Brain Teaser #18

A) Question:

You are treating an AP/PA shoulder at midplane. If you give 148 MU from the AP, what do you expect to give from the PA?

A) Answer:

You would expect to give 148 MU form the PA if you were at midplane.

B) Question:

You are treating a shoulder at midplane. You just completed the AP field with 148 MU. During the PA field the machine goes down and it goes down hard. It turns off at 92 MU delivered out of 148 MU. If the total dose that was supposed to be delivered was 250 cGy, what was actually delivered?

B) Answer:

If 92 out of 148 MU were only given from the PA, 148 MU were given from the AP, and you were supposed to give 250 cGy, you actually gave 203 cGy.

Follow Up:

You can solve this by setting up a proportion.

      total dose                            dose given
      ------------------------       =       ------------------------
    total MU                             MU given


250 cGy                        X
      ----------------       =       ----------------
      296 MU                   240 MU

The total MU is simply 148 MU + 148 MU. This is what you were supposed to give. The MU given is simply 148 + 92 MU. This is what was actually given because the machine when down on the PA field.

C) Question:

If you were supposed to give 250 cGy and you only gave 203 cGy, how MUch dose are you lacking?

C) Answer:

If you were suppose to give 250 cGy and you gave only 203 cGy, you were lacking 47 cGy

250cGy - 203cGy = 47cGy

D) Question:

The next day the doctor wants to make up dose to the PA field only. So, the doctor wants you to only give 47 cGy from the PA field. If the original dose to that PA field was supposed to be 125 cGy, and to give 125 cGy, 148 MU needed to be given, what are the monitor units needed to deliver 47 cGy?

D) Answer:

To give 47 cGy, 56 MU would have to be given.

Follow Up:

This can be done by setting up a proportion. You can use the following:

   new dose                               new MU
      ------------------------       =       ------------------------
     original dose                           orginial MU


47 cGy                          X
      ----------------       =       ----------------
   125 cGy                    148 MU

E) Question:

If the doctor wanted to give 250 cGy plus the 47 cGy from the PA field in one day, what would the PA field monitor units be (assume "normal" MU = 148)?

E) Answer:

To give 125 cGy plus 47 cGy that was missed, you would have to give 204 MU from the PA field.

Follow Up:

This can also be done by setting up a proportion. The first thing is figuring how MUch dose to give. We know that we want to give 125 cGy + 47 cGy because the 47 cGy was missed. This gives us a total of 172 cGy that we MUst give from the PA field. Now we can put that into the following proportion.

   new dose                               new MU
      ------------------------       =       ------------------------
     original dose                           orginial MU


172 cGy                          X
      ----------------       =       ----------------
   125 cGy                    148 MU

There is a simpler way to do this. If you recall, with the last question, it asked how many monitor units were needed to give 47 cGy. The answer was 56 MU. If you took 56 MU + 148 MU it would give you 204 MU. Two ways of looking at it, same answer!

F) Question:

The doctor has finally decided that he wants to spread the missed 47 cGy over 5 days on the PA field with the original prescribe dose given at the same time. If the AP field is given 148 monitor units, what will be given to the PA field (again assume original total was 250 cGy)?

F) Answer:

To give 47 cGy over 5 days plus the original dose, you would have to give 159 MU from the PA.

Follow Up:

The first thing we can do is figure out how MUch dose we need to give. Simply take 47 cGy and divided this by 5 days. This will give you the dose for each day. Then take your original dose and add these together.
47/5 = 9.5 cGy.
9.5 cGy + 125 cGy = 134.5 cGy.

We can then set up the following propotion:

   new dose                               new MU
      ------------------------       =       ------------------------
     original dose                           orginial MU


134.5 cGy                          X
      ----------------       =       ----------------
   125 cGy                    148 MU

Another way to do this is to take 56 MU and divided this by 5 days. If you recall, we already know to give 47 cGy we need to use 56 MU. Therefor, if we then add this value to 148 MU you will get 159 MU. This is the same thing, just a diffrent way of looking at it!

56/5 = 11.2 MU
11.2 MU + 148 MU = 159 MU

G) Question:

If you adjust the monitor units to compensate for increase blocking, will the dose increase, decrease, or remain the same?

G) Answer:

If you adjust the MU to compensate for increased blocking, you would expect the dose to remain the same.

H) Question:

During the course of treatment, the doctor decides to increase the blocking on one of the fields. If nothing else changes, to adjust for this increased blocking, would you expect the monitor units to increase, decrease, or remain the same in your new calculation?

H) Answer:

If blocking increases, you would expect the monitor units to increase, and sometimes they may remain the same if the blocking increased very slightly, but the monitor units should never decrease with increased blocking.

I) Question:

If the doctor increases blocking, and you don't adjust your monitor units accordingly, what will happen to your dose being delivered? Will it increases, decrease, or remain the same?

I) Answer:

If blocking increases, and you do not adjust your monitor units, you would expect your dose to decrease.

Follow Up:

Do keep in mind that if it is a slight change in blocking, it probably won't affect your monitor unit calculation significantly. But, if it is a significant change you are likely to see a change in the monitor unit calculation.


Author: Adam Buell
Co-Author: Deb Zehel
Editors: Mary Hare
Date Created: January 2007