2B - Sim Technique

What kinds of sims were done before CT took over

Fluoroscopy and x-ray

What are the two steps of simulation

Localization and verification

Localization

Geometric definition of the position and extent of the tumor or anatomical structures by reference surface marks than be used for treatment set up purposes

Done at CT sim

Verification

Final check that each planned treatment beams from treatment plan covers the tumor volume and does not irradiate critical normal structures

Done on the treatment machine

CT sim

Views volumes in 3 dimensions (GTV, CTV, PTV, organs at risk)

Allows comparison of beams without patient present to get optimal beam placement

Beam’s eye view, DRR construction, calculation, and dose distribution

Can be used in image fusion

Limits for ct sim

Bore size

Patient centering

Dose per scan

Unable to see fields on patient during initial setup

Landmark for C3/C4

Hyoid bone

Landmark for C7

Bump on back of neck

Landmark for T2/T3

Sternal notch

Landmark for T4/T5

Angle of louis

Landmark for T10

Sternoxiphoid junction

Landmark for top of 12

pleural reflection

Landmark for L2

Lower external spinal cord

Landmark for L4

Iliac crest, umbilicus (hip projection)

Landmark for prostate/femoral heads

Pubic symphysis

Landmark for knee interface

Tip of patella

Steps of the CT sim process

1. Presim planning

2. Patient immobilization and positioning

3. Admin of oral contrast if needed

4. Mark reference point

5. Scout/pilot film

6. Selection of scan area

7. Admin of IV contrast if needed

8. Scan patient

9. Transfer of reconstructed images to treatment planning system

10. Contouring of critical structures and target volume localization

What kind of info can you find on the pre-ct sim encounter

Photo, diagnosis, weight, consent, site laterality, pregnancy status, oral/iv contrast, npo, allergies, bladder prep, marker placement, lab work, dental review

What kind of info can you find on the sim order

Protocol, patient position, immobilization, bladder, scan from and to, proposed field arrangement, number of isocenters, estimated number of fractions, when they will perform vsim, fusion needed, comments

Brain immobilization

Mask, head rests, knee cushion

SRS/SBRT = encompass mask

What is a neutral head rest

B and F

H&N immobilization

Mask, bite lock, head rest, knee sponge, hand pegs/straps

Thorax/abdomen immbolization

Knee sponge, wingboard, vacbag

Breast immobilization

Breast board, prone board

Pelvis immobilization

Headrest, vacbag, knee sponge

Prone board

Extremity immobilization

Vacbag, custom headrest/mask

SBRT immobilization

Full body vacbag with compression plate

Failure to center patient in the bore causes what

Image noise

Lateral and vertical mis-centering

Lateral = 3-6 cm causing 18-41% increase in dose

Vertical = 2-6 cm causing dose increase up to 140%

What is used to make reference point marking

Radioplaque markers

What usually determines the ct sim slice thickness

The department protocol

After the patient is scanned, what happens while patient still on table

Radiation oncologist has to approve it or make modifications

If iso is set in CT, what is the process

Reconstructed images sent to workstation

Physician contours tumor volumes and critical structures

Once tumor volume determined, isocenter is specified (most wait)

Coordinates of isocenter is transferred to laser system, then it is marked in ct

When can the final sim/treatment marks be made

Once the lasers and table have been moved to the appropriate isocenter location

What instructions need to be given to the patient

Time/date of treatments

Number of treatments

Machine if applicable

Special instructions like bladder/bowel prep

Contact number of front desk or scheduling

It may be necessary to change your treatment time, we will lyk if that happens

Don’t scrub away marks

Where does the reconstructed ct data go

To the treatment planning system and PACS

How is image fusion overlayed

Overlayed voxel by voxel

What kind of info is on the post ct sim encounter

Time out, sim photos, patient instructions, ct sim set up, sim note, treatment plan requests, schedule patient treatments, intiaite whiteboard, pacemakers, carepath, complete and bill appointment

What is determined in the computerized treatment planning process

The location of the isocenter, number of treatments, daily dose, energy, and orientation of beams

What thickness of ct scans is desirable for DRRs

No more than 5mm spacing between scans

Main goals in treatment planning

Maximize dose in the PTV

Minimize dose to surrounding uninvolved tissue

Provide homogeneous dose with less than 5% variation through the PTV

What is done in vsim

Evaluates treatment plan

Ensures planned beams cover anatomy or miss critical structures

Can be performed on sim or linac

Identify accurate transfer or treatment plan to linac

Ensure no errors in blocking that can cause local recurrence