According to X George Xu, professor of nuclear and biomedical engineering at Rensselaer Polytechnic Institute (Troy, NY), with the real threat of a patient developing a second cancer in his or her lifetime, it's prudent to ask whether advances in conformal radiotherapy procedures come at an ignored latent cost. Writing in a recent review article, Xu proposes that the radiation oncology and physics communities "keep their eyes open on these rapidly evolving radiation treatment technologies and ensure that improved local tumour control does not have to compromise the protection of patients against adverse long-term effects" (Phys. Med. Biol. 53 R193).

Xu and co-authors have performed a comprehensive review of dosimetry studies on external-beam radiotherapy, with respect to second-cancer induction. The modalities covered comprise classical radiation therapies (including 3D-conformal radiotherapy), IMRT and tomotherapy, stereotactic radiotherapy, plus proton and carbon-ion therapy. The authors also considered a wide range of epidemiological studies (which observe groups of patients and report findings in terms of dose-response functions).

Looking in particular at newer modalities like IMRT and proton therapy, the authors concluded that while these techniques can offer highly conformal target-dose distributions, better local tumour control does not necessarily equate to a lower scattered dose to distant regions. They suggest that for IMRT (with beam energies of 6 MV or less), a reduction in secondary photon dose should be strongly encouraged. Accelerator manufacturers, for example, could investigate ways to reduce the leakage and scattered radiation, possibly by increasing shielding.

Proton therapy brings the additional advantage of delivering a lower integral dose than treatment with photon beams - implying a lower risk for radiation-induced cancers. Here, however (and also for IMRT with higher beam energies), irradiation from secondary neutrons is the main concern. Xu suggests that measures such as the introduction of active scanning systems could reduce this secondary neutron production.

Xu discussed these issues at last month's 50th annual AAPM meeting in Houston, TX, speaking in a continuing education course co-presented by David Followill of the M D Anderson Cancer Center (Houston, TX). AAPM also saw several other M D Anderson researchers present recent results in this field. One study, for example, compared the second-cancer risks following proton therapy and IMRT for treating prostate cancer. The work concluded that proton therapy reduced risk of a second cancer by 17% compared to IMRT - attributed to the substantial reduction of dose to the rectum and bladder provided by the proton plan.

Another M D Anderson paper examined the risk of secondary fatal malignancies from CyberKnife radiosurgery, as compared with IMRT from a conventional, gantry-based accelerator, for paediatric brain and adult prostate treatments. Here, the researchers concluded that the overall risk of secondary fatal malignancies was higher for the CyberKnife, with the exception of critical organs adjacent to the treatment field for the paediatric case. This bias was thought to result from the greater number of monitor units and longer treatment times associated with CyberKnife treatments.

Writing in Physics in Medicine and Biology, Xu notes that, while there's a tremendous amount of data from dosimetry and epidemiological studies, most past dosimetry studies focused on determining the "out-of-field" dose, measured at positions relative to the target or the edge of the accelerator collimator. Such data are useful in comparing different accelerators or procedures - and should continue, particularly for evaluating new treatment modalities. But to estimate cancer risks for specific anatomical sites, Xu suggests that future studies should examine organ-specific absorbed or equivalent dose.

He also points out that patients irradiated by both therapeutic and diagnostic-imaging procedures will become a critical source of data for continuous refinement of dose-response functions. Such knowledge will ultimately enable the radiation oncology community to optimize radiation treatment procedures.