Monte Carlo assessment of computed tomography dose to tissue adjacent to the scanned volume

The assessment of the radiation dose to internal organs or to an embryo or fetus is required on occasion for risk assessment or for comparing imaging studies. Limited resources hinder the ability to accurately assess the radi ation dose received to locations outside the tissue volume actually scanned during computed tomography (CT). The purpose of this study was to assess p eripheral doses and provide tabular data for dose evaluation. Validated Mon te Carlo simulation techniques were used to compute the dose distribution a long the length of water-equivalent cylindrical phantoms, 16 and 32 cm in d iameter. For further validation, comparisons between physically measured an d Monte Carlo-derived air kerma profiles were performed and showed excellen t (1% to 2%) agreement. Polyenergetic x-ray spectra at 80, 100, 120, and 14 0 kVp with beam shaping filters were studied. Using 10(8) simulated photons input to the cylinders perpendicular to their long axis, line spread funct ions (LSF) of the dose distribution were determined at three depths in the cylinders (center, mid-depth, and surface). The LSF data were then used wit h appropriate mathematics to compute dose distributions along the long axis of the cylinder. The dose distributions resulting from helical (pitch=1.0) scans and axial scans were approximately equivalent. Beyond about 3 cm fro m the edge of the CT scanned tissue volume, the fall-off of radiation dose was exponential. A series of tables normalized at 100 milliampere seconds ( mAs) were produced which allow the straightforward assessment of dose withi n and peripheral to the CT scanned volume. The tables should be useful for medical physicists and radiologists in the estimation of dose to sites beyo nd the edge of the CT scanned volume. (C) 2000 American Association of Phys icists in Medicine. [S0094-2405(00)02310-5].