Proton beam vs. intensity-modulated RT: out on the battle lines

The relative merits of proton beam radiotherapy (PBRT) and the more commonly available intensity-modulated radiotherapy (IMRT) as forms of external beam radiation for prostate cancer are much debated … with minimal supportive evidence on either side of the debate because no comparative trial has ever been carried out.

A newly published paper by Fonentot et al. appears (again) to claim major “advantages” for PBRT, based this time on a theoretical model. The supposed advantages are founded on the assumptions made in the creation of the model. However, this doesn’t stop the authors from concluding that, “When considering exposure to primary and secondary radiation, proton therapy can reduce the risk of an [second malignant neoplasm] in prostate patients compared with contemporary IMRT.”

What Fontenot and colleagues are suggesting is that PBRT comes with less risk for a later, radiotherapy-induced cancer (as a side effect of the initial radiation) than IMRT. This may well be the case in theory, but until we have significantly more evidence to support the theory, it’s probably not the best reason to have PBRT as opposed to any other form of radiotherapy. This topic has come up for discussion before on The “New” Prostate Cancer InfoLink, following a media release at the ASTRO annual meeting last year.

In their study, what Fontenot et al. actually did was to develop two treatment plans for each of three patients with early-stage prostate cancer. (The patients’ actual clinical characteristics are not provided in the abstract.). One plan was for PBRT and the other was for 6-MV IMRT. They then calculated the theoretical doses of radiation that would be delivered to organs (other than the prostate) that would be at risk of developing a second malignant neoplasm (SMN) as a consequence of the treatment, using sophisticated (Monte Carlo) simulations and available measured data, respectively. The risk of an SMN was estimated from primary and secondary doses on an organ-by-organ basis by use of risk models from the Committee on the Biological Effects of Ionizing Radiation.

They report that, based on this model:

  • PBRT reduced the risk of an SMN by between 26 and 39 percent compared with IMRT.
  • The risk of an SMN for both forms of treatment was greatest in the in-field organs (e.g., the bladder and the colon or rectum).
  • The risks to the in-field organs were considerably lower with the PBRT plan than with the IMRT plan.
  • This reduction can be attributed to the substantial sparing of the rectum and bladder from exposure to the therapeutic beam by the PBRT plan.

The “New” Prostate Cancer InfoLink has little doubt that the model created by Fontenot and colleagues is correct — based on the assumptions that they have made. However, we also have little doubt that an equally accurate model can be created (based on equally valid assumptions) by proponents of IMRT, and that such a model would suggest there is little to no difference in risk for SMNs between the two techniques. The bottom line is that without actual clinical outcome data, we have no real idea whether there is a greater risk of secondary cancers from either one or other of these techniques.

We would suggest to patients that, if they start hearing from members of the radiotherapy community that “PBRT is less likely to result in secondary cancers than IMRT,” that they answer by saying, “Really, please show me actual clinical outcomes that prove this.” Given the strong feelings about PBRT and IMRT among some members of the radiotherapy community, we do expect the “results” of this study to be used to justify that type of promotional statement. And we do expect the general media to publish reports that suggest this is the case — even though there is no proof.

Let us be clear: PBRT is certainly a highly effective and safe method for the treatment of prostate cancer. Whether it is really any more effective — or safer — than contemporary IMRT is unproven at this time, and since the PBRT community seems unwilling to commit to a head-to-head trial of the two types of radiation, it seems likely that it will never be proven.

As usual, none of this will be in the best interests of the patients who need help, which is just sad. It is all about people’s revenue streams and academic scoring of points.

Oh … and just as a reminder … the risk of a second malignancy after external beam radiation therapy for prostate cancer is probably somewhere between 1-2 percent, usually some 7-10 years after initial treatment. Thus, at best, a 35 percent reduction of risk for such a second malignancy by using PBRT as compared to IMRT would reduce that risk to between 0.65 and 1.3 percent. Does anyone remember how many angels one actually can fit onto the head of a pin?

3 Responses

  1. If you are one of those 10 years out and having to undergo additional follow up for those kind of secondary problems, your views would probably be different. I have my doubts about the PBRT community (other than those still doing both and too widely committed to the IMRT side) being unwilling to commit to and/or openly compare the treatment methods. Give them time. The proof will be forthcoming.

  2. I had 42 IMRT treatments between October and December 2005. No complications of significance. PSAs continued to rise from 2006 to the present. Started ADT hormone treatments in February 2008 and recurrent prostate cancer is under control(?).

    I don’t really believe that IMRT treatments were 100% effective, maybe 50%.

    Just some comments.


  3. Dear Mr Ensch:

    I am sorry to hear that first-line treatment was not successful in your case. However, …

    The fact that IMRT was not effective in eliminating your prostate cancer is unlikely to have been the fault of the procedure. It is far more likely that you already had cancer that had escaped the area that was being radiated, but that this was not recognized at the time of treatment.


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