I think I've only seen this mentioned once at BCCA
for eye melanoma.

http://en.wikipedia.org/wiki/Proton_therapy
Pros and cons

The treatment method is of interest because of its ability to accurately
target and kill tumors, both near the surface and deep seated within the
body, while minimizing damage to the surrounding tissue.[1] For this
reason, it is favored for treating certain kinds of tumors where
conventional X-ray radiotherapy would damage surrounding radio-sensitive
tissues to an unacceptable level.[1] This is of particular importance in
the case of pediatric patients where long term side effects such as
residual occurrence of secondary tumors resulting from the overall
radiation dose to the body are of great concern. Because of the lower dose
to healthy tissue protons have less severe collateral side-effects than
conventional radiation therapy.

The logic for treating common cancers (for example lung, head/neck, etc)
with proton therapy is the same as saying that surgery alone should cure
most cancers, as surgery is the Definitive Local Treatment. Of course,
surgery does not - because most cancers spread microscopically very early
beyond the tumor ('local') site.

Historically, one area where proton therapy had considerable early
successful application was in treating choroidal malignant melanomas, a
type of eye cancer for which the only known treatment was enucleation
(removal of the eye). Today, proton therapy is one of the techniques that
are capable of treating this tumor without mutilation. Proton therapy is
used on cancers that have not yet spread.[3]

Reference 3.
# ^ http://www.snof.org/maladies/melanome-oculaire.html article in French
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