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Medical physics: the pick of the ASTRO papers
07.44 Wednesday ET: That authentic US election-night experience flagged up in the last blog posting turned out to be a somewhat pedestrian affair - i.e. pepperoni pizza and a couple of beers at the bar in Pietro’s (a top-notch Italian restaurant on the corner of Walnut and 17th Street). After a coffee from Starbucks, it was back to the hotel for another full-on assault from CNN, a channel which, over the past few days, has clearly decided that all other news bar the mid-term elections is cancelled. Though it’s worth noting that producers did make space for a breaking news flash Tuesday afternoon about pop babe Britney Spears filing for divorce from husband Kevin Federline (the subject of some debate among tired reporters and staff in the ASTRO press office yesterday evening). Seizmic shenanigans like this apart, the ASTRO meeting rolled on relentlessly…
PET insights on hypoxia
For this correspondent, Tuesday kicked off with a plenary session featuring the top three physics papers at this week’s conference. First up, Kelin Wang from the department of medical physics at Sloan-Kettering Cancer Center (New York, NY) presented the results of an 18FMISO-PET study in which he and his colleagues modelled transient and chronic tumour hypoxia (a low-oxygen state that makes tumours more resistant to chemo- and radiotherapies). The work secured Wang the resident clinical/basic science award from ASTRO’s scientific committee.
The New York team performed serial 18FMISO-PET pretreatment scans on 20 patients to ascertain possible changes in hypoxia distribution. Their conclusion: “These results, for the first time, provide information concerning the partition of chronic and transient hypoxia in human cancers, and as to how they may influence hypoxia imaging using PET.”
In a neat touch, and one that other scientific conferences could learn from, the ASTRO committee asked independent experts to provide brief supporting commentaries on each of the three plenary papers. The discussant on Wang’s paper, Mark Dewhirst (Duke University Medical Center, Durham, NC), noted that the work shows “intermittent hypoxia has a complex frequency distribution over time [and that] spatial distribution data suggest large networks of vessels are involved.”
Radiation sensitivity in the liver
Next, Yue Cao from the University of Michigan (Ann Arbor, MI) presented results of a study to predict radiation-induced liver dysfunction (RILD) using a local dose and regional venous-perfusion model. “We have shown that high dose conformal radiation combined with chemotherapy appears to prolong the survival of patients with unresectable intrahepatic cancers,” note Cao and her co-workers in their ASTRO paper. However, they add that “the ability to safely deliver higher doses is primarily limited by the development of RILD, characterized venous occlusion”.
During their research, Cao and colleagues developed a hypothesis that early monitoring of venous perfusion would have the potential to select patients with preclinical signs of perfusion changes prior to the onset of symptomatic radiation-induced injury. Preliminary data suggest that their local dose and regional venous perfusion model does indeed have the potential to “predict individual sensitivity to radiation”. Cao described the work as “the first step toward developing a method to delivery of higher and potentially more curative radiation doses to the patients who can safely receive those higher doses”.
In her supporting commentary on Cao’s talk, Laura Dawson (Princess Margaret Hospital, Toronto, ON, Canada) explained that the work is useful because it will help “to improve understanding of radiotherapy liver toxicity, with an ability to measure regional injury”. The challenge now, she says, is “correlation of perfusion with clinically important liver function [and] reproducibility of dose-dependent perfusion in larger patient populations”.
Radiation toxicity in the brain
The third plenary paper in the physics session was presented by Vijaya Nagesh from the department of radiation oncology at the University of Michigan. Using diffusion-tensor MRI, Nagesh and colleagues carried out a quantitative characterization of radiation-dose-dependent changes in normal-appearing white matter of patients with cerebral tumours. The study group comprised 20 patients who underwent 3D conformal radiotherapy.
The work is significant because radiation can have a number of effects on normal brain tissue, manifesting in neuroinflammation, demyelination and disruption of the blood-brain barrier. There can also be delayed neurological problems caused by white-matter necrosis. Yet these effects are poorly described so far, plus there may be multiple compounding factors (e.g. use of steroids, addition of chemotherapy, tumour recurrence and even the use of anti-epileptics).
Nagesh and her team found that quantitative diffusion-tensor MRI revealed acute and sub-acute effects of radiation in normal-appearing white matter (specifically, the genu of the corpus callosum). They conclude: “The decreased anisotropy of water diffusion indicates [a] reduction in restriction of water in the direction perpendicular to the fibre axis, possibly due to demyelination. The increase in mean diffusivity suggests overall structural degradation of white-matter fibre.”
In summary, Nagesh’s work shows that the ability of diffusion-tensor MRI to evaluate the susceptibility of white matter to radiation injury may well be valuable for predicting delayed neurological dysfunction. What’s more, as MRI technology improves, greater resolution may well lead to a more detailed understanding of cause and effect.
• Sharp-eyed readers of Monday’s blog post might be wondering what happened to the article on image guidance in radiotherapy. It’s still a work in progress. I have to catch up with a couple of vendors at the exhibition today, after which there’ll be a round-up of different approaches being pursued in IGRT later on this week.
