SSD dose calculations week 5

Why is it important to understand the factors that affect radiation treatment delivery?

• Wrong settings = Too much or too little radiation → Bad for the patient!

• factors include field size, beam energy, and things in the beam’s path (like trays, wedges, and compensators).

• • Tiny mistakes = Big dose changes

  Using the right dose rate, scatter factors, and absorption factors keeps treatment safe and accurate.

• how tissue absorbs radiation

Dose delivery type: photon beam radiotherapy is carried out under two set up conventions

Constnce source surface distance- SSD

Or isocentric source axis distance- SAD

Non isocentric

Not established at axis of the beam

• SSD = Source-to-Skin Distance

• You keep the distance from the machine to the patient’s skin the same for every beam.

• This means:
  

  • The patient may need to be moved for different beam angles.

  • The isocenter is placed on the skin, not inside the body.

  • You still deliver dose to a certain depth—just calculated differently.

SSD (Non-Isocentric) Technique – When to Use It

Use SSD setup for:

• Electrons – always use SSD.

• Shallow tumors (superficial treatments).

• Emergencies – faster setup.

• Single beams (like AP chest field).

• When distance needs to be longer (extended SSD). designed around surface distance, not an internal point (like SAD/isocenter techniques).

What do Mus represent

like a gas pump for radiation therapy!

⏺ MUs = The setting on a linear accelerator.

⏺ They control how much radiation is given.

⏺ Ensure the correct dose reaches the patient.

⏺ Adjust for dose rate and treatment conditions.

State the three points that are needed for a treatment (dose) calculation.

Think of it like filling a glass of water:

1. Dose at a Point – How much water you need (prescribed radiation dose).

2. Dose Rate at That Point – How fast the faucet flows (rate of radiation delivery).

3. Same Medium – Make sure it’s the right glass (dose and dose rate must be in tissue).

Describe a SSD/Nonisocentric treatment

• Fixed Distance: 80 cm (Cobalt-60), 100 cm (Linac)

• Non-Isocentric: Beam stops at skin, not deep inside.

• Field Size: Measured on the skin, not internally.

• Uses PDD (Percent Depth Dose) for dose calculation.

• Good for superficial tumors & skin treatments.

From a calculation standpoint, what is a disadvantage to SSD treatments?

1. Distance Changes? Extra Math!

• If SSD isn’t exactly 100 cm, PDD must be recalculated.

2. Needs More Corrections!

• Mayneord Factor adjusts PDD for different SSDs.

• Inverse Square Correction adjusts for dose rate changes.

3. More Calculations = More Work!

• SSD treatments require extra steps if the setup isn’t perfect.

When is Extended Distance Treatment Needed?

For Large Fields – Covers a bigger treatment area.

✔ When You Can’t Get Close – Physical obstacles in the way.

✔ Useful for Whole-Body or Large Tumor Treatments.

Tissue Absorption Factors – What You Use Depends on the Setup

If you’re using SSD (Non-Isocentric):

• Use PDD (Percent Depth Dose)
  

  • Based on distance from the skin.

If you’re using SAD (Isocentric):

• Use TAR, TMR, or TPR
  

  • Based on distance from a point inside the body (isocenter).

  • No need to keep SSD constant.