Download SPINAL STEREOTACTIC BODY RADIOTHERAPY

Ugur Selek M.D.
Ugur Selek M.D.
1. Introduction
Metastasis to the spinal column is an emergent problem in cancer patients which is not rare and occurs
in almost 40%. (1) The aim in management of metastasis to the spinal column is to decrease the sequelae
including pain, instability, and neurological deficit to
avoid progressive myelopathy resulting in the loss of
motor, sensory, and autonomic functions. Although
the treatment is palliative, the durability of the progression free period without any sequel is essential.
As the treatment options increase in number, comprehensible multidisciplinary approach needs to be in
charge to effectively implement the proper treatment.
Surgery is inevitable in case of cord compression,
however if there is no vertebral instability in immediate requirement of reconstruction and stabilization of
the spinal column, radiotherapy has developed into
a noninvasive and effective modality by Stereotactic body radiotherapy (SBRT) option. Conventional
radiotherapy of 30 Gy in 10 fractions or 45 Gy in 25
fractions can be effective in acute palliation of pain
and neurological symptoms of spinal metastases; however the outcome might not be robust in many consequences without the conventional reirradiation possibility. Spinal SBRT has the advantage of minimizing
dose to the spinal cord while delivering higher doses
to the spinal tumor; and is a potential treatment option for preferably low volume vertebral metastases
as a primary choice, whereas it can also be offered in
postoperative setting or in salvage setting subsequent
to previous irradiation or recurrence after surgery.
Stereotactic body radiotherapy (SBRT)
SBRT is a non invasive image-guided process of radiotherapy intervention which uses a three-dimensional
coordinate system to locate small targets inside the
body in order to deliver high precision and an increased load of radiation for highest local control possible. “Stereotactic” (or “stereotaxic”) means “solid
ordering” in Greek. The American Society of Therapeutic Radiology and Oncology (ASTRO) defines
SBRT as an external beam radiation therapy method
used to very precisely deliver a high dose of radiation to an extracranial target within the body, using either a single dose or a small number of fractions with high targeting accuracy and rapid dose
falloff gradients encompassing tumors. (2,3) Theoretically, any location inside the body can be subjected
to SBRT with required reliable set up and imaging.
Single-fraction SBRT with simulation, planning, and
treatment on the same day is called as Stereotactic
body radiosurgery (SBRS), while SBRT covers all fractionated treatment sessions using larger daily doses
of radiation than conventional fractionated doses of
1.8-3 Gy/fraction/day.
Single fraction SBRT has been reported by many
centers with various dose ranges, (4-6) as growing literature for multiple fraction SBRT exist. (7-10) The reasoning of single or multiple fractionations in spinal SBRT
depends on clinical judgment; small volume tumors
with enough safety margins from spinal cord might
be preferred to be treated with single fraction SBRT,
while tumors with paraspinal extension, or having
multiple level or prevertebral involvement, or being
close to bowel could be elected to be treated with 3 to
5 multiple fractions SBRT. General conservative consideration is the capability of more than one fraction
to balance possible positional set up inaccuracies in
comparison to single fraction SBRT. Therefore, the
safety band of fractionated SBRT with adequate confidence could be the first option in conditions with
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high setup or normal tissue toxicity risk despite its
disadvantage of inconvenience. Examples of Intensity modulated SBRT are given in Figures 1-5 as our
current clinical approach of delivery in three fractions. Single fraction SBRT is mainly built on strict
one-time immobilization for same day simulation,
planning and delivery of treatment while increasing
real time imaging capabilities such as cone beam CT
mounted on the linear accelerators for exact setup
provides flexibility in the timing of simulation and
treatment on different days.
Experience on spinal stereotactic radiosurgery
was set off with the early series of Hamilton et al
at University of Arizona that used an invasive rigid
fixation device for immobilization of the spine in
nine patients with recurrent spinal tumors and prescribed median 8 Gy. (11) One of the initial important
studies demonstrating targeting accuracy within 1.5
mm for actual patient treatment was reported by
Ryu et al. (12) These were establishing clinical feasibility of the accuracy and precision of SBRT to treat
tumors adjacent to the spinal cord. Chang et al detailed near-simultaneous computed tomographic image-guided SBRT at M. D. Anderson Cancer Center
for 15 patients with metastatic spinal disease using
a comparatively conservative regimen of 30 Gy in 5
fractions with a maximum dose constraint for spinal
cord of 10 Gy with no neurologic toxicity in median
9 months. (13) Bilsky et al reported on their preliminary Memorial Sloan Kettering Cancer Center clinical experience in treating 16 paraspinal tumors with
stereotactic IMRT where used 20 Gy tumor dose in
4 or 5 fractions with a maximum dose constraint for
spinal cord of 6 Gy. (14)
Succeeding clinical studies with spinal SBRT demonstrated the efficacy for rapid pain relief and improvement of neurological function. (4,12,15-19) In general
palliation of pain is the most invested and reported
main cause of irradiation of spinal column metastases with complete pain relief after SBRT was reported in various series at 33% to 86%. (5,12,20,21) Other
cancer related symptoms besides pain also need to
be investigated. Quality of life is also improved secondary to pain relief. (19) SBRT could help in reduction of symptom burden in terms of other cancer
related symptoms such as fatigue, pain, sleep disturbance, drowsiness, and distress with better local
control and activity of daily living, ability to work,
and mood. (13)
Ryu et al. at Henry Ford Hospital reported on
49 patients treated with single doses of 10–16 Gy
with a complete pain relief of 46% at 8 weeks and
both complete and partial pain relief of 85%, while
pain relapsed in 7% and tumor progressed in 5%. (4)
Gerszten et al published SBRT of 12.5–25 Gy with
long-term pain improvement in 86%. (5) Yamada et
al. noted that patients without local failure after single fraction SBRT doses of 18–24 Gy had long term
symptom palliation. (6) Gibbs et al. used 16–25 Gy in
one to five fractions and disclosed 84% improvement
of initial symptoms. (7,8) Nelson et al reported 39% and
51% complete and partial pain relief at one month
respectively with three fractions of SBRT (range, one
to four fractions). (9) Gerszten et al at the University
of Pittsburgh treated 500 cases of spinal metastases
with 12.5 to 25 Gy Cyberknife based single-fraction
SBRT. (5) Long-term pain improvement occurred in
86% while tumor control was achieved in 90% of
treated lesions. Ryu and colleagues defined that median duration of pain control with spine SBRT was
13.3 months. (4) The currently ongoing RTOG 0631 trial
will assess pain relief as its primary endpoint where
patients with spinal metastases are randomized to receive either 8 Gy x1 fraction of conventional radiation
therapy or 16 Gy x 1 fraction of SBRT. (22)
Conventional palliative radiotherapy has recognized role on pain control for bone metastases for
an intermediate period of time, while robust tumor
control on spinal metastases likely require higher
doses. Spinal SBRT might be a promising solution
to decrease pain and neurological complications including metastatic epidural spinal cord compression
(MESCC) due to inadequately conventionally treated
spinal metastases. (23-25) Higher doses than conventional
fractionation of 30 Gy in 10 fractions might have a
better chance of preventing bony destruction of the
spinal column leading to spinal instability. The major limiting factor avoiding higher dose prescription
to spinal column is the low threshold of spinal cord
radiation tolerance.
SBRT is a competent technique with ability of image-guidance to precisely deliver higher doses to secure a safe dose gradient at the spinal cord and tumor
interface. This noninvasive modality achieves more
than 80% of objective radiological tumor control at
the treated spine. (5,10,15) Thus, a new era of therapeutic window with low risk of spinal cord injury is in
charge with SBRT. Spinal SBRT could become a smart
Ugur Selek M.D.
2. Indications
Rationale of the large dose per fraction in SBRT is to
ablate tumor cells and/or overcome radioresistance
resulting in greater log cell kill. There are four major indications:
• spinal metastases without instability
• postoperative adjuvant approach for high risk
local recurrence
• postoperative salvage approach for clinical progression
• salvage approach for local disease progression
or recurrence after conventional radiotherapy
to the spine
Conditions of Patient Eligibility
• Localized (solitary or not more than two contiguous spine levels) spinal column metastasis.
• Distance to spinal cord is important for eligibility
and SBRT is considered if ≥ 3 mm gap is present
between the spinal cord and the edge of the epidural lesion. Surgery is preferable if closer than
3 mm to cord. SBRS might be opted if surgery
is not an option despite underdosage of epidural component.
• Paraspinal mass component ≤ 5 cm in the greatest
dimension contiguous with spine metastasis
• Well-controlled systemic disease
• Karnofsky Performance Status > 40
• Previous irradiation not more than 45 Gy (1.8-2
Gy/fraction) or 30 Gy (3Gy/fraction)
• Radioresistant tumors metastatic to spinal column
such as renal cell carcinoma, melanoma, and sarcomas are considered to benefit more in comparison to other cancers.
3. Contraindications
• Radiosensitive metastases such as plasmacytoma,
lymphoma, and germ cell primary: Conventional
fractionation radiation therapy with 1.8 - 3 Gy/
fraction/day to a total dose of 45-30 Gy is the
standard first choice of treatment in these histology due to tumor control curve lying to the left
of the normal tissue complication curve.
• Tumors enclosing the spinal cord, compressing the cord or thecal sac, involving the epidural space require surgical decompression which
subsequently requires SBRT after resection of the
proximal disease to the spinal cord.
• Cord or cauda equina compression with neurologic deterioration: Emergent surgery is encouraged
• Unstable spine (e.g. compression fracture; > 50%
loss of vertebral body height): Emergent surgery
is encouraged
• Non-ambulatory patients
• Previous radiotherapy dose greater than 45 Gy
(1.8-2 Gy/fraction) or 30 Gy (3Gy/fraction)
4. Procedures
Spinal SBRT requires extensively improved immobilization, imaging and delivery precision to limit target movement during planning and delivery. Delivery can be via planar or non-coplanar multiple static
beams or rotational fields of varying degrees with or
without beam intensity modulation. Stereotactic localization of the lesion is essential by appropriate imaging modality, such as bony landmarks, fiducials,
or computed tomography (CT) to ensure accurate
beam placement. Quality assurance (QA) must be
followed strictly for SBRT accuracy. The quality of
a SBRT treatment depends on the coordinated team
effort along an accurate treatment planning and delivery process with reliable verification.
The responsible radiation oncologist (RO) carefully evaluates the planned treatment impact through
benefits and potential risks, educated design and
conduct of treatment, and cautious follow-up after SBRT. (2) RO needs to determine patient-specific
and reproducible positioning, and stability of setup
appropriately; and to supervise simulation process
with spatial accuracy and precision of the imaging
modality. After defining the target, RO prescribes
dose, sets limits according to normal tissue dose constraints, approves the final treatment plan prepared
by the medical physicist, and closely directs the actual treatment process.
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alternative to surgery in selected cases with avoidance
of possible operative risks, besides a proper way of
reirradiation of spinal column metastases which are
generally not irradiated by conventional means.
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SBRT targeting and treatment ensures adequate
dose coverage of the target with rapid falloff to normal tissues by numerous coplanar or non-coplanar
beams or large arcs of radiation with apertures, as
well as by intensity-modulated radiation delivery.
Intensity-modulated radiation therapy (IMRT) is a
sophisticated form of three-dimensional conformal
radiation therapy granting necessary and appropriate coverage of tumor with falling doses to the spinal cord in constraints. Computer-controlled multileaf collimation of the modern linear accelerators
provides precision and conformality to irradiate
any shape of tumors such as surrounding the spinal cord. The use of a newly developed fusion and
matching capacities of real time on board imaging as
KV portal films and cone beam CT accurately identifies planned target for treatment.
4.a. Immobilization
As SBRT is designed to deliver high ablative doses
to the spinal column tumor in one to couple of fractions; limiting the volume and maximum dose of
the spinal cord that is irradiated is crucial to avoid
potentially catastrophic toxicities. Tumor motion in
general is a very complicated challenge for SBRT
in other body SBRT approaches due to respiratory
movement of organs. Fortunately, spinal column is
a comparatively less mobile axis with respiration. If
a reproducible minimization of target motion via immobilization is ensured, physiologic tumor motion
is not an issue to be accounted via tracking or gating which is an obligation such as for lung or liver
tumors. Precise imaging and positioning are mandatory to accompany and unify immobilization.
Despite conventionally fractionated approach, irradiating a large volume of normal tissue to account
for setup uncertainty is unacceptable in SBRT dose
range. Therefore, the basics of spine SBRT are patient positioning, targeting, and delivery with minimal dosimetric margins.
A reproducible immobilization method is required which facilitates the real-time imaging and
positioning at the SBRT linear accelerator. The immobilization is to decrease the set-up uncertainty in
multiple fractions while verification of isocenter is required at each fraction by cone beam and/or KV image guidance. Immobilization should be in a stable
supine and comfortable position to prevent patient
movement. Stereotactic whole body cradle vacuum
bag or alpha cradle with or without shrink-wrapping
is used in general for customized immobilization to
surround the patient on three sides to conform patient’s external contours with reference to the treatment delivery coordinate system. There is couple of
commercial systems used to ease the reproducible
immobilization such as Body Pro-LokTM or BodyFIX®
Stereotactic Systems. A rigid head and neck immobilization device such as thermoplastic mask should
be used for metastasis to cervical spine or cervicothoracic junction.
4.b. Target and Critical Structures
delineation
The gross tumor/target volume (GTV) is contoured
with relevant imaging studies; GTV is expanded to
the clinical target volume (CTV) due to the pattern
of microscopic spread, and coordinate the proper
planning target volume (PTV) beyond the CTV
with information of the mechanical and setup uncertainty. (10,26,27)
• Gross tumor volume (GTV): Contour gross lesion
• Clinic tumor volume (CTV): GTV=CTV
• Planning tumor volume (PTV): CTV + 1-2mm
• Lower-dose CTV (CTV lower-dose): Contour remaining entire vertebral body and posterior elements and ensure more generous margin posterior to diseased vertebrae
• The spinal cord volume is contoured 5-6 mm superior to 5-6 mm inferior to the GTV. (28)
• Critical structures are contoured starting at 10
cm above the target volume to 10 cm below the
target. (29)
4.c. Physics and Dosimetry of SBRT
SBRT is performed with high conformality of the
prescribed potent dose to the tumor volume, with
steep dose fall off to decrease normal tissue toxicity. This delivery requires multiple, non-opposing,
non-coplanar beams or arcs. (30,31) While minimizing
the entrance dose to prevent severe acute skin toxicities, beam angles, beam number and beam weighting to isocenter are arranged to reduce the volume
of tissue at intersection of beams to maintain an acceptable maximum dose in target and circumferential dose falloff. SBRT physics seek to provide 95%
planning target volume coverage with an appropriate prescription isodose line (Figures 1-5).
Figure 1a
Ugur Selek M.D.
Figure 1b
Figure 2a
Figure 1b:
8 coplanar beams are assigned to this prescription: 180, 205, 225, 245, 095, 115, 135, 155.
Figure 2a:
Intensity modulated SBRT was delivered for breast carcinoma metastasis to T8 vertebral body to 27 Gy
in 3 fractions at 9 Gy per fraction using 6 MV photons with multiple coplanar beams. Dose distribution
in axial, sagittal and coronal view. Prescription is to 92% of GTV.
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Figure 1a:
Intensity modulated SBRT was delivered for thyroid carcinoma metastasis to T11 vertebral body to 27
Gy in 3 fractions at 9 Gy per fraction using 6 MV photons with multiple coplanar beams. Dose distribution in axial, sagittal and coronal view. Prescription is to 95% of CTV.
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Figure 2b
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Figure 2b:
Dose volume histogram demonstrating the doses to regions of interest.
Figure 3
Figure 3:
Intensity modulated SBRT was delivered for metastatic colorectal cancer to C6-C7 vertebral bodies,
neural foramina and posterior elements to a total dose of 27 Gy in 3 fractions at 9 Gy per fraction using
6 MV photons with semi-coplanar beams.
Figure 4a
Ugur Selek M.D.
Figure 5:
Intensity modulated SBRT was delivered for renal cell carcinoma metastasis to L5-S1 vertebral body to
27 Gy in 3 fractions at 9 Gy per fraction using 6 MV photons with multiple coplanar beams. Dose distribution in axial, sagittal and coronal view. Prescription is to 96% of GTV.
The medical physicist is responsible for the technical aspects of SBRT. This requires acceptance testing
and commissioning of the SBRT system for its geometric and dosimetric precision and accuracy; monitor and assure proper functioning of the linear accelerator and CT simulator of SBRT system, the image
guidance system, the image-based 3D and/or intensity-modulated treatment planning system, determine
and check the appropriate beam-delivery parameters,
and calculation of the radiation beam parameters consistent with the beam geometry according to the plan
approved by the radiation oncologist. (2)
4.d. Quality Assurance (QA)
QA is basically verification process of radiation isocenter, cone beam isocenter, CT-CT matching software.
SBRT QA secures the combined testing, proper functioning and communication of image-guidance and
treatment delivery systems within acceptable tolerances which guarantees the information from the
imaging system matching the planned beams to the
exact position within the patient. This is performed
with designation of image guidance system to define ≥1 test points of predefined coordinates and then
testing treatment planning system to irradiate these
same test points. Adequate image guidance is mandatory according to coordinate the stereotactic targeting of tumor between imaging system and delivery
system, and body frames based only on frame fiducials are not adequate without proper image guidance. The frame-based SBRT includes a stereotactic
body frame with adequate imaging system, and the
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Figure 4:
Figure
5.
Intensity modulated SBRT was delivered for pancreas carcinoma metastasis to T8-9 vertebral body with
paraspinal extension to 27 Gy in 3 fractions at 9 Gy per fraction using 6 MV photons with multiple coplanar beams. Dose distribution in axial, sagittal and coronal view. Prescription is to 96.5% of CTV.
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frameless SBRT includes one or more of the following, implanted metallic seeds within or close to the tumor; or landmark bone anatomy as a fiducial marker
for matching with the planning digital reconstructed
radiographs (DRR), or volumetric data obtained from
on board CT to match the planning CT.
SBRT is basically an image-based and guided
treatment with relevant image fusion, and/or localizing of target volumes. The images are necessary to
delineate the gross tumor volume and normal tissue
margins besides defining target coordinates of treatment beams. CT is practical and spatially undistorted
to be used for SBRT where a virtual patient model for
treatment planning can be established for the treatment plan evaluation and dose calculation. MRI is
also helpful for exact delineation and MRI with CT
images fusion is used to minimize geometrical distortions inherent in MR images. These images input
are used in 3D planning process.
It is viable to mention that treatment planning
software estimates doses to critical structures which
is definitely restricted by the accuracy of irradiation
beam data measurements by ion chambers under
effect of size or volume of the system, and possible
leakage of multileaf collimators besides leaf shape. (13)
Therefore, one should be aware of the potential risk
on treatment planning of over or under-estimating
the actual doses delivered in order to be conservative to limit doses to the spinal cord. It is essential to
establish QA of the delivery system to increase the
confidence of safety and setup data demonstrating
consistent daily patient setups within 1mm of treatment isocenter.
4.e. SBRT Equipment
Currently, many commercially available treatment
units capable of refined image guidance can perform
spine SBRT. (31) Decreasing the uncertainty with near
real-time imaging for positioning and delivery is the
major requirement, and patient positional imaging
is obtained to align the patient in the treatment position in order to perform shifts at each fraction set
up. None of them has a clear superiority to others
in spinal SBRT. However, the key is proper training
and experience of the team in SBRT.
The treatment devices can be categorized according to their imaging capabilities for positioning
and delivery. Integrated gantry mounted cone beam
CT and KV & MV on board imaging systems (The
Trilogy™ Stereotactic System from Varian Medical
Systems, The Elekta Synergy®), CT scanner linked
to a linear accelerator via a shared tabletop (The Siemens Primaotom), orthogonal X-ray cameras (The
CyberKnife, Accuray, Inc., Sunnyvale, CA, USA, the
Novalis, Brainlab, Ammerthalstrabe, Germany) and
megavoltage CT mounted in the system (The Tomotherapy HI-ART, TomoTherapy, Inc., Madison, WI,
USA). The SBRT could be delivered as three major
ways of application:
(a) Irradiation with a circular approach with a
moving source or table, named arc treatment
(b) Irradiation with stationary fields shaped by a
multileaf collimator with gantry-based systems
(c) Irradiation with multiple microstationary
fields by cylindric collimator with the CyberKnife
concept
4.f. Radiotherapy set up and Patient
positioning
Acceptable and appropriate image-guidance (IG) for
localization includes the accuracy of less than 2 mm
from simulation/planning to the end of treatment.
There are mainly four ways of IG noted below for
image guided radiotherapy (IGRT):
• Cone-beam CT equipment mounted to the linear
accelerator performing with the treatment beam
or an auxiliary kV x-ray head to acquire multiple
images for volume reconstruction to fine tune patient set ups with ultra-precise CT scans;
• Spiral dose delivery equipment using the treatment beam to collect helical CT information for
image guidance;
• Any equipment that can produce stereoscopic
planar views of the patient in the treatment position using either the treatment beam with a standard electronic portal imaging device (EPID) or
a kV x-ray source with opposed imaging panel
in order to localize anatomic points in space or
implanted fiducial markers to reposition patients
quickly and accurately
• A standard CT scanners linked to a linear accelerator via a shared tabletop (e.g., on rails) in the
same room with the treatment equipment.
4.g. Fractionation
The prescription dose changes in SBRT with the fraction number from single to five. (10,26,27) CTV lower-
Ugur Selek M.D.
• 16-24 Gy in single fraction
• 27 Gy in 3 fractions
• 30 Gy in 5 fractions
• Lower total doses with lower dose per fraction
(21 Gy in 3 fractions or 25 Gy in 5 fractions) is
prescribed owing to the discretion of the responsible RO (e.g. lesion below L2 adjacent to psoas
muscle to decrease plexopathy risk, etc)
5. Complications and Avoidance
Spinal cord is the major dose-limiting critical organ at risk in SBRT and radiation-induced myelopathy (RIM) could be a debilitating complication to
be avoided. The dose constraints for other organs at
risk are detailed in the literature. (29)
5.a. Spinal Cord Tolerance
An extremely sharp fall-off of radiation dose beyond
the tumor is required and this makes it possible to
safely treat spinal column metastatic lesions with high
total doses in single or fractionated SBRT.
Although radiation-induced myelopathy (RIM)
is one of the most critical complications associated
with radiation therapy, the factual spinal cord tolerance to SBRT has not been clearly defined due to
highly conservative constraints followed in general.
The RIM is based on white matter injury with demyelination and necrosis of the spinal cord, as well
as on vasculopathies and glial reaction. (32) In spinal
SBRT, therapeutic ratio is increased with greater separation between the spinal cord normal tissue complication curve and the spinal tumor control probability curve. The spinal cord is highly sensitive to
fraction size as being a late responding tissue where
the actual fraction size to the spinal cord needs to be
relatively small in comparison to hypofractionated
high stereotactic radiation doses. The reported incidence of spinal cord complications is too low in the
radiotherapy literature to project threshold of a safe
dose of the spinal cord.
Because of different fraction sizes in a mainly palliative setting, it is difficult to conclude spinal cord
tolerance to hypofractionation. A cornerstone study
from Medical Research Council by Macbeth et al.
used various regimens to palliate non-small cell lung
cancer patients with 8.5 Gy X 2, 4.5 Gy X 6, 5 Gy X 6,
3 Gy X 10, 3 Gy X 12, and 3 Gy X 13 and estimated
RIM as 0.6% (3 patients) with 8.5 Gy X 2 fractions
at 8 to 42 months and 1.3% (2 patients) with 3 Gy X
13 fractions at 8 to 10 months. (33,34) It is also encouraging to see that no RIM was reported after 3 Gy X
15 fractions (=45 Gy) in two studies. (35,36) Kramer et
al compared palliative regimens of 30 Gy in 10 fractions and 16 Gy in 2 fractions in a Dutch trial of nonsmall cell lung cancer and reported no RIM. (37) On
the other hand, there are other palliative studies stating occurrence of RIM up to 13.3% with 40 Gy in 10
fractions to the spinal cord and up to 11.4% with 33.5
Gy in 6 fractions. (38-41)
As RIM is a very debilitating complication, it is
logical to hold spinal cord constraint conservative
and schemas limiting the maximum spinal cord dose
to less than total 10 Gy in case of prescription dose
of 27 Gy in 3 fractions, or 30 Gy in 5 fractions need
to be preferred. (6,13,14,42) Single session SBRT series reported safety of limiting the surface dose of the spinal
cord to 10 Gy, 12 Gy, and even 14 Gy appears safe,
however it sounds rational to keep it to less than total 10 Gy due to the fact that it is currently unclear
with limited follow up. (4,5,11,12,15-18,20,43-45) Data supporting the single dose constraint of 10 Gy stating no RIM
was the report of the Medical Research Council of
114 patients by Macbeth et al that estimated the risk
of RIM to be zero by two years, (34) and single dose
constraint of 8 Gy stating no RIM were the reports of
the Hamburg University Hospital of 199 patients by
Rades et al and The Radiation Therapy and Oncology Group (RTOG) of 455 patients including spinal
patients by Hartsell et al. (46-48) The most recent and
solid data is by Ryu et al revealing Henry Ford Hospital experience on a partial volume tolerance of spinal cord after single dose of SBRT in 230 procedures
of 177 patients, where the spinal cord volume was
defined as 6 mm above and below the SBRT target.
They have concluded that partial volume tolerance
of the human spinal cord is at least 10 Gy to 10% of
the cord volume. (28)
As the data on reirradiation is also very limited,
Rades et al. reported their reirradiation of the spine
experience of 62 patients after initial course of 8 Gy
in one fraction or 20 Gy in 5 fractions with no incidence of RIM after second course of 8 Gy in one
Minimally Invasive Procedures In Spine Surgery
dose includes entire vertebral body and posterior elements. Two thirds of prescription dose is planned
to be prescribed to entire vertebral body and posterior elements.
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fraction, 15 Gy in 5 fractions, or 20 Gy in 5 fractions
with a median follow-up of 8 months after reirradiation (range 2-42 months). (49) Although retrospective, Rades et al noted that the cumulative BED2
≤120 Gy of spinal reirradiation appeared to be effective and safe. (50)
Conclusion
SBRT has becoming to gain a significant and promising role as a non-invasive modality to be used in
conjunction with other complementary modalities
in the management of spinal metastasis along with
the important collaboration of spine surgeons, and
radiation oncologists.
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