Tech Review - 3

The past few weeks have been as hectic as I've seen in months. Exams and travel always make for squashed personal time! That said, I scoped out my favorite journals and found an article* co-authored by W. Newhauser out of M. D. Anderson discussing secondary neutron dose generated during proton therapy of the prostate. The topic is interesting to me because I did a more generic study of such secondary dose last term for a project in which I first reviewed the literature and then assimilated basic aspects into a single model. That this new study is prostate-specific is interesting, as I'm currently performing research in (an unrelated aspect of) the area. Finally, I was set to collaborate with the author two summers ago but ran out of time; perhaps in the next half year or so I will have time to rehash the proposed work.

The article first points out that over 200,000 new cases of prostate cancer are found every year in the U.S., certainly an important factoid for the gents out there. The big ticket treatment options have been brachytherapy, external-beam radiotherapy, chemo, and the more drastic removal of the prostate. Joining the group as of late has been proton radiotherapy.

In general, proton beam radiotherapy offers a good method by which to deposit quite locally a given dose. However, the high energy used leads to production of secondary radiation, most noticeably being neutrons. The paper's main goal is to determine the equivalent dose Hi to the ith sensitive organ and the total effective patient dose E from secondary radiation. (Note, I had to review what exactly these quantities denote: Hi = dose*weightA; E = sum(Hi*weightB)).

In calculating H for given organs, the authors point out an interesting fact. Typical ICRP protocol assumes only an external neutron fluence, whereas in this treatment a significant neutron population is generated in the patient. They argue then that "weightA" is actually a function of the local neutron fluence, not of the fluence incident on the patient. ( I am confused at this point, for "weightA" is defined simply by incident neutron energy, irrespective of where that neutron happens to be. They give little insight as to what exactly changes. I would think that in a given locality (i.e. organ) the neutron energy spectrum is available, and using that, any set of weights could be applied properly.)

To quantify equivalent and effective doses, the article employs the ratios (H/D) and (E/D), read "equivalent dose per therapeutic absorbed dose" and "effective dose per therapeutic absorbed dose", respectively. D is simply the dose deposited by protons in the target region.

Their results indicate a typical treatment yields an (E/D) value of roughly 5.5 mSv/Gy. (H/D) values range from just under 2 mSv/Gy in the esophagus to nearly 13 mSv/Gy in the bladder. As one might imagine, these values are very dependent on proximity; the bladder neighbors the prostate, whereas the throat lays far from it.

These values match seemingly well with the study I performed last term. While I did not model a full patient phantom (I used a simple Lucite spherical phantom), I found the (H/D) value internal to the target to be roughly 50 mSv/Gy. Given this region could be broken into organs and assigned weights (as shown above), it is conceivable that I would have found a value near 5 mSv/Gy as the article presents.

* Fontenot, et al. "Equivalent dose and effective dose from stray radiation during passively scattered proton radiotherapy for prostate cancer", Phys. Med. Biol. 53 (2008).