Proton beam dose in water

I just played a very simple model for the proton beam dose in water, but the result is not as expected.

I set a 10cm x 10cm x 30cm water phantom. The incident proton beam is along the Z direction (the 30cm long dimension). I set the USRBIN to collect DOSE with 1mm voxel.

I shifted the primary 0.5 mm from (0,0), so the primaries will hit the center of the bin #(10,10) (index on the X-Y plane) as in the figure below.

The input is attached here (proton-water.inp (1.8 KB))

The figure below shows the dose along the beam direction from the incident bin #(10,10) and from the adjacent bins. The “Bragg peak” is roughly at the correct depth (maybe a little bit deeper), but the surface dose is so high.


Any hint?

Dear @xiaosj,

it is easy to explain. You have a pencil beam, impinging only at the +0.5, +0.5 bin. As this beam goes deeper, the protons start to scatter to the side, populating the neighboring bins as well. This distributes the deposited dose into a larger volume, lowering the maximum in one bin.

This can be seen in you plot as well, the lines (except #(10,10) don’t have dose deposition in the first centimetres.

Using a wide beam would create a result matching the expectations.



You have a very ideal beam. you can set dx dy some number and take more bins to plot.