson, Houston SRS Planning and Evaluation Techniques available: •Non-coplaner arcs •McGill dynamic arc •Static beams •IMRT •Vmat SRS Planning and Evaluation Cone planning: • Sharp penumbra but can be tricky • on GK, CK and maybe Linac, prefer spherical target SRS Planning and Evaluation Conformal index: CI= Rx dose Volume/ PTV, Range: 0 - ∞, ideally 1. but, 2 V PTV ( D ) Paddick Index: CI Paddick VPTVVTotal ( D ) Range: 0 – 1, ideally 1 RTOG wants CI <2, dose homegeneity index, DHI = Max Dose/Rx dose <2. For most cases better indices are achievable. Quality Assurance of SRS Program Equipment QA: •Daily QA – Winston-Lutz test (mechanical / rad isocenter) – Verification imaging isocenter •Monthly – Winston-Lutz – Couch position •Annual – End-to-end test (including Localization/dosimetry) Patient/Process QA: Apply checklist Quality Assurance of SRS Program: Daily QA From AAPM TG-142 Quality Assurance of SRS Program: Daily QA From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Daily QA From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Monthly QA From AAPM TG-142 Quality Assurance of SRS Program: Monthly QA From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Annual QA From AAPM TG-142 Quality Assurance of SRS Program: Annual QA From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” Quality Assurance of SRS Program: Patient QA Checklist Example From Astro white paper, “Quality and safety consideration in SRS/SBRT” References: • “Stereotactic Radiosurgery”, Dr. Michael Schell, et al. AAPM TG-42 (report 54) • “ Quality and Safety Considerations in Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy”, Dr. Timothy D. Solberg, et al. Practical Radiation Oncology: August 2011. (Astro white paper) • “Stereotactic body radiation therapy”, Dr. Stanley Benedict, et al. The report of AAPM Task Group 101 • “Small fields and non-equilibrium condition photon beam dosimetry”, AAPM TG-155 (in progress) • “Intracranial stereotactic positioning systems”, AAPM TG-68 • “Use of MRI in treatment planning and stereotactic procedures”, AAPM TG-117 (in progress)