[Intl_DxMedPhys] CTDIw - why 2:1 periphery/center weighting?

[Peter Watson, Mr]

Thanks to Mena Shenouda and Tim Stick for pointing out that the 1/3, 2/3  CTDIw weights come from assuming a linear dose profile from center to periphery and calculating the average dose over the CTDI phantom area.

Now that you know this, feel free to quiz your grad students/residents 🙂

-P


Peter Watson, PhD, MCCPM

(masculine pronouns)

Medical Physicist, McGill University Health Centre

Assistant Professor, Medical Physics Unit and Department of Oncology, McGill University

________________________________
From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> on behalf of Peter Watson, Mr via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Sent: April 13, 2026 11:18 AM
To: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: [Intl_DxMedPhys] CTDIw - why 2:1 periphery/center weighting?

Hi List, I was recently contemplating the definition of CTDIw, and wondered why weighting factors of 2/3 periphery to 1/3 center CTDI100 were chosen. Anyone know the history here? If CTDIw is meant to estimate the average dose in the axial slice,
Hi List,

I was recently contemplating the definition of CTDIw, and wondered why weighting factors of 2/3 periphery to 1/3 center CTDI100 were chosen. Anyone know the history here?

If CTDIw is meant to estimate the average dose in the axial slice, then of course the periphery must have a higher weighting than the center due to geometric arguments (the area of the "periphery " region is larger than the "center" region) but it seems rather arbitrary and simplistic to pick a 2:1 ratio. In a CTDI phantom of radius R, if you consider the "center" region to have radius R/2, and thus the "periphery" to be an annulus of outer-inner radius R-R/2, then the areal ratio would be 3:1 (or ¾ to ¼ periphery to center weighting). Is it one of those things where a 2:1 ratio is "close enough" to experimental measurements and MC simulation data, and easy to remember?

Curious to hear what people know!
-P




Peter Watson, PhD, MCCPM

(masculine pronouns)

Medical Physicist, McGill University Health Centre

Assistant Professor, Medical Physics Unit and Department of Oncology, McGill University
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osu.edu/pipermail/intl_dxmedphys_wd_osu_list/attachments/20260413/046424e6/attachment.html>