Measurements and simulations of focused beam for orthovoltage therapy.
OBJECTIVE
Megavoltage photon beams are typically used for therapy because of their skin-sparing effect. However, a focused low-energy x-ray beam would also be skin sparing, and would have a higher dose concentration at the focal spot. Such a beam can be produced with polycapillary optics. MCNP5 was used to model dose profiles for a scanned focused beam, using measured beam parameters. The potential of low energy focused x-ray beams for radiation therapy was assessed.


METHODS
A polycapillary optic was used to focus the x-ray beam from a tungsten source. The optic was characterized and measurements were performed at 50 kV. PMMA blocks of varying thicknesses were placed between optic and the focal spot to observe any variation in the focusing of the beam after passing through the tissue-equivalent material. The measured energy spectrum was used to model the focused beam in MCNP5. A source card (SDEF) in MCNP5 was used to simulate the converging x-ray beam. Dose calculations were performed inside a breast tissue phantom.


RESULTS
The measured focal spot size for the polycapillary optic was 0.2 mm with a depth of field of 5 mm. The measured focal spot remained unchanged through 40 mm of phantom thickness. The calculated depth dose curve inside the breast tissue showed a dose peak several centimeters below the skin with a sharp dose fall off around the focus. The percent dose falls below 10% within 5 mm of the focus. It was shown that rotating the optic during scanning would preserve the skin-sparing effect of the focused beam.


CONCLUSIONS
Low energy focused x-ray beams could be used to irradiate tumors inside soft tissue within 5 cm of the surface.
Department of Therapeutic Radiology, Yale University School of Medicine, Yale-New Haven Hospital, New Haven, 344 Lane Street Hamden, Connecticut 06514.
Hassan
H
Abbas
Intel Corporation, Mail-Stop RA3-410, 2501 NW 229th Avenue, Hillsboro, Oregon 97124.
Dip N
DN
Mahato
Department of Radiation Oncology, Albany Medical Center, 43 New Scotland Avenue, Albany, New York 12208.
Jahangir
J
Satti
Department of Physics, University at Albany, SUNY, 1400 Washington Avenue, Albany, New York 12222.
C A
CA
MacDonald
D009010:Monte Carlo Method; D011879:Radiotherapy Dosage; D012399:Rotation; D014964:X-Ray Therapy / Q000379:methods*