Download Brittany Check Clinical Oncology June 23, 2015 Craniospinal

Brittany Check
Clinical Oncology
June 23, 2015
Craniospinal Assignment
Craniospinal treatment typically involves two lateral whole brain fields and two long posterior
spinal fields. Setup can be challenging because of the beam divergence and matching fields over
the spinal cord. Historically, patients were treated prone to allow for gap measurements on the
patient’s back. With CT simulation, supine positioning has become more common. At
Gundersen Health System, supine positioning is used for craniospinal patients. This assignment
refers to supine positioning when applicable and more specifically, techniques employed at
Gundersen Health System. A typical collimator setting of 0 degrees, or a slight rotation off of 0
degrees, is assumed when specific jaws are referenced in equations.
1) If the patient were positioned prone, a board is usually placed under the chest to build up the
lower torso. What does this accomplish?
When lying in a prone position, the natural curves in the spine cause the treatment distance to
vary from the cervical, thoracic, and lumber regions of the spine. When treating the spine with a
posterior field such as in craniospinal irradiation, it is best to have a relatively constant treatment
distance to achieve a more uniform dose.1,2 To help level the spine, devices such as a board or
Styrofoam may be used under the chest to help align the head and torso.3 This may also allow the
patient to more comfortably extend their chin away from their chest to prevent divergence from
the upper spine field exiting through the mouth.
2) How do you match the spine and head ports for a craniospinal setup? BE SPECIFIC. Give me
the formulas used to determine any angles and give an example of using the formula(s). Provide
a diagram or drawing.
In order to match the brain and upper spine ports for a craniospinal setup, collimator and couch
rotations are used to account for beam divergence. The collimator rotation is necessary to align
the inferior border of the brain ports with the divergence from the superior jaw of the upper spine
field. This alignment prevents overlap from the upper spine field and the anterior-inferior region
of the brain fields.3 The matching of the divergence can be seen in the Figure 1 below.
Figure 1. Collimator rotation matches divergence of upper spine port.
The degree to which the collimator is rotated depends on the superior jaw setting of the upper
spine field and the treatment distance (SSD or SAD). Trigonometry can be used to determine the
collimator rotation by using the following equation:
tan−1 (
𝑌2𝑈𝑝𝑝𝑒𝑟 𝑠𝑝𝑖𝑛𝑒
)
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒
For example, if the superior jaw setting of the upper spine field is 19 cm and an SAD setup is
used, the collimator rotation can be found as demonstrated below:
19
tan−1 (100) = 10.8°
The collimator rotation is opposing for the right and left brain ports. The calculated value is
subtracted and added to the collimator setting of 0 degrees to determine the setting used to match
the upper spine field divergence.
A couch rotation matches the divergence from the inferior jaw of the brain fields to the superior
jaw of the upper spine field. This matching prevents overlap in the superior and lateral aspects of
the upper spine field. The matching can be seen in Figure 2 below.
Figure 2. Couch rotation matches divergence of brain fields.
The degree to which the couch is rotated depends on the inferior jaw setting of the brain fields
and the treatment distance (SSD or SAD). Likewise, an equation can be derived using
trigonometry to determine the rotation of the couch:
𝑌1𝐵𝑟𝑎𝑖𝑛
tan−1 (
)
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒
For example, if the inferior jaw setting of the brain fields is 11 cm and an SAD setup is used, the
collimator rotation can be found as demonstrated below:
11
tan−1 (
) = 6.3°
100
The feet of the patient are rotated towards the gantry for each of the brain ports as seen in Figure
2. The calculated rotation is added to 0 degrees to determine the couch rotation setting for the
right brain port in the supine position, and subtracted from 360 degrees to determine the couch
rotation setting for the left brain port in the supine position. The combination of the collimator
and couch rotation allow for matching of the brain ports and upper spine field. The longitudinal
shift between the isocenters is discussed in question 4.
3) If you wanted to remove any divergence from the eyes in the cranial port, how would this be
accomplished? Why would you do this? Show a formula and how it can be used. Provide a
diagram or drawing.
Irradiating the eye can cause blindness or cataracts, so it is important to avoid the eye when
possible. Divergence into the eyes can be reduced by placing the central axis near the eye.
However, rotating the couch will move the contralateral eye inferiorly and into the field. To
account for the divergence near the anterior jaw, a gantry rotation can be used. Unfortunately, the
superior-inferior orientation of the eyes can only be adjusted with a couch rotation which is
already defined in the setup.3 This concept is demonstrated in Figure 3 below:
Figure 3. Gantry and couch rotation effect on eye motion.
To align the eyes in the anterior-posterior direction with the gantry, the gantry must be rotated to
the degree in which the anterior jaw of the brain field diverges. This is only effective if the
patient’s anatomy and simulation result in both eyes an equal distance from the patient support
assembly. The distance from the isocenter to the posterior aspect of the eyes as well as the
treatment distance (SSD or SAD) are used to determine the gantry angle. The following equation
can be used to find the degree to rotate gantry anteriorly of the lateral field in the brain ports:
sin−1
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒𝐼𝑠𝑜𝑐𝑒𝑛𝑡𝑒𝑟 𝑡𝑜 𝑒𝑦𝑒𝑠
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒
4) In your own words describe the setup for a CSI adult patient (specify prone or supine) where
two spine ports must be matched that extend to the bottom of S2.
The following parts to question 4 are in reference to a supine setup.
Pretend you are telling the therapist everything that is needed during the CT simulation. (How
the patient should be positioned. Do not forget to include all the devices used, head position,
chin position, arms, etc.)
To setup a patient for craniospinal treatment at Gundersen Health System, place the patient in a
supine position and straighten them using the lasers. Remove their dentures if they have any.
Extend the patient’s chin superiorly to prevent the upper spine field from exiting through the
mouth. Make a custom neck rest and immobilization mask of the head to prevent turning or
moving of the head. Be sure that the mask is indexed to the table for reproducibility. Place the
patient’s arms at their sides and be sure they are supported by the table. If the patient needs
boards to help support their arms, be sure that the boards are not under the patient. Make a vacloc bag to immobilize the patient’s pelvis and legs. Make sure that the patient is comfortable so
that they are able to remain still for the duration of treatment. Once the patient is positioned,
mark the central axis and place radiopaque markers in a stable location in the center of the head
using the room lasers. Add leveling marks in the patient’s chest and lower spine area. Use the
sagittal laser to add straightening marks from the patient’s torso through their pelvis to use to
straighten the patient prior to treatments. Scan the patient with 2.5 mm slices from the top of
their head through the bottom of the sacroiliac joints. Take a reference photograph of the
patient’s position and record the table top measurement.
For treatment planning, approximately where will you place the isocenter for each field for the
patient above, will the isocenters be moved? Why or why not? What are the approximate field
borders?
Although the brain ports, the upper spine field, and the lower spine field will all have separate
isocenters, it is ideal to keep the couch lateral and vertical the same if possible. This helps
minimize extra steps and reduce confusion during the treatment. The isocenter for the brain field
will typically be placed at about the center of the eye to minimize beam divergence across the
eyes. Additionally, a slight gantry angle may be utilized to minimize this divergence. Therefore,
the vertical location of the isocenter may correspond with the level of the eye, otherwise may be
placed at the depth of the upper spine field. The lateral position should be approximately
midbrain. Generally, the reference markers are a good starting point since the therapists have
centered, straightened, and marked the patient along this plane during simulation. After the brain
isocenter is set and the appropriate couch and collimator angles used, the brain fields and
blocking are set by the physician. The longitudinal position of the upper spine field depends on
the inferior border of the brain fields since they are abutting. The upper spine field length is set
with maximum Y-jaw settings of 19 cm to allow for feathering. The longitudinal shift from the
brain isocenter to the upper spine isocenter follows this equation:
𝑌1𝐵𝑟𝑎𝑖𝑛
+ 𝑌2𝑈𝑝𝑝𝑒𝑟 𝑠𝑝𝑖𝑛𝑒
cos(𝐶𝑜𝑙𝑙𝑖𝑚𝑎𝑡𝑜𝑟 𝑎𝑛𝑔𝑙𝑒)
Asymmetric jaws or multi-leaf collimation (MLC) may be used to shape the field if the spine
field is not exactly centered on the patient. The superior edge of the upper spine field should
align with the inferior edge of the brain fields. The lower spine isocenter is selected by setting
the superior jaw to match at the depth specified by the physician and setting the inferior jaw to
the bottom of S-2. The depth is most commonly mid-cord. Again, the lower spine isocenter is at
the same vertical and lateral as the brain and upper spine isocenters. The longitudinal shift from
the upper spine isocenter to the lower spine isocenter follows this equation:
𝑌1𝑈𝑝𝑝𝑒𝑟 𝑠𝑝𝑖𝑛𝑒 + 𝑌2𝐿𝑜𝑤𝑒𝑟 𝑠𝑝𝑖𝑛𝑒
The field borders of the brain fields encompass the brain and one to two segments of the cervical
cord.4 The field is designed similarly to that of whole brain irradiation; to flash posteriorly,
superiorly and anteriorly, and blocking along the orbital ridge, the lateral canthus of the eye,
through the external auditory meatus, to the mastoid tip. The subsequent spine fields abut to the
previous fields and encompass the vertebral bodies with about a 2 cm margin laterally.
Pretend that you must give the therapist a detailed description for treatment (feathering the
gaps) for the patient above. How will the fields be feathered during treatment?
The prescription dose for craniospinal treatments is often close to the spinal cord tolerance.
Because of this, matching fields on the spinal cord can be worrisome because of the risk of hot
and cold spots. In order to spread out the dose in the junction region, the fields may be feathered.
To feather the fields, the isocenters are not changed. Rather, the field sizes are adjusted in the
superior and inferior directions by 0.5 cm or 1 cm. For example, for treatment one the field sizes
may be at their original locations. For treatment two, the junctions between the brain and upper
spine as well as the upper spine and lower spine may shift inferiorly 1 cm. For treatment three,
the junctions may both shift superiorly 1 cm. Tables 1 and 2 demonstrate the jaw changes for an
inferior junction shift and superior junction shift.
Brain fields
Y1 + 1
Upper spine
Upper spine
Lower spine
superior
inferior
superior
Y2 - 1
Y1 + 1
Y2 - 1
Table 1. Jaw changes for inferior junction shift.
Brain fields
Y1 - 1
Upper spine
Upper spine
Lower spine
superior
inferior
superior
Y2 + 1
Y1 - 1
Y2 + 1
Table 2. Jaw changes for superior junction shift.
If there are modified fields, it is important to ensure that they are shifted as well and that the
multi-leaf collimation (MLC) is opened when the jaw are opened. Since the fields with the
junction shifts are separate fields, it can be helpful to set up a treatment calendar or schedule only
the fields that should be treated on a given day to avoid confusion.
References
1. Discussion with John Wochos, Medical Physicist at Gundersen Health System. June 26,
2015.
2. Discussion with Casey Abing, Medical Physicist at Gundersen Health System. June 26,
2015.
3. Bentel GC. Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996.
4. Khan FM, Gerbi BJ. Treatment Planning in Radiation Oncology. 3rd ed. Philadelphia,
PA: Lippincott, Williams & Wilkins; 2012.