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15+ Year Member
Feb 21, 2003
Attending Physician
Hi all, wondering if someone would humour me enough to answer some very simple questions about radiotherapy? Mainly for stuff I can't quite find a clear answers for in books, etc.

To start -- how wide do you generally make the field/beam to cover the PTV and make up for any penumbra at the edges? For example, say you had a chest wall met you wanted to treat; you contour/volume it, give say a 0.5cm CTV margin, another 1cm PTV margin, etc. Then how much bigger should the beam be to make sure the PTV is covered? Another 1cm? More? Less?

This is actually a very fundamental question.

The best source for this is a report, ICRU 62 (International Commission on Radiation Units and Measurements (ICRU)) that explains how you use the setup uncertainty to create your margins.

This report is the "ur-text" for TV definition and is one of the 1st things folks should read to get the idea of how that get volume concepts/nomenclature work...its technical, but readable.

It does a good job defining CTV as a target encompassing microscopic subclinical disease using anatomic boundaries and PTV margins based on geometric/spatial uncertainty (the key difference in CTV vs PTV margination strategies ).

Hope this helps!!
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Staff member
7+ Year Member
Oct 10, 2011
Resident [Any Field]
Generally 7mm from PTV to block edge but can minimally be dependent on energy of your photon beam. This should be discussed in your physics lectures if you're a Rad Onc resident.

If you're talking about electrons we generally do 7mm to 1cm as well, although we sometimes do more as we have a CTV we're interested in, with a combination of thinking to PTV margin + beam penumbra.
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10+ Year Member
Apr 6, 2006
Resident [Any Field]
As big as you need to cover your ptv. I guess technically it could be no margin for an srs case up to 7-10 mm for a 3d conformal plan.


2+ Year Member
Jun 25, 2016
Attending Physician
I have typically seen 5 mm.Here is the fundamental issue: if you have no block (or even negative) margin, for example, then you will need to prescribe to lower ( 70-80%) isodose line to cover your PTV, so your PTV becomes much "hotter," but the dose falls off is steeper! there is a tension here between homogeneity and dose fall off.

Historically in 3d radiation, the dogma was that greater than 10% hot spot in all cases = a bad plan. (I believe there are exceptions to this, but that is a different discussion) Some planners may use greater than 5-7mm block margins to make the plan metrics/homogenity look better (lower hot spot) but the high isodose line dose fall off will suffer.

There are select cases in 3d radiation where you may actually prefer a hot spot in the tumor, or you can manually slide the leaves in some beams in closer near a critical structure (selectively reducing your block margin) and you may get quicker fall off near the critical structure at the expense of prescribing to a lower isodose line/ increasing hot spot. I find that in the IMRT era, some of us are more tolerant of the magnitude hot spots , more concerned about the location than size.
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