$EVWUDFW,'7LWOH$PXOWLSRUWDOFRPSHQVDWRUV\VWHPIRU,057GHOLYHU\ We have developed a multi-portal compensator system for IMRT delivery, comprising a rotational compensator mount for a linac head,
cylindrical compensator enclosures positioned in the mount, a vacuum-formed thermoplastic sheet with heavy alloy granules inside the
enclosure, and a vacuum thermoforming device. The mount rotates like a revolver by a stepping motor, thus allowing automatic multi-portal
IMRT without exchanging compensators by human operators during treatment. The thermoforming device has servo-motor-driven 10 by 10
metal rod elements to actualize an arbitrary intensity profile. The thermoplastic sheet is pre-heated by a built-in biplanar heater and then it is
placed over the rod elements. Subsequently, vacuum forming is performed through corner cut-outs of the rod elements. After forced
cooling-down, the heavy alloy granules are fed into the formed sheet. Preliminary experiment using solid water phantoms and an X-ray film
has shown that the intensity profile on the film agrees reasonably well with the desired profile. Clinical advantages of the present system
against the MLC-based IMRT include (1) much less treatment time which expectedly approaches that of conventional multi-portal open beam
treatment, and (2) MU correction and QA process is as easy as conventional open beam or wedge treatment. We also expect that the
compensator-based IMRT will be more frequently used in the near future when breathing-gated IMRT is routinely performed, because
MLC-based IMRT is not practical in terms of treatment time.