Produce An IMRT Plan

I expect that you think that this is going to be hard, right?


Why not? Well, for one, once your volumes are complete ….. oh, by the way I have forgotten to give the mantra of "first read make a proper GTV, make a proper CTV, make a proper PTV and properly assess a plan before going on?"

No? …then consider yourself told! And for good measure look at how to produce a 3DCRT plan for good measure.


Yes, "why?" is a very good question here. All of the activities needed to undertake good IMRT are based on the activities that produce good 3DCRT and good voluming. If you are not taking this voluming activities seriously, then IMRT just becomes a way for you to improve bad planning, rather than a way to make good planning really excellent. For instance, you should be able to spare the parotids with a 3DCRT approach to the Head & Neck - somewhat. IMRT will be better, but don't expect IMRT to spare the parotids if your really excellent 3DCRT plan is creaming them.

Remember that IMRT is an extension made possible by earlier developments. Interestingly, the IMRT techniques have been around for years …. yes, years! I don't claim extensive experience in this matter, but I personally saw a compensation system capable of delivering IMRT in Saskatchewan in 1997. The modern IMRT improvement has been to deliver IMRT using MLCs without compensators or blocks, i.e., more efficiently through software approaches.

It is important to point out here that there is an increasingly large split between duties that were once spelled out by oncologists. Our job as an oncologist is to have the volumes contoured excellently, and have set our target volume parameters (95-107%!) and our contoured organ constraints. When this has been done, then you call the dosimetrist/RT planner over and ask if you have missed anything. If not, leave, walk away and leave them to it! When let free, you will be surprised!

They should set the beams to gantry angles that work well with 3DCRT approaches, then, and only then, they will tell the planning system to produce an IMRT plan. Simple! Before you all gaffaw and say "what would he know?", please consider the following. The role of the dosimetrist/planner in this process now begins and that person is a professional who is capable of producing truly exceptional plans - stuff we used to dream of in the mid-1990s. They will use techniques such as split beams, dose dumping, regions of exclusion and a whole lot of 'tricks' to get the planning system to produce replicas of those lovely pictures printed in the red journal. But that is their job, and until plan assessment is required, the first part of my job is complete.

Now just a few additional issues that you will want to come back and read as you get the IMRT issues sorted out. Once you have the voluming/contouring under control (I do go on about this, don't I? That's because it's important!) your IMRT expertise will also develop. There are four areas where you will start to alter your practice (I have described this in another document on this site):

  • firstly, you will concentrate on getting the high dose area to conform to the PTV you have marked. Once that worry is over, you will start to look at other things.
  • secondly, you will start to concentrate on reducing the high dose area that covers normal tissue contours (introduce the use of the PRV) without reducing coverage of the PTV. Once that worry is over, you will start to look at other things.
  • thirdly, you will start to worry about the effect of shrinkage of patient and GTVs on your plan. Once that worry is over, you will start to look at other things.
  • fourthly, you will start to worry about ways to more effectively kill the cancer by manipulating the deposition of dose into the parts that have no normal tissue (i.e., the middle of the PTV!)
  • fifthly, when this has been done, you will get a warm feeling inside when you complete a plan knowing that your conformality and coverage are maximal, and then ……. you should be ready to retire!

A case of Nasopharyngeal IMRT

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