# Medical physics question

1. Nov 30, 2008

### medphysics

This is a homework question that is due on Monday and I'm not sure how to proceed. Any help at all would be appreciated. I do not have a background in physics and this is my second quarter as a grad student. please help.

Two 10 MV beams treat a patient in an SAD. The isocenter is given. Determine the dose the patient receives from both beams at points A, and B, and C if the dose to the isocenter is 200 cGy per day. The isocenter is 10 cm deep from the anterior beam and 12 cm deep from the lateral beam. The field size is 12 x 16 for both fields.

A: 2 cm off axis from CAX of AP beam and 12 cm deep (diagonal).
B: depth of dmax from AP beam, on central axis
C: 20 cm deep from lateral beam on central axis

we were told to break up the question into separate parts,

a) dose from AP to point A, b) Dose from AP to point B, Dose from AP to point C, dose from lateral beam to point A, etc

I know that we need to determine the field size by finding the equivalent square and then take the ratio of the TMR's multiplied with the ratio of the Sp's and the ISF........but I am not sure how to calculate the dose from the AP beam to points A and C and I do not even know how to approach the lateral beam.

This is what I did for dose at B taking into account dmax:

Dose from AP to pt B:
AP beam
Depth = 100 – 10 + 2.5 = 92.5cm
Field size = 13.7 x 92.5 = 12.7cm
100
FS at pt B (dmax) = 13 (approx)
Dose = 100 x (ratio of TMRs) x ratio of Sp’s x ISF
= 100 x 1 x 1.009 x (100/92.5)^2
0.847 x 1.012
= 137.6cGy
I do not know if I even did this right. Please someone help me

2. Dec 6, 2008

### turin

First of all, I appologize on behalf of all of us who have the good intention to help with homework. I would love to help you with this, but I'm afraid this is just too far outside of my realm of understanding/experience, and I did not try to help you with this question because I thought maybe someone else here could do much better, and I didn't want to get in the way. I have no idea about medical physics, so your terminology is quite thick for me, but I imagine that is why you have not received any responses from anyone else here, either. I would actually appreciate the opportunity to learn some medical physics myself, and helping you with this question would do that. Since no one else is stepping up, if you still want, I will try to help, but first you need to define, or at least help me understand, your terminology:

- SAD (I have no idea)
- isocenter (wikipedia has very vague definition, and, in particular, refers to a line as a point, which confuses me)
- dose (too many possible definitions)
- cGy (is this "centigray", i.e. a unit of specific absorbed radiation = 1 Joule per 100 kilograms?)
- "deep from" - I have no idea
- "12 x 16" - are there units for this, e.g. square centimeters?
- CAX (I have no idea)
- "AP beam" (I have no idea)
- "dmax" (I have no idea)
- central axis (is this the axis around which the arm of the machine rotates?)
- points A, B, and C (I have no idea)
- "the equivalent square" (I have no idea)
- "TMR" (I have no idea)
- "Sp" ("simulated/standardized patient"? - but if so, I still have no idea what this is supposed to mean)
- ISF (I hvae no idea)
- FS (I have no idea)

There is a book called The Physics of Radiation Therapy by Faiz M. Khan. Is this a good source? There is also a book called Radiation Therapy Planning by Gunilla C. Bentel. Is this a good source?

Last edited: Dec 6, 2008