Resident: Samir Laoui, Ph.D. 06/28/2016

Stereotactic Radiosurgery and Stereotactic Body Radiation

Therapy

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Overview

▪ Stereotactic radiosurgery (SRS): Is a single fraction therapy procedure for treating intracranial lesions using a combination of a stereotactic apparatus and narrow multiple beams

▪ Stereotactic radiotherapy (SRT): utilizes the stereotactic apparatus and radiation beams for multiple fractions (up to five)

▪ Concentrated dose in the lesion

▪ Steep dose gradients external to the treatment volume

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Overview

▪ A high degree of dose conformity is a hallmark of SRS

▪ Accuracy of beam delivery is crucial

▪ Involves: ▪ Imaging, target localization, head immobilization, and treatment setup

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The History of Stereotactic Radiosurgery: Summary

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The History of Stereotactic Radiosurgery

▪ 1929, Ernest Lawrence from UC Berkeley invented the cyclotron

▪ 1950s, his brother John began investigation of the use of heavy particles (proton beams, then helium ion beams) for the treatment of patients with pituitary and other intracranial disorders

▪ 1962, Raymond Kjellberg, a neurosurgeon at the Harvard/Massachusetts General Hospital facility, began the use of proton beam treatments in SRS treatments of AVMs

▪ The expense of building and maintaining a cyclotron has limited the use of heavy-particle SRS to a few centers 5

The History of Stereotactic Radiosurgery

▪ 1951, Lars Leksell coined the term stereotactic radiosurgery (SRS)

▪ 1968, Leksell stopped experimenting with heavy ions due to their expense and their impracticality and ultimately designed the Gamma Knife

▪ Treatments were limited to patients with arteriovenous malformations

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The History of Stereotactic Radiosurgery

1981-1989: First Gamma Knife in U.S. This is the original Gamma Knife designed by Larson and Leksell and donated to UCLA by Lars Leksell

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The History of Stereotactic Radiosurgery- Acceptance

▪ Mid-1970s, the advent of CT, and magnetic resonance opened up the possibility of direct targeting of tumors and other “soft tissue” targets inside the skull

▪ 1980’s, adoption of linac based SRS

▪ 1984-1985, working independently, in Buenos Aires, Argentina, and in Vicenza, Italy, respectively, Betti and Colombo reported the successful adaptation of linacs for SRS • Betti O, Derechinsky V. Hyperselective encephalic irradiation with a linear accelerator.

Acta Neurochir 1984; Suppl 33:38– 390.

• Columbo F, Benedetti A, Pozza F, et al. External stereotactic irradiation by linear accelerator. Neurosurgery 1985; 16:154–160.

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The History of Stereotactic Radiosurgery- Acceptance

▪ 1986, Winston and Lutz described the use of a commercially available stereotactic frame for linac radiosurgery

▪ 1994, CyberKnife for SRS delivery ultimately came into being

▪ 1995, Loeffler and Alexander demonstrated how a linac dedicated to SRS could be a practical alternative to a GK • Loeffl er J, Shrieve D, Wen P, et al. Radiosurgery for intracranial malignancies. Semin

Radiat Oncol 1995; 5:225–234

▪ At the present time, clinical and physics studies seem to have settled the issue (GK versus linac) in that SRS can be delivered effectively and accurately with either method

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SRS Accuracy

▪ The uncertainty in dose delivery is a result of 2 processes: ▪ Target definition: Depends on image resolution, pixel dimensions

▪ Machine tolerance: Gantry, collimator, couch within 1mm sphere

Summed in quadrature 10

SRS program Acceptance tests

▪ Accurate localization • Localization of a well defined object within 2 mm diameter

▪ Mechanical precision • Alignment of frame coordinates with linac

• Gantry, collimator, couch, lasers

▪ Accurate and optimal dose distribution • Uncertainty less than 5%

• Dose gradient in penumbra from 80% to 20% should be within 3 mm

▪ Patient Safety

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SRS techniques: Linac based

▪ Use of multiple noncoplanar arcs

▪ A spherical dose distribution is shaped to fit the lesion by the following • Shaping the beam’s eye aperture dynamically with MLCs

• Changing arc angles and weights

• Using more than one isocenter

• Combination of stationary beams with arcing

beams

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Linac based: Beam Collimation

▪ Small lesions require much smaller fields

▪ Penumbra must be as small as possible

▪ A tertiary collimation can reduce the penumbra by bringing the collimator diaphragm closer to the surface (Cones)

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Novalis®

▪ The Novalis platform incorporates two complementary imaging systems that work together to enhance treatment

▪ ExacTrac® room-based X-ray imaging system provides real-time imaging and fine-tuning of a robotic couch that moves in six dimensions to ensure that the targeted lesion is aligned with the treatment beam during treatment

(Jefferson Radiation Oncology, 2010)

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Gamma Knife

▪ Isocentric gamma rays Co-60

▪ Activity of each cobalt source (at time of installation) = 30 Ci

▪ Total of 201 sources (195 sources in the Perfexion model)

▪ Total activity of radioactive cobalt ~ 6000 Ci

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Proton beam

Bragg Peak is very attractive to SRS especially for larger and irregularly shaped targets

• Pituitary adenomas • Meningiomas • Acoustic neuromas • Cavernous sinus tumors • Arterial venous

malformations (AVM)

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Accuray CyberKnife®

Radiosurgery delivered using an x-band linear accelerator mounted on a robotic arm.

Uses a frameless approach and is capable of intracranial and extracranial radiosurgery.

Real time image-guidance is accomplished using 2 KV imagers.

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Choice between SRS and SRT

▪ Tumor volume: As the target size increases, normal tissue toxicity increases with SRS

▪ SRS is not recommended for lesions > 3cm (UCI practice)

▪ Proximity to cranial nerves: Neurotoxicity. SRT should be considered when SRS may jeopardize cranial nerves function

▪ Location of the lesion: SRT is preferred for lesion in the deep gray matter of the brainstem

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Dosimetry

▪ The dosimetry of x-ray small fields is complicated: The relationship between detector size and field size and lack of lateral charged particle equilibrium

▪ 3 quantities of interest in SRS dosimetry: • PDDs /TMRs

• Off axis-ratios

• Output factors (Sc,p or dose per unit [MU])

▪ Detector size is important because of the steep dose gradient at the field edge

▪ Detectors: Ion chambers, film, TLDs, and diodes

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Detectors in SRS dosimetry

▪ Detectors include: The choice depends on the quantity to be measured • Ion chambers: Precise, least energy dependent, but has size limitation

• Film: Best spatial resolution but energy dependence and statistical uncertainty

• TLDs: Little energy dependence, small size but suffer from statistical uncertainty

• Diodes: Small size, but energy and directional dependence

▪ Cross-Beam profiles: Diodes and films

▪ Depth dose: Parallel plate chambers less than 3 mm in diameter, film and diodes for very small fields

▪ Output factors: 12.5 mm and larger fields, cylindrical and parallel plate. For ultra small fields (10 mm or less), films, diodes and TLDs.

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Quality Assurance

▪ Treatment QA and routine QA

▪ Treatment QA:

• Frame accuracy, imaging data transfer, frame alignment with gantry and couch, congruence of target point with radiation isocenter

▪ Routine QA:

• Hardware/ Software

• Linac based: AAPM report 142 and 54

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TG-142

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SBRT

▪ Effective in controlling stage primary and oligo-metastatic cancers in thorax and abdomen, spinal and paraspinal sites

▪ Delivery of high dose in few fractions

▪ Results in high BED

▪ SBRT has been used as a boost in addition to regional nodal irradiation

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Treatment planning

▪ MRI, PET, PET/CT, CT

▪ Slice thickness of 1-3 mm is adequate for most clinical sites

▪ Primary sources of organ/tumor motion during simulation imaging are respiration (up to 5 cm), cardiac function, peristaltic activity, and organ filling and emptying

▪ Unlike conventional treatments requiring uniform dose within the target, hot pots are acceptable in SBRT

▪ Sharp dose fall-off is required to limit normal tissue toxicity

▪ In SBRT, GTV and CTV are often considered to be identical

▪ Variation in size and position due to respiration is accounted for by the ITV

▪ PTV accounts for the geometrical variations

▪ CTV to PTV: 0.5 cm in the axial planes, 1.0 cm in the superior/inferior, isotropic when 4DCT is used

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Dose heterogeneity, gradient, and beam geometry

▪ Dose prescription are often specified at low doses (80%) to improve dose fall-off outside target.

▪ This increases dose heterogeneity within the target when functional normal tissue is not involved

▪ Hot pots within the target are desirable because they help eradicate radio resistant hypoxic cells

▪ Non overlapping beams

▪ Dose fall off is affected by beam energy and resolution of beam shaping (MLC width)

▪ The higher the energy the larger penumbra (Lung for example)

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Dose heterogeneity, gradient, and beam geometry

▪ More arcs, better conformity. However, for practical reasons, the number of arcs should be limited

▪ Use of beam arrangements employing five to eight coplanar or noncoplanar static conformal beams shaped by 5–10 mm MLCs for targets in the thorax and abdomen have been reported

▪ It has been reported in the literature that a 2.5 mm isotropic grid produces an accuracy of about 1% in the high-dose region of an IMRT plan consisting of multiple field

▪ This report recommends the use of an isotropic grid size of 2 mm or finer. The use of grid sizes greater than 3 mm is discouraged for SBRT.

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Treatment plan reporting

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PATIENT POSITIONING, IMMOBILIZATION, TARGET LOCALIZATION, AND DELIVERY

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PATIENT POSITIONING, IMMOBILIZATION, TARGET LOCALIZATION, AND DELIVERY

▪ For SBRT, image guided localization techniques should be used to guarantee the spatial accuracy of delivered dose distribution

▪ Gantry mounted kV units capable of fluoroscopy, radiographic localization and cone beam imaging

▪ Implantation of fiducials

▪ Ultrasound imaging

▪ Radiofrequency tracking • Stereoscopic infrared cameras

• Video photogrammetry

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PATIENT POSITIONING, IMMOBILIZATION, TARGET LOCALIZATION, AND DELIVERY

▪ Respiratory gating techniques • Delivery of dose at certain phases of breathing

• Issue of reproducibility

• Patient-specific tumor motion assessment for thoracic/ abdominal targets

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Patient safety

▪ At least one qualified physicist be present from the beginning to end of the first treatment fraction

▪ For subsequent fractions, it is recommended that a qualified physicist be available

▪ A radiation oncologist approve the result of the image guidance and verify the port films before every fraction of the SBRT treatment

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QA: ISO Calibration

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QA: Winston Lutz

Target simulator and the floor stand are independently set to the patient’s target coordinates. If everything is aligned and set correctly, the ball bearing will remain in the center of the radiation beam regardless of the angle of the gantry /turntables

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Thank you

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