Physics News Update No. 686

The American Institute of Physics Bulletin of Physics News Number 686

May 28, 2004 by Phillip F. Schewe and Ben Stein

TO STOP TUMORS, KNOW HOW THEY GROW
Stimulating the immune system in a certain way can cause immune-system cells to surround tumors and stop them from growing, researchers have found (Antonio Br? Espino, Environmental Sciences Research Center, Spanish Research Council, antonio.bru@ccma.csic.es,).

Demonstrated in mice, the finding is a direct result of applying a new universal model of tumor growth developed over the last ten years in a collaboration between scientists at the Spanish Research Council and medical research centers in Spain. The researchers have evidence to show that all tumors grow in the same way, irrespective of the tissue or species in which they develop (Br? et al., Biophysical Journal, November 2003).

In a previous paper, these researchers reported that tumor growth, rather than being exponential as commonly believed, is a much slower "linear" process similar to the growth of certain crystals and other natural phenomena (Br? et al., Phys. Rev. Lett, 2 November 1998).

Tumor cells, they have found, grow through the diffusion or migration of cancer cells at the tumor's outer edges. Only the cells close to the edge of the tumor proliferate--those inside the tumor do not, contrary to previous assumptions. According to the researchers' observations, cells formed at the edge of the tumor diffuse at the border of the tumor mass until they settle in curved depressions where the competition for space is lowest and where they are best protected from the immune system.

In their new paper, Br? and co-workers show that the mechanical pressure exerted by immune-system cells known as "neutrophils" around mouse tumors can prevent the diffusion of these cells and thus prevent tumor growth. In 16 mice with a tumor mass in the muscle, the researchers induced neutrophil production by administering an immune system booster known as GM-CSF over two months.

In a short time, they observed that GM-CSF altered the growth dynamics of the cells. The tumors of two mice regressed completely and 80-90% tumor-cell death was seen in the rest. If the growth dynamics of tumors are universal, there is every reason to be hopeful the same result could be obtained in humans. Knowing how tumors grow, by cell diffusion at the surface, opens up the possibility of developing new and far more efficient ways of preventing their enlargement and spread. (Bru et al., Physical Review Letters, upcoming)

MAGNETIZATION INCREASES WITH TEMPERATURE for antiferromagnetic nanoparticles. This odd experimental finding, made a few years ago, is now explained, for the first time, by physicists at the Technical University of Denmark.

The experimental behavior is odd for two reasons: first because antiferromagnets, whose tiny neighboring magnetic moments generally line up in an alternating down and up pattern, are supposed to sustain no significant net magnetization of their own in an applied field; and second because magnetism itself, which arises at the microscopic level from the aligned magnetic moments of many atoms (the atoms act as tiny bar magnets), should tend to decline as the disruptive action of higher temperatures takes effect.

The Danish physicists explain why "thermoinduced magnetization" is missing in bulk antiferromagnetic samples (which accounts for their being nonmagnetic), but become more noticeable in dots with size below 10 nm. Steen Morup (morup@fysik.dtu.dk) and Cathrine Frandsen (fraca@fysik.dtu.dk) argue that antiferromagnetic nanoparticles might be engineered into a new class of material, one in which magnetization can be switched quickly and without energy loss, making it valuable for use in high-frequency electronic devices. (Morup and Frandsen, Physical Review Letters, 28 May 2004)

STRONTIUM-76 IS ONE OF THE MOST DEFORMED NUCLEI in its ground state and is the most deformed of all nuclei in which the number of protons (Z) equals the number of neutrons (N). This finding comes out of a new experiment in Switzerland.

The lighter N=Z nuclei, such as He-4, C-12, O-16, and Ca-40 are quite stable and among the most important nuclear species on earth, especially where life is concerned. But as the number of proton and neutron inhabitants of the nuclear abode increases distortion begins; the electric charges on the protons leads to mutual repulsion, and this leads to disintegration of the nucleus.

Nuclei struck by another nucleus can be sent into a rapidly spinning superdeformed state, but what about the quiescent shape of nuclei that haven't been hit? Earlier evidence suggested that Sr-76 should be about as deformed a nucleus as one can have in its ground state.

In a new study carried out at the CERN-ISOLDE facility in Geneva, a new method for measuring this deformation has been put into practice. First, the rare Sr-76 nuclei were made by smashing a proton beam into a target of niobium. The newly made Sr nuclei then diffused out of the target, ionized, and were swept away and sent to the heart of a spectrometer called "Lucrecia." (http://isolde.web.cern.ch/ISOLDE/).

There the fragile nuclei are directed up a slender hole in the world's largest crystal of pure sodium-iodide. It is in that sanctum that gamma rays from the fragmentation of the Sr-76 nuclei are observed. Not only the lifetime---7.89 seconds---can be deduced, but even the approximate shape of the nuclei can be worked out from the pattern of emergent gammas.

Sr-76 was not only shown to be highly deformed, as expected, but its shape is now determined to be highly prolate (its equatorial axis being some 40% less than its longer axis) rather than oblate. (Nacher et al., Physical Review Letters, upcoming article)

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