Brain Teaser #3
A) Question:
A patient is in the prone position and is being treated for rectal cancer. A three-field technique (posterior, Rt-lat, and Lt-lat) is being used. Using basic dosimetry knowledge, would you typically expect to see a wedge or wedges on this set up?
A) Answer:
Yes, you would expect a wedge or wedges.
B) Question:
On what field(s) would you expect a wedge(s) (PA, RT-LAT, or LT-LAT) to be used?
B) Answer:
You would expect a wedge on the right lateral field, and you would also expect a wedge on the left lateral field.
C) Question:
Anatomically, what way would you expect the wedge's heel to be oriented on the the left lateral? And, what way would you expect the wedge's heel to be oriented on the right lateral?
C) Answer:
You would expect to see the heel oriented in the posterior direction for the right lateral, and you would also expect to see the heel oriented in the posterior direction for the left lateral.
D) Question:
For one day the wedges were forgotten by the therapist. Anatomically, where would you expect the dose to be the "hottest"?
D) Answer:
The "hot spot" would be in the posterior portion of the patient.
Follow Up:
This drawing is to help show the orientation of the wedges in a cross sectional slice.
E) Question:
If the patient has not had surgery, what critical structure would get an increase in dose if the wedges were left out for one day?
E) Answer:
The rectum is the critical structure that would get overdosed if the wedges were missing.
F) Question:
If the rectum dose being carried is 175 cGy per day, and the rectum got 43.2 % more dose for that one day the wedges were missing, then what dose did the rectum receive for that one day?
F) Answer:
The rectum received 250.6 cGy for that one day the wedges were missing.
Follow Up:
You can set up a simple proportion to help you find this answer.
--------- = ---------
100 % 143.2 %
G) Question:
You are carrying the rectal dose at 175 cGy per day, and there will be 25 fractions for the first phase of treatment. What will the rectum receive in overall total dose if for that one day the rectum received 250.6 cGy?
G) Answer:
The rectum would receive 4450.6 cGy.
Follow Up:
The best way to do this is take (24 fractions)(175 cGy/fraction) = 4200 cGy.
Then 4200 cGy + 250.6 cGy (1 fraction only) = 4450.6 cGy (for 25 fractions)
H) Question:
Will the dose of 4450.6 cGy be a high enough dose to go over the TD5/5 for the whole rectum?
H) Answer:
No, this will not be a high enough dose to go over the TD5/5
I) Question:
What is the TD5/5for the whole rectum?
I) Answer:
The TD5/5 for the whole rectum is 60 Gy or 6000 cGy.
THE END
Author: Adam Buell
Editors: Mary Hare and Janelle Plavchan
Date Created: February 2005
