Make A Proper PTV

Nearly there now, at the complete planning process!

Finally we come to the PTV and the end of the oncologist's input! Now, I should warn you that this is quite a controversial stand. You should have appreciated already (if not, go and work in another unit!) that there is a variable difference between the theoretical process and the actual process that relates to expertise and professional demarcation. I suggest that we ignore these issues and carry on with our theoretical discussion about how the world ought to be. Although I would say that if the logic is sound, why would you do anything else?

The PTV is the Planning Target Volume and is defined as a "geometric construct" based on the completed CTV. The relationship between the CTV and PTV is a fixed margin (this is what ICRU50 defined). The margin is made of two components which are the

Internal Margin
this margin relates to the intra-fraction
Set Up Margin
which related to the inter-fraction

movement of the organ during treatment, and the movement of the patient at each treatment.

What is really nice about the CTV to PTV margin is that it can be very accurately determined by a departmental QA process designed to record the size of the measures. We already know that there is quite a difference in the margins depending on body site and fixation device, so the numbers used can be variable.

There are some corollaries that flow from this definition.

  • the expansion can be done by the software by anyone proficient in its use and should be informed by the QA process.
    • In many departments, constructing the PTV is seen in the same light as the GTV and CTV - it is done by the doctor. The definitions however give this position no support.
  • the PTV cannot be modified once the CTV has been described.
    • Unless your name is "Dr Geometry", the definition of the PTV prevents you or any staff member from altering it. This is because it is an automatic expansion based on measurable movements. If you change measured movements by employing an image guided approach, then the expansion is less, BUT the process is unchanged, isn't it? You, the doctor, still can't alter the generated PTV because you name wasn't changed by the image guidance!
  • Usually the PTV will determine the position of covering isodoses.
    • Now you will note that this statement is worded rather strangely. Surely the PTV determines the field size, and the field size determines where the isodoses lie. Well, I'm sorry to say that this thinking is not in line with the logic of ICRU50 and ICRU62. This author has had made the conceptual leap from the thinking used in conventional planning to that used when manipulating 3D image sets.
      • Conventional treatment is based on geometric planning where the oncologist defines field sizes (as we were usually not able to see the tumour in relation to the field, the field determined dose and we just hoped that this would cover the tumour. Recent data has shown very well just how much we were kidding ourselves!).
      • 3D treatment is based on dosimetric planning where the oncologist selects the position of isodoses, and the issues such as the field number, energy and size become irrelevant. Sorry, not entirely irrelevant, but really there are many solutions in 3D space.
    • the conditions now exist that having defined the GTV, CTV and margins for PTV, that you now find that the PTV-covering isodose now encroaches into a critical structure which was also encompassed by the PTV. This situation requires discussion as it is of critical importance to the advancement of radiotherapy.

What do I do when …. a PTV-covering 95% isodose covers a critical structure?

There are only two replies to this question, and for one of those replies there are two ways to achieve the desired outcome, which I will argue have profound differences for the future of radiotherapy technique.

  • the first option is to say "tough luck for the critical structure" and accept the plan as is. That's OK, it's a clinical decision that you will have to live with (higher expectation of side effects). Just make sure you have discussed it with the patient and documented the discussion and outcome.
  • the second option is to say "I don't want to give the critical structure that high dose, so I need to manipulate where the isodoses are. This too is OK, and it's a clinical decision that you will have to live with (higher expectation of recurrence). Just make sure you have discussed it with the patient and documented the discussion and outcome. But there are two ways of achieving this out come.
    • the first, and I believe the most common, is to alter the PTV to exclude the critical structure and then ask for a new plan. This is wrong because:
      • the definition of the PTV does not give you the leeway to alter it. The only way a PTV can change is because the Internal Movement or Setup Margin have been reduced by a control process. And this is not what has happened!
      • as a result all plans look 'perfect' with respect to the DVH assessment - but why is this important? Clearly the plan is not perfect, so what is the gain for the oncologist? I am at a loss to know.
      • And isn't it 'dishonest' too? After all the plan is NOT PERFECT because you have deliberately compromised dose to the GTV/CTV.
      • and finally from an analysis of the PTV, you have no way of picking up whether there is any clinical detriment to your actions (it just looks like a perfect plan failed!). In particular you can't work out what happens when you reduce this dose - do you lose some or all of your local control? What if the shaving of a PTV were to make the recurrence rates the same as no treatment? Would you still alter the PTV in the future?
    • the second option is to hand the plan back to the planner and say "This plan is unacceptable because of the critical organ dose. Please re-plan but re-order your number one priority so that the dose received by the critical structure does not exceed 45Gy. Then give the best 95% coverage you can to the rest of the volume away from the critical structure." This is the correct thing to do because:
      • The end result of this will be an isodose EXACTLY THE SAME AS option 1 above. Yes, the PTV will be different, but the isodoses will be identical.
      • the PTV, which you are not permitted to alter anyway, has not been changed
      • it hasn't made the dosimetrist's job any harder at all.
      • the plan is NOT PERFECT AND looks to be NOT PERFECT, because you have deliberately compromised dose to the GTV/CTV.
      • from an analysis of the PTV, you can identify plans where you deliberately altered PTV doses, and so you have a way of picking up whether there is any clinical detriment to your actions. Because you can group plans on the basis of adequate V inadequate 95% PTV coverage, you can work out what happens when you reduce this dose - you can quantify loss of local control with PTV dose deficiency? This way we could improve out techniques while knowing the clinical import of the changes rather than basing it on some 'feeling in the water' or asking a colleague equally as uninformed as yourself.
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