Brain Teaser #2

A) Question:

A large patient is being treated for prostate cancer. The patient is supine, and on the right lateral an SSD of 78.5 cm was measured. On the left lateral an SSD of 78.0 cm was measured. In the lateral plane, what is the separation for this patient?

A) Answer:

The separation in the lateral plane is 43.5 cm

Follow Up:

Looking at the diagram below will hopefully explain how to come up with this answer. You know isocenter to the target is 100 cm, and that the SSD also called TSD is from source/target to skin surface. If you can not do this problem in your head, drawing it out helps a lot!

B) Question:

If the target to blocking tray distance is 58 cm, then on the left lateral, which had an SSD of 78 cm, what is the distance from the blocking tray to the patient's skin?

B) Answer:

The blocking tray to the patient is 20 cm away.

Follow Up:

This can be answered best by doing a digram. Hopefully, you will be able to follow my digram to help explain how to come up with this answer.

C) Question:

Do you have to worry about electron contamination, yes or no?

C) Answer:

no, you don't have to worry about electron contamination.

D) Question:

What is electron contamination?

D) Answer:

Electron contamination is the term for electrons being created in the photon beam. It occurs because of the photons interacting with air, the collimator, and anything else in the way of the beam, such as blocks, trays, and wedges, thus creating electrons which are then scattered.

E) Question:

At what distance do you have to worry about electron contamination possibly minimizing the skin sparing effect observed by photons?

E) Answer:

The blocking tray to patients skin should be greater then 15 cm, to keep the skin dose to an acceptable level using photons.

Follow Up:

This answer is based on Khan, the Physics of Radiation Therapy, page 325, second edition.

F) Question:

Is electron contamination ever useful, yes or no?

F) Answer:

Yes, electron contamination can be useful.

G) Question:

When is electron contamination useful?

G) Answer:

Electron contamination can be useful for Total Body Irradiation (TBI).

Follow Up:

In a TBI in order to get an adequate skin dose a spoiler may be used to create electrons thus causing electron contamination in the photon beam. For this reason, it can also be useful when treating tangential breast fields with a high energy beam. If the physician is concerned that too much sparing is ocurring, then a beam spoiler may be used to ensure the skin is getting adequate dose.


Author: Adam Buell
Editors: Mary Hare and Janelle Plavchan
Date Created: February 2005