Doses near the surface during total-body irradiation with 15 MV X-rays

An extended source-surface distance (SSD) is often employed in total-body irradiation (TBI) to obtain fields sufficiently large to encompass the total body. It is clinically desirable to administer a uniform dose to all tissues, including the surface, especially to counteract the skin-sparing of high-energy X-rays. The electrons generated from the air contribute more to the dose near the surface at extended distances than at regular SSDs. However, if further increase of the dose at shallow depths is desired a spoiler can be employed. Monte Carlo simulations were used to study these effects for the electron component of the beam and the parameters needed for the pencil-beam dose calculation of this component. Measurements were performed to confirm these studies. The energy spectrum of electrons generated in air is independent of SSD. All the electrons in the beam, however, originate from the spoiler when one is used. The spectrum of these electrons is nearly independent of the SSD. The pencil-beam dose kernels were calculated using the derived electron spectra. To calculate doses in the buildup region with and without a spoiler, we use one pencil-beam kernel for the electrons generated in air and one for the electrons generated from the spoiler. Measured depth-dose data agreed with the calculation results.