Lecture 10: Craniospinal Axis

The CNS is surrounded by_____

meninges

List the meninges from innermost to outermost

1. Pia Mater - innermost

2. Arachnoid Mater

3. Dura Mater- Outermost

Cerebrospinal fluid flows inside the_______

subarachnoid space

Where does the spinal cord stop in

1. adults

2. children

1. Adults: ~L2

2. Children: may extend beyond L2

Main side effect we are concerned when treating Craniospinal Axis

Overtreating spinal cord may cause paralysis

Treatment/Immobilization Devices, Positioning

for craniospinal axis

• correction: bolster

A patient is generally treated (supine/prone)

Prone

It is possible to treat a patient's supine if:

They are treated with tomotherapy

Why do we use a vac-lok?

To build up the body to raise the chest & abdomen so that the face can be placed in a prone headrest WITHOUT bending back

Why must the chin be extended?

Reduce mandible exposure from PA spine field exit dose

Fields Used in CSA

1. Lateral Opposed Cranial Fields (1/2 Beam or Full Beam)

2. 1 or More: PA Spine Fields

Blocking used in lateral opposed cranial fields

German Helmet Blocking

If you treat the lateral opposed cranial fields with a full beam you will need:

To angle the couch to create a non-diverging beam inferiorly

____ of PA Spine Field will abut ____ of Cranial Field

Superior edge of PA Spine Field will abut inferior edge of Cranial Field

If more than 1 PA Spine Field is needed to cover the entire spine, what will be need?

A second abutment area between inferior and superior PA Spine Fields

What 2 things do we do to prevent a hot spot (overtreating the spinal cord) between the match lines?

1. Gap Formula: for the PA Junctions

2. Junction Shifts (FYI: field feathering): for the PA Junctions & the PA-Cranial Junctions

Explain what is going on here with the short and long fields

It is an example of Junction Shifts:

1. Junction Day 1: short cranial field length + long sup spinal field length + short inf spinal field length 

2. Junction Day 2: long cranial field + long sup spinal field + short inf spinal field

True or False: Generally, the same guidelines for brain and spine field borders are used

True

Blocking for cranial borders will be tighter. Why?

To assure that no portion of the brain is missed

Cranial borders:

Inferior edge will extend down to the spine itself, enough to allow for an abutting field and additional room for shifts

Daily Treatment:

What do you treat first. Cranial Fields or Spinal Fields

Cranial fields

Treating Cranial Field Steps

1. Align to lateral marks on mask

2. Assure patient is straight along ENTIRE length of body

3. Angle table

4. Make sure eyes are covered by block

5. Mark inferior matchline

6. Treat 1st lateral

7. Rotate Gantry: mirror table kick, check eye block, mark match line

MASA Escasa Me Trago Raza

Assure the patient is straight along the entire length of the body using sagittal laser. Marks you will align to:

1. L/R marks on mask

2. Marks on patient for spine field (may use lateral level marks)

3. Palpate spine to assure patient is straight

How should you mark the inferior match line (i.e. Cranial-Spine match line)

1. Use a dotted line

2. Use different colors for each day OR erase old marks

Treat Spine Steps

1. Rotate gantry to top

2. Move patient to align the spine (i.e. go to spine CR)

3. Set field size

4. Align superior border - marked match line from cranial fields

5. Set prescribed SSD

6. Match to superior marks, then shift to create gap (or mark the gap on skin and use a ruler instead but it’s less precise than reading the table & shifting to create the gap)

7. Visually check gap is correct (ruler)

8. Double check SSD

9. Mark inferior border for your next match line

10. Treat PA #1

11. Move Patient to next spinal field & repeat

RAFAS

Good

Chili

Sause

Must

Taste

Magnificent

Junction Shift Rules

1. Make sure you are using the correct marks (recap: by using different color marks for different days or by erasing old marks)

2. Assure the correct set of shifts are used together

3. Field size changes, but CR WILL REMAIN THE SAME

Field #1 with an increase in field length will equal

A field decrease in field 2 and an increase in field 3 (long, short, long)

↑ Field1 = ↓Field2 = ↑ Field3

Decrease in field #1 length will cause

an increase field #2, decrease in field #3

↓ Field1 = ↑Field2 = ↓ Field3

CSA Simulation

Which field should be simed first?

The most inferior spine field

Why should we sim the most inferior spine field first?

all field borders are tied to the inferior border of this field (note: makes sense since the most superior border is the brain border which is flash)

Procedure for Conventional Simulation of CSA overview

Part 0: Setup

Part 1: Inferior PA Spine Field

Part 2: Superior PA Spine Field

Part 3: Lateral Opposed Cranial Fields

Part 0: Setup (Sim)

1. Consult physician & Meet Greet Evaluation

2. Initial Positioning

Part 0: Setup (Sim): Consult physician & Meet Greet Evaluation

1. Evaluation will involve Physicist, Dosimetrist & Physician.

• Whenever possible, most decisions will be made in advance

• The rest will be made with the patient present to ensure proper customization

2. Evaluate patient: see if more than 1 spinal field is needed

3. Evaluate placement of spine-cranial junction

4. Evaluate the type & amount of immobilization needed

Part 0: Setup (Sim): Initial Positioning

1. Get Patient patient on table in prone position & on correct immobilization device(s)

2. Assure patient is straight using sagittal laser. Following a line between: feet, legs, center of pelvis, sacrum, spine & neck.

3. Assure head tilt is sufficient to keep mandible out of the PA spine exit dose

Part 1: Inferior PA Spine Field

1. Ballpark: Start with inferior spine field to include physician-defined inferior edge

2. Fluoro

3. Notate

4. Mark CR

5. Verify SSD

6. Film

7. Get films approved by Dr

8. Mark the rest of relevant field marks on patient (mark everything specially superior border I guess)

Part 2: Superior PA Spine Field

1. determine field length

2. Ballpark

3. Fluoro to assure you are straight etc.

4. Determine gap & execute

5. In gapped position: film it, check it, mark it

6. Place wire on superior border of the superior spine field

FYI for 4:

1. You have set a specific field size for the inferior PA spine. Which gives us a superior border for the junction with the middle PA spine field.

2. We kinda know where the superior junction between the middle PA spine field & the cranium is going to take place, so we can set a field size there too.

3. Now, we got 2 field sizes, if we have 2 field sizes the dosimetrist can calculate the gap with the gap formula. Remember the gap formula requires knowing the field Length (and SSD & depth in tissue) so you need to know all your field sizes in order to calculate for the gap

Lazy

Bears

Fry

Green

Fish

Weekly

Part 2: Superior PA Spine Field: In gapped position: film it, check it, mark it. What marks might we add

Lateral level marks on hips & thorax

Note: Lateral level marks must be on the same plane for both hips & thorax

i.e. don't put lateral leveling marks at one depth for one, and at another depth for the other. Just have it run all the way along. It does not need to be SSD dependent as long as you have the marks on the ribs and down the hips. )

Why do we place a wire on the superior border of the superior spine field?

will help determine table kick and collimator rotation for the cranial fields)

Part 3: Cranial Fields

1. Swing to lateral

2. Ballpark whole brain borders with inferior edge at the wire (fyi:Superior PA-Cranial field wire)

3. kick table so inferior brain border is clinically aligned with wire, then turn collimator to complete match

   1. stair step table & collimator until both are aligned clinically

   2. fluoro to check for wire match

      

4. Notate

5. Mark CR

6. verify SSD

7. film

8. get films approved by Dr

9. Swing to opposing lateral: mirror table kick & collimator angle

10. Repeat steps

11. Take photos

12. Educate

Procedure for CT sim

the same