Radiation absorption properties of orbital implants

PURPOSE: To determine the radiation absorption properties (RAP) of three commonly used orbital implant materials, namely, methylmethacrylate (MM), hydroxyapatite (HA), and porous polyethylene (PP). METHODS: Eighteen (18)-mm spheres of MM, HA and PP were tested with 1.25 MV gamma-rays from cobalt-60, 6 MV X-rays, and 9 MeV, 12 MeV, and 16 MeV electron beams. The implants were immersed in a water phantom, and the measurements were obtained on X-omat V(R) film; the scanning was done with a computerized laser densitometer, CADSCAN(R). RESULTS: The RAP of all three materials appeared to be very close to those of water. The density of the MM implant was calculated to be the closest to that of water at all photon and electron energies. PP had a higher transmission than water at all electron energies (9, 12 and 16 MeV); the transmission through HA, however, was lower than through water. CONCLUSION: When postoperative radiation is indicated for an orbit containing an implant, the RAP of the allograft material play a significant role in the planning of the radiation treatment. Our study indicated that MM implants have RAP equivalent to those of water when treatment of orbital tumors is undertaken at the commonly used photon and electron energies. The RAP of the other allografts were either higher or lower, which may lead to unreliability in irradiation planning.