This may seem a little … sorry, a lot presumptuous! Is there really a way to improperly assess a plan? Well not really, we oncologists develop our individual, idiosyncratic ways of doing things and we are all specialists, etc… BUT … these methodologies are informed by the software we use and the reports of external bodies, who incidentally continue to amaze me with their forethought and forward thinking at a time when technology was less mature than today. So I bow to those gurus and try to use their thinking because I think that it will put me more in tune with the way things are built and therefore (and this is what I really want!) I'll expend less effort in doing the job well!
So, back to the problem at hand! To properly assess a plan, you must first properly draw the volumes that require irradiation. So if you haven't been through make a proper GTV, make a proper CTV and make a proper PTV, then now is the time to make a detour and get that out of the way.
The underlying assumption of ICRU50 and ICRU62 is the process of dosimetric planning as opposed to the previous process of geometric planning.
Geometric Planning
In the days of conventional planning, field sizes were determined by the bony anatomy, the oncologist specified the field directions and then a plan was produced to the prescription dose.
So what is wrong with this?
Nothing, … and everything! This will still produce a plan, and this remains acceptable (and the best that you can do) if you are unable to see the cancer, the 'at risk' areas and critical normal structures. If however you have the increased imaging capability of a CT scanner and the improved 3D dose calculation capability of a modern planning computer available to you, then it is in the patient's interest to utilise that information for their benefit.
Why deliver 40Gy to the posterior rectum with a 4 field prostate plan when you can deliver 25Gy to the same place with a different plan?
The mammoth assumption of the geometric approach that produces conventional plans is that you can accurately determine the cancer's position from the bony landmarks. There are enough papers published to be able to categorically state that this assumption is hogwash!
Geometric planning is the comfort zone that we are leaving in Radiation Oncology. The ticket to leave is being able to volume the GTV, CTV and PTV - yes, flogging that old horse again!
Dosimetric Planning
There are several ways to ensure that you can't achieve dosimetric planning. Firstly, tell the radiation therapist in planning the field size that should be used. Secondly, when you write a script tell the radiation therapist in planning (also called a 'dosimetrist' overseas) what field arrangement to use. Thirdly, neglect to volume the critical structures, or having volumed them don't issue a dose constraint for the structure. That should cruel the process efficiently!!
Dosimetric planning for the planning radiation therapist is an exercise in finding a solution to a problem. The high dose area is defined (PTV) and the low dose area is defined (critical structures) by the oncologist and then the radiation therapist tries to develop a plan which satisfies the need for high and low dose areas.
Does it matter what field arrangement is used? So long as it is achievable (and who better than the radiation therapist to know this?) and not wasteful (why use 50 fields to achieve what 5 can?), it shouldn't! Which of course begs the question - when you assess a plan, are the field outlines turned on or off? Is your assessment of a plan easier with them on or off? Now I know the answer to that question? [I'll add some images so that you can reach the same conclusion!]
When it comes to assessing the plan, what are you looking for? The bible of dosimetric planning (ICRU50) says that there should be homogeneous dose coverage (more about this another day!) of the volumed area (PTV), which is defined as coverage of the PTV by 95% of the intended dose and no more than 107% of the same intended dose. So when you come to assessing a plan, why would you be interested in 98%, or 102%?
Some planning systems like Pinnacle give you a colour wash. This looks like modern art. I don't like modern art. It makes me nauseated. It also doesn't tell me what I want to know. I'm a male - so my little brain has trouble holding more than 2 or 3 items at once (yes, I know my wife thinks that this is an over-estimate!). So when I assess a plan, I turn off the beams, turn off the colour wash, turn on the 95% and 107% isodose lines and turn on the PTV.
That's all! Nothing else!
I then look to see if the 95% isodose goes all the way around the outside of the PTV. If it does not, I use the little jigger that can tell you dose at any pixel on the CT image to find out how much I am missing. If I think the missing bit is significant, the planner goes and fixes it up. If it is insignificant, then I move on! I look to see if any 107% areas ("hot spots") appear. The hot spot has to be over 2cm2 to be significant.
Oh and one more thing! The margin between the PTV and 95% isodose line should be MINIMAL. Yes, like Spandex. For those trainees born after 1970, Spandex was a very popular clothing material of the 1980s - you would have been young to have afforded such luxuriant items, but you should ask your [older!] consultants if they have any pictures of themselves in Spandex (ONJ in Xanadu???). The more recognizable name is Lycra. For the pedants, Lycra and Spandex may well have been different paths to the same fashion statement, but I writing and I choose to ignore the distinction because either material should get the point across. And the point has nothing to do with clothing! However to keep those who feed information in visually happy, I have included a tame picture of Spandex to show you what I mean! See no gap! If you have followed the ICRU50 methodology then there is no place for 'a little bit extra'.
| Normal Shorts | SPANDEX shorts!!! |
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