What Is Oncological Importance Of Contour Change
Name Answer
Santam The first response that comes to the mind is of course u dont contour the tumour the TPS doesnot see it also and in most instances with conformal radiotherapy you end up missing the tumor with a resultant relapse. However the answer will be deeper than it appears. First of all we have to consider which contour is actually changing? If its the GTV then of course the consequence of the miss can be devastating. I believe the best evidence to this regard can come from the recurrence data in brachytherapy patients treated after external beam radiotherapy. The consequence of a change in the CTV contour is more difficult to appreciate. Logically speaking CTV is the anatomical area where there is a significant chance of subclinical disease being present. So if your knowledge of pattern of spread is accurate then perhaps you can conceive of a situation where changing the contour with the changing anatomy carries minimal penalty in terms of oncological outcome. On the other hand in the same scenario it must be appreciated that the majority of failures in malignancies treated with radiation with curative intent occur at the site of GTV itself. The very nature of subclinical disease and the tail of Holthousen's curve ensure that CTV failures will remain exceedingly difficult to quantify. The question of PTV is more complex. We routinely subtract PTV from the skin in order to reduce dermal toxicity in IMRT but have yet to see a failure at the site. Changing the PTV to be inside the body contour with progressive weight loss makes a lot of sense too. The impact of these changes on the oncological outcome is close to nil or atleast not shown to be detrimental in my experience. Changing the PTV expansion in the region of the rectum for IMRT in prostate is another pertinent example where the oncological outcome in terms of toxicity and control dont match with the conventional wisdom of not trimming the PTV manually
Andrew This is an area where there is very little knowledge. "The patient shrank by 1cm last week, what happens to the treatment." And there are two issues. Firstly what happened to the dose deposition pattern from the beams I accepted at the start. Secondly what happened to the structures/volumes? Are they still in the dose deposition area that they were supposed to occupy? Because we don't have any knowledge about how the dose changes or the target areas change, might it not be the case that reason for many of the GTV failures results from movement of the GTV into a low dose area (which has arisen because of contour change, or because of dosimetric inaccuracy due to contour change)?
Indranil Agree with Andrew, but this is still largely in theoretical domain. Don't know of any publications linking these two. If I may add to this - we underestimate the importance of interfraction and intrafraction target motion in HN - some marginal failures may well be due to motion rather than change in size or position.
Andrew We are conducting a prospective protocol trying to find answers, but the real confounder is the voluming by the RO. Our early data is showing that the interfraction/intrafraction motion which we account for is a much smaller problem than the morphology changes. We have seen changes from 98% of the 95% isodose covering PTV to 87% of 95% isodose covering PTV in one week.
Santam Well this is what we have indeed observed when we did the serial imaging in head and neck cancers. However we did not do the weekly scans as logistics dont allow. We attempted to do scans two or three times spaced across the course of the treatment and have observed that yes there is a definitely a good amount of shrinkage and wrapping of tissues. I have one question indeed what is the oncological consequence of this isodose shift for the PTV? Isnt the PTV designed for this express purpose to ensure CTV get the prescribed dose. I would like to know what happened to the CTV here too.
Santam Going by our own protocol we have observed that after fusion the major change occurs at the level of the neck CTV. The GTV actually is not so well defined. One worrying thing we have noticed is the significant movement of the tongue , larynx etc. When you are contouring a base of tongue tumour then the change in the tongue position will shift your GTV. How do we overcome this? We tend to contour the entire pharyngeal space in the region of the GTV to account for this movement expressly.
Andrew there are two things there.

Firstly the PTV coverage of change. Nice theory but neither the design not the build of the PTV incorporates any concept of contour change, only movement. The object is assumed to be invariable. We don't really take into account rotation either. SO the answer is "NO", the PTV is not designed to cover the contour change which hasn't been quantified before.

Secondly, the difficulty of the GTV with time is why the accurate CTV is so important, because the anatomy remains much easier to define reproducibly. The issue of tongue and larynx movement is well known but not coped with. Treating tongue tumours on this protocol is a nightmare for this reason, the upper portions vary greatly. Does a bung help? My own experiences are that they make patients very uncomfortable and very motion-prone. The voluming laterally is rarely the problem, it is the superior extent when the tongue shape changes which is the problem I have appreciated. I don't yet have an answer for how to treat it.
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