Intrafractional Junction Shifts Utilizing Multileaf Collimation: A Novel CSI Planning Technique Rodney Hood RT(R)(T)CMD
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Quiet Room Beam me up Scotty!
What is CSI?
CSI-DURHAM!
CSI Craniospinal irradiation (CSI) is a technique used in radiation therapy to deliver a prescribed amount of radiation to the entire cranial-spinal axis to achieve curative measures in the treatment of intracranial tumors.
Why is CSI Necessary? Curative surgical excision of malignant intracranial neoplasms is rarely possible This tumor spreads malignant cells throughout the subarachnoid space via the cerebrospinal fluid (CSF) Entire central nervous system must be treated (CSF) Evans, A.E. et al. The Treatment of Medulloblastoma. J. Neurosurg. 72:572-583; 1990
CSI Craniospinal Irradiation Treats anywhere CSF flows Treatment fields typically include the brain to the thecal sac
CSF Cerebrospinal fluid Clear bodily fluid found in the brain and spine Produced by choroid plexus of the brain Provides mechanical and immunological protection for brain Cerebral auto regulation of cerebral blood flow Occupies the subarachnoid space between arachnoid and pia mater Constitutes the contents of the cisterns, sulci and brain ventricles as well as the central canal of the spinal cord.
Cerebrospinal Fluid (cont.) Brain produces around 500ml of CSF daily Fluid is constantly reabsorbed Usually about 100-160ml present at any one time CSF turns over around 4 times a day As always, all info ripped directly from Wikipedia
Craniospinal irradiation (CSI), combined with chemotherapy and surgery, is the current choice of treatment for malignant brain tumors - medulloblastoma - ependymoma - Germinoma (prophylactic)
CT Simulation Setup and Scanning
Wedge and sponge combination to extend mandible
Styrofoam pad used to build up patient to reduce cervical and thoracic spine curvature
Long Orfit mask to immobilize head and shoulders
CT zero marked along midline
No Styrofoam pad to build up spine
Styrofoam pad to build up spine
Why do we feather the junction? Uncertainty Possible setup error Possible patient movement
Can you identify the pathology?
According to a study performed at Harvard Medical School that assessed the accuracy and precision of patient setup during treatment with a plan involving feathered fields, the magnitude of the spatial stochastic and systematic setup error was determined to be approximately 3 and 2 millimeters respectively. Holupka, E.J. et al. Effect of Set-Up Error on the Dose Across the Junction of Matching Cranial-Spinal Fields in the Treatment of Medulloblastoma. Int. J. Rad. Onc. Biol. Phys. 27:345-352; 1993
Moving the junction (feathering) is a common technique utilized in radiotherapy to smooth out any dose inhomogeneity across the junction of the abutting fields Doses along a border of two fields - too much dose (a hot spot) due to an overlap - under dosing (a cold spot) from too wide a gap The gap must be feathered in order to decrease the possibility of such hot or cold spots from occurring
Feathering 2 Types In order to evenly distribute dose along field junctions the junction/gap is moved 2 ways to feather Inter-fractional Intra-fractional
Difference between inter and intra The prefix inter means between The prefix intra means within
Inter-fractional Shifting the junction in-between treatments
Intra-fractional The fields decrease or increase during the treatment to move the field junction
Dosimetric Advantages Utilizing Intrafractional shifts is superior dosimetrically because it is much more forgiving for setup error and patient movement. When errors are introduced, hot and cold spots are reduced utilizing this technique
Treatment Advantages In conventional CSI treatments each junction shift requires a new setup Intrafractional treatments use the same plan throughout the treatment so the long initial setup only needs to be done on the first day Amount of imaging is reduced greatly
Field Setup Given the entire spine and brain must be treated, multiple fields must be used to encompass the entire volume comprising the brain and the spinal cord
Traditional method
FiF (Field-in-Field)
Brain field creation Set iso as low as possible while maintaining flash on vertex of skull (y2=20)
Brain field creation I try to set iso around c3
Brain field creation Set couch to match divergence
Brain field creation
Brain field creation Add MLC
Upper spine field creation Ensure no exit through mandible
Upper spine field creation Keep same x & y-coordinate ( left, right and ant/post ) as brain field
Upper spine field creation I like to set lower border at bottom of L2
Upper spine field creation Collimator of brain field is set using upper jaw size 18/100=.18(inv_tan)=10.2
Lower spine field creation I like to half beam this field
Lower spine field creation Set iso maintaining same x&y coordinates
Lower spine field creation Kick table to 270 Rotate gantry to match divergence of upper spine field Length of upper spine field will determine gantry angle Matching divergence is dosimetrically advantageous
Angled Lower Spine Field
Angled lower spine vs. Pa low spine PA fields converging anterior to cord Post field matched to upper spine field divergence
Don t forget the Golden Rule ALARA As Low As Rodney Achieves!
Setup Field Junctions First decide # of junctions needed This is dependent on total dose We feather every 9Gy or once a week So a 27Gy course would require 2 junction shifts Jaws must be opened to largest aperture size because we will be merging fields to create a step and shoot delivery Utilizing 1cm junctions feathering would require 2cm of field overlap We use a 5mm safety gap so the overlap region is 1.5cm at both junctions 1.5cm overlap
Setup Brain Subfields First subfield is open to largest aperture size For next subfield pull two MLC s to decrease aperture size by 1cm For the last subfield pull an additional 2 MLC s to decrease aperture by another 1cm Fields are closed by 1cm to match the larger MLC in upper spine field Dose is calculated and subfields are merged
Subfields merged
Setup Upper Spine Fields Setup upper spine field so the aperture shrinks on both superior and inferior borders
Setup Lower Spine Fields Setup lower spine field similar to upper but only modify aperture at superior aspect
Dose Calculation Dose is calculated to midplane of brain fields Dose to each spine field is normalized to a specific depth by the Rad Onc by averaging depths along the spinal axis. Two separate spine fields are desirable when spinal canal depths vary greatly from upper spine and lower spine segments
Gap Calculations
Upper/Lower spine Junction verification During planning the PA upper spine field as well as the PSO lower spine fields are set to gantry 0 The distance between the lower border of the upper field and the upper border of the lower field is measured in the TPS I also like to contour the location of the BB s so that DRR s can be created from the treatment fields This eliminates hand calculations
Calculated Gaps Gap and Couch Position Report
Single Spine Field
Treatment
First Day (Dry Run) Pt is setup identical to CT simulation. Make indicated iso shifts Start with the brain and work inferior From this iso all shifts are longitudinal only Image brain (I prefer KV) Adjust pt as needed-i.e. shifts or pt adjustment Make calculated inferior shift from brain iso to upper spine field Image and verify match Make calculated shift from upper spine iso to lower spine field Image lower spine No adjustment to spine iso s are made only pt postition
Dry Run (cont.) After all initial kv imaging is complete an MV of each tx field is taken After MV imaging of the brain a BB is placed on the center of the inferior border We now shift inferiorly to the upper spine field The field is imaged to verify blocks and the position of the BB s placed on the inferior aspect of the brain field This field is now rotated to 0 and the field is drawn onto the patient A BB is placed at the inferior aspect of this field We now shift to the lower spine field This field is imaged to verify blocks and the location of the BB. This field is rotated to 0 and drawn on The anterior skin gap is measured between the upper and lower spine field and verified against the calculated distance
Junction Verification-Brain/Upper Spine BB s are placed on inferior border of brain field, portal image of upper spine field is taken and BB distance is verified in off-line review.
Anterior Skin Gap Verification Rotate gantry to 0 and mark field edges on skin verify the gap on the skin against the numbers instructed
Upper/lower spine verification
5mm gap Dosimetry No gap
Dosimetry-evaluating junction doses
Aligned/Misaligned Static 1cm overlap
Aligned/Misaligned FiF 1cm overlap
Introduced Errors Perfect Alignment 3800 3750 3700 Dose (cgy) 3650 3600 3550 FiF Static 3500 3450 3400 0 1 2 3 4 5 6 Distance (cm)
Introduced Errors 5mm Gap 5000 4500 4000 Dose (cgy) 3500 3000 2500 2000 1500 FiF Static 1000 500 0 0 1 2 3 4 5 6 Distance (cm)
Introduced Errors 5mm Overlap 8000 7000 6000 Dose (cgy) 5000 4000 3000 Static FiF 2000 1000 0 0 1 2 3 4 5 6 Distance (cm)
Introduced Errors 1cm Overlap 9000 8000 7000 Dose (cgy) 6000 5000 4000 3000 FiF Static 2000 1000 0 0 1 2 3 4 5 6 Distance (cm)
Introduced Errors 1cm Gap 4500 4000 3500 Dose (cgy) 3000 2500 2000 1500 FiF Static 1000 500 0 0 1 2 3 4 5 6 Distance (cm)
TREATMENT VERIFICATION Gotta Be Right! 74
Delivery Verification-Portal Dosimetry Brain Upper Spine Lower Spine
Delivery verification Be mindful of where MLC are parked
Park MLC outside of patient or under jaw
Brain/Spine Junction Film Dosimetry
Spine/Spine Junction Film Dosimetry
Conclusion Shifting the junction intrafractionally reduces treatment setup time by limiting the number of setups needed. This technique is also superior dosimetrically because it reduces the variation in dose across the junction. Lastly, shifting in this manner is more forgiving to setup error and patient movement.
Questions?
Acknowledgements Justus Adamson, Ph.D. Taoran Li, Ph.D. Irina Vergalasova, Ph.D. Kevin Kirby, CMD Scott Green, CMD https://www.youtube.com/watch?v= dui8novkqbq