Lecture 8 B: Head and Neck: non-monoisocentric/non-half beam

True or False: In a non-monoisocentric setup. The jaws are typically NOT asymmetrical jaws.

True

A non-monoisocentric AP SC field will have some way of blocking the superior edge of the field that you would normally just close down with an asymmetrical jaw.

What would we use to create a half-beam for the AP SC?

A cerrobend block

True or False: Because we don’t have asymmetrical jaws, you will ballpark the AP SC with an open field

True

how does a non-monoisocentric setup look?

Steps for Setting up a Non-Monoisocentric Setup.

Steps: Consult Dr

Steps: Meet, Greet, Eval

Steps: Position

1. Get Patient on table: Swing legs, center on table, lay them back.

2. Assure patient is straight using sagittal laser: line between feet, legs, center of pelvis, xiphoid process, SSN, chin, nose, glabella.

3. Create tongue blades, bite blocks, or mouth devices

4. Create Mask

How do you create the mask?

1. Apply shoulder retractors

2. Assure that chin is extended (up) and straight 

3. Assure that laser follows along the same lines 

4. Apply mask.  Two therapists may be required to properly make long mask.

   

Step: Ballpark AP SC

1. Set CR sup/inf: at thyroid notch

2. Set field length: just inferior to SSN

3. Set field width: to include the lateral aspect of both clavicles

4. Set ant/post: just Posterior to the angle of the mandible OR Set 100 SSD at your AP SC CR

Step: Fluoro AP SC

Step: Call Dr

Step: Notate

What do you notate?

Everything INCLUDING table vertical

Step: Mark AP SC CR

1. Mark AP SC CR

2. Mark inferior border

3. Put wire along superior border

Why do we put a wire along the superior border?

It will determine the couch angle for the laterals

After doing all the marks on AP SC, what step is next?

set supraclav dose point

Where is the Supraclav dose point?

3 cm superior & 3 cm lateral to SSN

What steps do we take after setting dose point?

step: Verify SSD is at 100

Step: Film AP SC

Step: Have film approved by Dr

Step: Mark rest of relevant field borders

Step: Rotate to lateral

Step: Ballpark lateral

1. Set length: by moving patient sup/inf & changing f.s.

   1. Superior border: at EAM

   2. Inferior border: flashing across wire

Note: CR won’t be in the same place since you are moving the patient in/out

2. Reduce width: posterior to lips

3. Rotate table

You rotate the table, feet (away from/towards) the gantry until:

Towards

until inferior border matches wire with no divergence- double check this on flouro

Steps: Fluoro lateral

Steps: Call Dr

1. Adjust field length

2. Adjust depth if needed

What must you do if you adjust the field length?

Compensate by moving the table sup/ inf as well.

FYI:

Remember: your jaws are not asymmetrical. If what you are ballparking doesn’t match radiographically, you’ll have to change your field size. Both Y1 and Y2 will shrink down or open up. So you will have to move the table in/out to make sure your superior border is at the EAM and inferior border is flashing across the wire.

Steps: Notate

What do you notate?

Everything Including table vert & Table Angle

Note: you will have 2 table verticals. 1 for the APSC and another for the lats

Steps after notating:

Step: Mark Lateral CRs

Step: Set Cervical Dose Point

Where is the Cervical Dose point located?

1. ½ way between sup and inf border

2. 1-2 cm ant to post border.

   
   

Steps after setting up Cervical Dose point.

Step: Read Lateral CR SSD

Note: It will be in the middle of the patient’s head instead of the neck

Step: Film

Step: Have film approved by DR

Step: Mark rest of relevant field marks on patient

Step: Rotate to opposing lateral

What must you do when you rotate towards the opposing lateral?

Mirror table angle/kick

Repeat steps: ballpark → mark rest of relevant field borders on patient

Skip Cervical Dose Point step

True or False: If you already filmed your AP SC, you are pretty much done

TrueW

What step is next?

1. Check SSDs @ each dose point

2. Take separation at each dose point (no mask)

Remaining steps:

Step: photos

Step: Educate

What is the tiny box with crosshatching on the bottom right?

A safety block

Where does the doctor draw the safety block? Why?

On either lateral or AP SC

To protect the spinal cord from divergence/ hot spot

Even with a safety block in place, you might still have a:

Junction Shift

FYI: aka feathering

What is the purpose of the junction shift?

Move the junction/matchline every couple of weeks to reduce hot spots

What moves in a Junction shift? What remains the same?

1. Match line will be moved to a different location

2. The overall sup/inf aspect of the treatment volume remains the same

   i.e. the inferior SC border and superior lateral border remain the same. Those borders cannot move

Junction shift steps

1. Lateral field will have a decrease in field size by 1 cm

2. AP SC field will have an increase in field size by 1 cm

3. CR & match line for each field will be moved by 0.5 cm in the same direction

4. This will equal to an overall match line shift of 1 cm, maintaining the original Sup/Inf border

An Electron Boost is needed for:

Posterior Cervical Nodes

Add a cord block after ____cGy

4500

True or False: After you add a cord block, you still need to treat the cervical nodes to a higher dose.

TrueW

How do you treat the cervical nodes after adding the cord block at 4500cGy?

With an electron field

What size and shape is the electron field?

Same size and shape as the cord block

What are the field borders for the block & electron boost?

Anterior: Abuts to the new posterior photon field

Posterior: Where it used to be

Superior: Where it used to be

Inferior: Abuts to the SC matchline

Simulation for the cord block and electron boost is done on

A. Simulator

B. Treatment Machine

C. Neither, Drawn on radiographs

B. treatment machine

When you are simulating on the tx machine:

1. Draw the cord block.

2. These constraints are simulated for an electron field.

What other things might you need to do?

1. May need to tx at an extended SSD (in order to have shoulder-tray clearance)

2. May need to pour the block as far posterior & inferior as possible in the template to allow for more clearance