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Definition |
In theory, proton beams are an attractive alternative to photon-beam radiotherapy for PCa, as they
deposit almost all their radiation dose at the end of the particle’s path in tissue (the Bragg peak), in contrast to
photons, which deposit radiation along their path. There is also a very sharp fall-off for proton beams beyond
their deposition depth, meaning that critical normal tissues beyond this depth could be effectively spared. In
contrast, photon beams continue to deposit energy until they leave the body, including an exit dose.
Two recent planning studies comparing conformal proton therapy with IMRT have yielded
conflicting results; one study suggested that the two are equivalent in terms of rectal dose sparing, but that
IMRT is actually superior in terms of bladder sparing; the other study suggested a clearer advantage for
protons.
One randomised trial on dose escalation (70.2 vs. 79.2 Gy) has incorporated protons for the boost
doses of either 19.8 or 28.8 Gy. This trial shows improved outcome with the higher dose, but it cannot be used
as evidence for the superiority of proton therapy per se. Thus, unequivocal information that shows an
advantage of protons over IMRT photon therapy is still not available.
Studies from the SEER database, and from Harvard, describing toxicity and patient
reported outcomes, respectively, do not point to an inherent superiority for protons - indeed, in terms of longer
term GI toxicity, proton therapy might even be inferior to IMRT.
A retrospective 2:1 matched-control analysis of 27,647 US Medicare patients compared 314
men receiving proton therapy with 628 men who had IMRT. Despite the considerably higher costs for proton
therapy, there was some improvement in GU-tract toxicity after 6 months, but not after 12 months, and not at
the GI tract.
A randomised trial comparing equivalent doses of proton-beam therapy with IMRT is needed
to compare the efficacy of protons vs. photons; a study of this type is under consideration by the RTOG.
Meanwhile, proton therapy must be regarded as a promising, but experimental, alternative to photon-beam
therapy.
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