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Photodynamic therapy (m) using Photofrin ItR (PII) as a photosensitizer is currently being evaluated as a new Veahnent modality for superficial bladder cancer. An optimum therapeutic ratio requires uniform illumination of the whole bladder wall and accume light dosimeby. The first clinical light dosimetry results (16 patients) m reported, obtained using a system which allows in situ measurement and conhol of the light Ruence at the bladder wall.The true light Ruence at the bladder wall (i.e. non-scattered incident light plus scattered light) appeared to be on the average a factor , 9 = 4.8 * 1.2 (mean -k SDI larger than the non-scattered incident light fluence. The latter is often, but incorrectly, used in reporting light Ruence. The factor ,9 varied between patients with extremes of 2.5 and 7.1. Because such large variations were unexpected, but may have significant clinical consequences, experimenCi in plastic bladder models were performed to study separately the various factors (e.g. bladder shape, air bubble) aKecting the dosimetry in clinical trealments. The results imply that the clinical variations are most likely to be the result of variations in optical properties of the bladder wall mucosa, probably due to the disease and prior treatments. If light dosage is based on non-scattwed light only (without in situ light dosimetry, according to a current clinical protocol) the present results indicate variations in the true (total) light fluence between patients by a hctor of at least 2. At the least this may cause unnecessary discomfort to a number of patiem. 0031-9155/93/050567+l6$07.50 @ 1993 IOP Publishing Ltd 561 Star W M, Marijnissen J P A and van Gemen M I C 1988 Light dosimeq in optical phantoms and tissues. 1 : Multiple flux and Vanspofi theory Phys. Med. Biol. 33 437-54 the bladder, a preliminary lepon 3. Urol. 131 884-7 of bladder carcinoma 3. Photochem Photobid 8: B i d 6 183-7 cancer. First clinlcal experience Umloge(A) 24 31&9 of the bladder 3. Uml. 115 150-1

We propose a mechanism for the fast dissipation of magnetic fields which is effective in a stratified medium where ion motions can be neglected. In such a medium, the field is frozen into the electrons and Hall currents prevail. Although Hall currents conserve magnetic energy, in the presence of density gradients, they are able to create current sheets which can be the sites for efficient dissipation of magnetic fields. We recover the frequency, ωMH, for Hall oscillations modified by the presence of density gradients. We show that these oscillations can lead to the exchange of energy between different components of the field. We calculate the time evolution and show that magnetic fields can dissipate on a timescale of order 1/ωMH. This mechanism can play an important role for magnetic dissipation in systems with very steep density gradients where the ions are static such as those found in the solid crust of neutron stars. PACS number(s): 52.30. Bt, 47.65.+a, 97.60.J

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