Radiotherapy and Monte Carlo simulation

Britain's lack of investment in particle physics may ultimately entail that the chances of surviving cancer under the National Health Service are significantly less than in Europe or the US.

Conventional radiotherapy attempts to kill cancerous tumours with a dose of X-rays. Unfortunately, a significant dose is also delivered to surrounding tissues and organs. In contrast, particles with a non-zero mass, such as protons and heavy ions, deliver a dose of radiation according to something called a Bragg peak. As a consequence, proton therapy and heavy ion radiotherapy offer a more precise tool for the treatment of cancer.

The link with particle physics is that proton and heavy-ion therapy requires particle accelerators, such as the synchrocyclotrons used in pure physics research. And, whilst Europe, the US, and Japan are forging ahead in this medical technology, here in the UK we have next to nothing. In fact, at the M.D. Anderson Cancer Centre in Texas, children suffering from a particular type of cancer are each given: (i) an individual CAT scan, to precisely identify the tumour location, shape and size; (ii) an individual MCNP (Monte-Carlo N-Particle) simulation, to ascertain the required proton dose distribution.

In this context, it is worth noting that in the US, 66.3 per cent of men and 62.9 per cent of women have a five-year cancer survival rate, whilst in Europe the survival rate is 47.3 per cent for men and 55.8 per cent for women (Eurocare statistics).

As a technical aside, MCNP is an interesting type of simulation because it represents particle interactions as a type of probabilistic billiards. Rather than evolving a quantum mechanical wave-function, particles are given determinate particle tracks between collisions, but the interactions are probabilistic. The distance between collisions is probabilistically selected, the nature of the interaction is probabilistically selected, and the outcome of a collision, (the direction and energy of an outgoing particle, for example) is selected according to the quantum mechanical cross-sections. (In quantum theory, the probability of an interaction is specified by something called a cross-section; the greater the cross-section, the greater the probability).

For an introduction to these issues, my own paper on the subject, Dosimetry, Scattering theory, and Monte Carlo simulation, might be of some assistance.

The Tegmark interview

In dread of the 10-hour Virgin Atlantic flight back from the US last weekend, I tried to purchase as much reading matter as I could at Albuquerque airport. In doing so, I came across this excellent interview in Discover magazine with Mr Multiverse, Max Tegmark. Max's big idea is that physical existence coincides with mathematical existence.

In particular, I reverberated to the wisdom of Max's career strategy:

"I learned pretty early that if I focused exclusively on these big questions I’d end up working at McDonald's," Tegmark explains. "So I developed this Dr. Jekyll/Mr. Hyde strategy where officially, whenever I applied for jobs, I put forth my mainstream work. And then quietly, on the side, I pursued more philosophical interests."

Artifactitude

There is, reputedly, a greater density of art galleries and museums in Sante Fe than in New York City. Moreover, entry to all of the galleries in the centre of Sante Fe is completely free on a Friday night. It was for this reason, then, that I paid full admission to gain entry to the New Mexico Museum of Art last Sunday.

The museum itself is a stunning mock-adobe construction, and currently houses several interesting exhibitions, including Flux, a collection of glass sculptures. This really appealed to me, because in such sculpture, the art is so clearly built upon the craft, and one cannot imagine being able to produce such works oneself without a degree of learning and dedication.

As ever, I was amused by some of expository captions which accompanied the exhibits. One such explained that whilst many glass sculptures attempt to mimic other artifacts, there is an alternative trend in which the sculpture adopts the attitude of being an artifact in itself, an attitude which it dubbed 'artifactitude'. A further caption spoke of a nascent tendency for glass sculptors to produce works which are not vessels, but, rather, anti-vessels. Sadly, no definition was furnished of what an anti-vessel is, but perhaps the answer lies with this paper by Hans Halvorson and David Baker on the concept of antimatter.

Los Alamos

Los Alamos is distributed across the top of several canyon-separated mesas, at an altitude of 8,000ft. Thankfully, it's quite cool here, at only 89 degrees Fahrenheit. In contrast, Albuquerque, through which we passed on Saturday, is 98 degrees today.

The journey across New Mexico on Interstate 40 was truly eye-boggling. The road often stretches for miles, in a straight line, across a Road-Runner landscape, towards the horizon. And what is truly remarkable here is the speed-discipline of American drivers. The speed limit is only 75mph at most, and yet the vast majority of drivers rarely exceed the limit by more than 5mph. Most drivers in the UK, confronted with an open road stretching literally miles into the distance, would hit 100mph within minutes.

Vegas

There's no way of putting into words just how mental Vegas is. The interactive television service in the rooms of the Las Vegas Hilton affords guests the opportunity to book a prostitute, without anything appearing on the hotel bill.
It's also hot. Hotter, in fact, than a cauldron in a furnace inside a steel factory.




I'm now in Flagstaff, Arizona, having driven yesterday from Vegas to the Hoover Dam, and then on to the Grand Canyon. Onward to New Mexico today...

The risks of having a partner

Singletons of the world rejoice! It transpires that sleeping with someone can, over the course of a year, increase your annual radiation dose by up to 2 millirems! There seems to be some disagreement, however, because this article from Discover Magazine claims that the dose is 50% lower:

40 millirems of our annual dose is internal, generated from the decay of isotopes incorporated into the molecules of our being: a potassium-40 atom in the brain firing off a gamma ray here, a carbon-14 atom in the liver spitting out a beta particle there. Enough radiation escapes our bodies that sleeping nightly with another person adds 1 millirem to your annual dose.

I propose some research to uncover the true figure...

Licking the bowl of an old pipe

"Och, that's horrible!" she exclaimed. "There's no whey I'm drinking that!"

If I recall correctly, my fellow attendee to the 21st LH Gray Conference in Edinburgh this week, was sampling the second Whisky on offer, 'Sophia Lauren in a mink coat.' This one was only 49.1% alcohol, but it was, indeed, completely undrinkable. As an inexperienced Whisky drinker, however, I should add that I was very much in the minority of the contingent gathered at the Scotch Malt Whisky Society on Queen Street. And, as a low sun sparkled through the window on a long, balmy Summer evening, our urbane host explained to a rapt audience the craft of Whisky production, and the art of Whisky drinking, with exemplary enthusiasm and clarity.

The third Whisky, 'Sticky toffee pudding and baked apples', was, at 62.6% alcohol, also a little strong. So I added a wee dash of water. After which it was still too strong. The fouth sample ('Tinned peaches and tobacco pipes'), however, was rather interesting, evoking a pleasant initial flavour, but leaving the residual taste of "licking the bowl of an old pipe."

Despite being clearly out of my depth here, it was a delightful evening, and I would like to thank our hosts, and recommend the experience to anyone who gets the opportunity!

Which is a good deal less than I would say about the experience of flying from Terminal 5 at Heathrow. Now, to be honest, I'm not sure that the Terminal itself can be blamed on this occasion, but after boarding our plane, we then sat for fully 2 hours in our cramped cigar tube while 'engineers' struggled to replace a damaged panel on the underside of the plane. Our BA Captain kept us informed of progress throughout the delay, even exhibiting at one stage physical specimens of the small screws which the engineers were struggling to insert, as we all sat reading Business Life magazine and other sub-journalistic deposits.

And the conference? Well, there were a fascinating array of topics, including an account by Nick Gent from the Health Protection Agency on the implications of the Litvinenko polonium poisoning incident; interesting developments and ideas in oncology and mammography; the emerging epidemiology from the Chernobyl accident; and a raging debate on whether low doses of radiation can actually be beneficial to human health (so-called hormesis). The quality of the presentations was excellent, if a little data-intensive. I shall perhaps expand upon some of the issues in days to come, but for the moment, the final word must go to our Whisky guide, and its interpretation of the third sample:

A thick, chewy texture; burnt toffee; sweet, but also tannic dry, with hints of Oloroso sherry; a long finish, and an aftertaste of fruit cake and blowing up 100 balloons.

Cosmic time

It has become almost de rigeur in quantum gravity to question the meaning of time in the Planck regime, before the universe was 10^-43s old. As Henrik Zinkernagel and Svend Rugh point out, however, there are serious doubts whether our concept of time can be applied before the electroweak phase transition, when the universe was 10^-11s old. The basic problem can be put like this:

1) Massless particles have no proper time. Massless particles trace null curves in space-time, and are invariant under conformal transformations (i.e., they are invariant under local scale transformations).

2) Before the electroweak phase transition, the weak and electroweak forces were unified, the electroweak Higgs field(s) were zero, and as a consequence, all particles were massless. According to the Standard Model, the elementary quarks and leptons, and the interaction carrieres of the weak force, only possess mass today by virtue of non-zero Higgs field(s).

3) Hence, before the electroweak phase transition, there was no meaningful measure of time.

I think Zinkernagel and Rugh have a point...