Pitfalls in the use of high energy X rays to treat tumors in the lung.

The problem of central axis dose reduction for high energy photon beams of small cross-sectional area traversing normal lung tissue is well known. An additional problem, which may be not as well appreciated, is the loss of electronic equilibrium on the periphery of high energy photon beams, resulting in an increase in the penumbra occurring in lung. We have compared profiles of x-ray beams ranging in energy from 4 MV to 18 MV. The profiles were measured at 10-cm depth in unit-density and lung-density (0.26) phantoms. At the highest energy the 20% to 80% physical penumbra width was measured to be 7.5 mm in the unit-density material, whereas in the lung phantom the width was 18 mm. At 4 MV the situation was reversed; that is, the penumbra was slightly smaller in the lung phantom. Most computer programs for radiation therapy treatment planning do not take into account this change in beam profile when calculating dose in the lung. As a result, unanticipated underdosing inside the field and greater dose outside the field can occur when high-energy X rays are used.