Unusual increase in the background dose

Dear experts,

To check absorbed energy from proton beams, I used following
coding (please see the attachment #1). My USRBINs are
0.5*0.5*0.1 cm^3 in X, Y, Z respectively. Iām using an annular
beam, where radius is 0.3cm. If I used 108MeV/p proton beams,
Iām getting a nice Bragg peak. But, if I use 158MeV/p, Iām
getting an unusual increase in the background dose before the
Bragg Peak. (Attachment #2).
Also I reduced the radius up to 0.15cm, in that case I got a
higher absorbed dose, where as I should get a lower dose than
the previous case (attachment #3). Could you please explain
this to me?

Thank you,

Dulitha.