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Sparing the Hippocampus: Lessons from 2015 PLAN Challenge™

October 08, 2015

Greg Robinson M.S.

One thing I love about radiotherapy treatment planning is there's inherently a pursuit of excellence. As professional treatment planners, we have a strong desire to do our absolute best, because we know the result ultimately impacts a human being.  

The PLAN Challenge was born from that spirit. Every year, it helps advance best practices in medical dosimetry because we are able to learn from each other—lessons we can apply in real clinical practice.  

This year was no different. Here are the top three things we learned from this Challenge's brain case.  

  

1) We Can Create High Quality Plans that Spare the Hippocampus   

It's not often that treatment planners, striving to keep up with the demands of a busy clinic, have time to test out new protocols and techniques. However, the annual PLAN Challenge gives us an opportunity to discover and test new techniques.  

This Challenge explored a simultaneously integrated boost (SIB) technique with an emphasis on hippocampal sparing

SIB plan with hippocampal sparing submitted for the 2015 Challenge

Historically, patients with whole brain metastases were given very poor prognoses. Today, our treatments are getting better, and patients are living much longer. That's been the driving premise behind elegantly carving out the dose around the hippocampus, instead of treating the whole brain with wide-open beams.  

SIB is one way to achieve hippocampal sparing and deliver an accelerated dose to the tumor. These techniques can help prevent dementia and other neurocognitive side effects of whole brain radiotherapy.  

It all sounds great in theory, but actually conforming dose to the metastases, while avoiding the sea-horse shaped structure located right in the center of the brain, would take some rather fancy beam work.

PLAN Challenge High Performer Shadonna Holmes said this year's project let her prove to herself that as fancy as this type of planning is, she could, in fact, do it:  

"I'd always hear of hippocampal sparing, and I read about it," said Holmes. "I'd think, 'that kind of sounds impossible,' but I never had the chance to think about it in-depth until the PLAN Challenge. Now I know that I'm capable of doing a plan like this. And I think that it will benefit my clinic in the future because we've already told our doctors about it, and I think we are going to start treating more patients with hippocampal sparing."   

  

2) We Know Where to Find Resources for Contouring the Hippocampus 

One of the added challenges in attempting this type of plan is accurately delineating the structures, especially uncommon ones like the hippocampus. The good news is there are very helpful resources that walk you through contouring this specialized structure.  

RTOG 0933 Contouring Atlas 

RTOG 0933 is the go-to guide for this type of protocol, which includes how to define the target volumes and critical structures. For clinical reference, there is a great contouring atlas embedded within this protocol. It also includes detailed recommendations for MRI-CT fusion. 

"A Radiation Oncologist's Guide to Contouring the Hippocampus" by Chera et al 

This research paper arranges the principles of hippocampal sparing in a helpful 3-step framework. The researchers include very clear axial MRI scans with 1.5 mm thickness as is recommended in the RTOG 0933 atlas.   

In radiotherapy, we are constantly inundated with new information. So the expectation is not to have all this stuff memorized.  The key is to know where to turn. So, having these resources is invaluable. 


3) We Realized Technology Can Help Personalize Radiotherapy Protocols 

Every patient's anatomy is different—the tumor volume and location may provide a big enough discrepancy that can drastically affect DVH curves.  Yet we apply standardized protocols, mostly using clinical experience to determine how relevant that protocol is for the particular patient’s anatomy.  

In order to build the clinical goals for this year’s Challenge, we were essentially acting as a virtual physician. Certainly, using the RTOG 0933 recommendations provided a great starting point. However, this year I discovered from using PlanIQ™ software that there is a better way to develop clinically relevant goals that are patient-specific.  

PlanIQ has a unique feature—Benchmark Dose™— that allows you to “paint” 100% dose on the identified targets. Benchmark Dose calculates the steepest dose gradient possible for every voxel outside the target area. The calculation is based on known photon dose fall off for a specific energy.

This process produces an idealized dose cloud, and a Feasibility DVHTM for each structure. 

The Feasibility DVH removes the guesswork, and tells you upfront what is going to be impossible per organ at risk (OAR) for a given patient’s unique anatomy.  

This impossible-to-achieve dose cloud essentially lets you work backwards to identify, for each structure, clinical goals that approach the limits of what’s possible.  

As a result, the minimally acceptable goals in the Challenge were inspired by the RTOG 0933 protocol, and PlanIQ established the ideal goals. Consequently, a perfect score was not going to be achievable.  However, that is what drives continual improvement, and ultimately allowed Challengers to push the limits of impossibility.    

For more TPS-specific lessons learned from the 2015 Plan Challenge, watch presentations from some of the High Performers walking through how they achieved their high quality treatment plans: PLAN Challenge webinar series.


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