IEEE Spectrum October, 2016 - 29

unnecessary and possibly harmful. So one of the challenges
in cancer surgery is to figure out exactly how much lymphatic tissue to cut out and how much to leave intact.
One strategy makes use of the fact that the membranes
that surround cancerous cells are more penetrable and
reactive than healthy tissue. So surgeons inject the area
of the primary tumor with dyes or radioisotopes. After a
few minutes the surgeon can then determine which of the
surrounding lymphatic tissues are likely to contain cancer
simply by noting the presence of the colored dye or identifying them with a radiation detector.
The main problem with this approach is that the dyes
that are normally used for this purpose (isosulfan blue or
methylene blue) migrate quickly from the site of injection
through the cancer-containing lymph nodes and on to the
entire lymphatic system. So surgeons typically take a picture
of the tumor and surrounding areas soon after injection of
the dye, before it has migrated very far, and then operate
based on what that picture shows. As you can imagine, it's
hard to be accurate when you have to refer to memory or
a picture to guide your actions.
Another issue doctors can run into when using organic
dyes is premature fading: The dyes bleach quickly under the
strong lights used to illuminate them. Still another problem
is that, even before any fading occurs, those blue organic
dyes often cannot be seen through the skin or fatty tissue.
There is a different organic dye-something called indocyanine green-that does show through skin and fat because
it fluoresces in the near infrared, which penetrates better
through living tissue. Because the dye's emissions are outside the visible range, though, the surgeon must use special imaging equipment to see where the tracer has gone.
When a radioactive tracer is employed for such surgery, it's
typically a metastable form of technetium that emits gamma
rays when it decays and has a half-life of about 6 hours. So
time is not such a factor. But like dyes, it may migrate too
quickly. In any event, pinpointing where the technetium
has moved is hard to do with a handheld gamma probe.
As a result, radioactive tracers give a very coarse picture
of how cells shed from a tumor might travel through the
patient's lymphatic system.
It's evident that current methods need improvement, as
shown by a 2013 clinical trial intended to determine the
effectiveness of a technique called sentinel-lymph-node
surgery for women with a particular kind of breast cancer.
Normally, 10 or more lymph nodes in the woman's armpit
would be removed and analyzed for cancer. With sentinellymph-node surgery, however, a dye or radioactive tracer
(or both) is injected at the site of the tumor, which can lead
the surgeon to the first few armpit nodes along the line of
lymphatic drainage. If those "sentinel" nodes do not con-

tain cancer, there is a strong likelihood that the cancer has
not spread from the tumor site. This approach avoids having to remove a large number of lymph nodes in the armpit,
which often causes lymphedema (arm swelling), numbness,
and restricted arm movement.
So this seems like a smart strategy for treating breast
cancer, right? The question is how well it really works. The
2013 study found that 12.6 percent of the women got falsenegative results. That is, their sentinel nodes, as identified
using both a blue dye and a radioactive tracer, showed no
cancer, but cancer was, in fact, present in these women's
other armpit nodes. The study authors considered a falsenegative rate above 10 percent to be problematic. That
doesn't mean that the medical community should abandon sentinel-lymph-node surgery-not at all. Physicians
just need to figure out how to do it better.
Here's where quantum dots could really...well, shine.
Unlike organic dyes, quantum dots migrate relatively
slowly and can maintain their ability to fluoresce for many
hours. So the surgeon could perform the procedure based
on what the dots reveal in real time. The surgical team
would simply inject the quantum dots and then illuminate

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Table of Contents for the Digital Edition of IEEE Spectrum October, 2016