Fwd: Backscatter factor for cervical spine ap projection

[Chris Alston <alstonc@ODRGE.ODR.GEORGETOWN.EDU>]

>From: Keith Wise <keith.wise@HEALTH.GOV.AU>

>

>G'day

>

>Recently I extended a program I've written to do dosimtery calculations for

>general radiology and CTs based on mathematical phantoms and the EGS4 code.

>This extension was to do backscatter calculations for head and neck 

>projections

>(previously they were for projections involving the abdomen).  My tests of the

>code involve comparison against NRPB R-262 for head and cervical spine

>projections.  For posterior-anterior cervical spine and for lateral cervical

>spine projections at 90kV, 3mmAl and NRPB field sizes and positions I get 

>a BSF

>about 1.3 (as expected).  With anterior-posterior projections with the 

>centre of

>the beam 3cm above the shoulder (i.e. below the chin) I get 2.3.  My first

>thought is that the code is well and truly stuffed.  Basically the code 

>works by

>evaluating the primary component and the scattered component separately

>(technically this uses the LATCH facility in EGS4 or EGSnrc to distinguish

>unscattered and scattered photons) and the additional scatter is coming 

>from the

>chin and shoulder.  Maybe this is old news to many of you but I'd like to 

>learn

>from anyone who has dealt with this sort of thing (in the field or at the

>computer).

>

>Regards

>Keith Wise

>ARPANSA, Melbourne

>Australia                Fax Int + 61 3 9432 1835





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