The GTV is marked in the patient's CT images - it's an exercise that is radiographic and quasi-anatomic, or rather ANTI-anatomical (i.e., you only mark the ABNORMAL anatomy). GTV stands for Gross Tumour Volume and was defined by the ICRU in their 50th report, where they say it is defined by the oncologist on transectional images.
Deciding on where this GTV lies is the result of a clinical decision-making process. The process relies on your anatomy knowledge, because you are being asked to identify the tumour, an abnormal tissue. Unfortunately you can't identify the abnormal anatomy without being able to identify the normal anatomy! And since you are using a CT to look at all this anatomy, your radiological anatomy is the type that has to be very good (you are going to have to do a lot of this GTV marking if you want to be a 21st century radiation oncologist!). There are a couple of principles that can make CT interpretation less of a chore.
Lateralised structures occur in pairs (2 eyes, 2 ears, 2 testes, 2 legs, …..), and central structures are solitary (1 nose, 1 larynx, 1 brain, ….). Of course, there is the abdomen where all the solitary organs are thrown in higgleddy-piggleddy, and there you just need to know where it all goes!
As well as this, there are the 3D facts in the 2D world - any pipe-like organ (e.g., aorta) will appear as a long series of circles if cut transversely or ellipses if cut obliquely, with the corollary that you can tell whether a circle is part of a sphere (read 'node') or a cylinder (read 'vessel') by looking up and down at nearby slices. The sphere disappears while the cylinder doesn't.
There are a wide range of 'windows' or contrast settings (rain, lung, liver, bone) that can assist with interpretation. Feel free to use them liberally. I like to mark up the GTV on high contrast views (like 'brain' settings) as it makes the fat black and shows the tissue planes much more clearly.
And finally …. when you don't know, ASK a colleague or read a book! Why should a patient suffer because you didn't ask when you weren't sure.
TRY TO GET THE LOGIC, not just draw lines in the same place.
After this there are a couple of really obvious points to make that might seem a little stupid. Forgive me and humour me!
If there is no CT, then no GTV can possibly be marked.
If the tumour is not visible on the CT (such as when the surgeon threw it in the bin), then no GTV can possibly be marked.
Since the CT has many slices, the visible tumour (the GTV) must be marked on ALL slices. It is easy to interpolate, but check the interpolation!
Of course, software vendors try to be as helpful as possible by producing many helpful tools to help us do our job. You should be wary of any 'automation' feature that places volumes without your action. You are still responsible for what the computer does, and so any interpolation by software still has to be checked by you.
You can mark the GTV in two ways - as a positive mark-up, or as a negative mark-up. The positive mark-up refers to the clinician (yes, that's you!) marking the tumour, while the negative mark-up refers to the clinician marking up the normal structures that are NOT the tumour. Drawing the GTV is predominantly a POSITIVE MARK-UP exercise. Remember that the decision to draw the GTV is a pixel-by-pixel decision process that corrals all the tumour pixels. There is no place for extending your volume borders past this point because you suspect that there might be cancer there - that decision is made in the next step. Just draw what you can see which is abnormal!
Why be so pedantic about this process? Well, firstly I am pedantic, so you don't escape! Secondly, the literature already have shown a marked variation in the drawing of ICRU volumes. Obviously the cancer doesn't know who is voluming to make it want to change accordingly, so the variation must exist within the brains of the clinicians (which is you and therefore means that you have a brain!) - but who's brain is correct?? Unfortunately this question is too difficult for an answer, except to say, applying a brief set of mark-up procedures will go some way towards reducing the variation that we see.
How can the variation be reduced? Fortunately, I am responsible for me alone. I would make the controversial statement that it is my responsibility to know that I am well trained and voluming at a 'specialist' level. For me this means being observed during these voluming exercises and explaining the clinical reasons for where the GTV line is placed.
if fusion is available, I will typically use the MRI images after the first pass voluming on CT. The CT volume is only changed when the disparity is obvious and gross, and typically the GTV is only expanded by virtue of the MRI.
At the end of this you should have the cancer pixels corralled in a GTV. Initially the process takes a good deal of time but with time and the use of your software tools, a precise skilled delineation can be done in 5-10 minutes.