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
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.
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
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
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 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.
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
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
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.