Proton Therapy vs. Conventional Radiotherapy

Eric Shinohara, MD, MSCI
Last Modified: August 7, 2008

Dear OncoLink "Ask The Experts,"
I would like your opinion regarding the risk/benefit of proton beam radiotherapy vs. conventional radiotherapy in a patient with a low-risk cancer status-post radical prostatectomy who now has rising PSA levels (biochemical failure) without any imaging evidence of recurrent tumor- hence, suspected local recurrence. Is there really less morbidity to proton beam therapy, without compromise in efficacy?

Eric Shinohara, MD, MSCI Radiation Oncology Section Editor for OncoLink responds;

The major advantage of protons is that the area being treated with radiation can be specifically targeted while sparing the surrounding normal tissues, thanks to unique properties of the proton beam. Learn more about the differences between proton therapy and photon therapy (3-D conformal and IMRT Radiation) in this article. Sparing of normal tissue may be important when a tumor is near critical normal tissues that cannot tolerate high doses of radiation. There are ongoing studies evaluating protons for treatment of prostate cancer.

However, in a man who has already had a radical prostatectomy and now has a recurrence, the entire area where the prostate gland was located needs to be treated. Because the bladder and rectum fall into the hole where the prostate gland was previously located, there is no way to spare these structures from radiation by using protons.

There may be a reduced risk of secondary cancers due to radiation when protons are used because they spread less radiation dose to normal tissues. However, this is theoretical, and most models available at present suggest that a special type of proton beam, known as a scanning beam, would be best at reducing dose to normal tissues. Unfortunately, this beam is not yet widely available.

Hence, at present there is not a clear benefit to protons (and for that matter IMRT) in the treatment of recurrent prostate cancer after prostatectomy over 3D conformal radiation therapy.