Variation in calculated effective source-surface distances with depth

Effective source-surface distances (ESSD) are assessed at the depth of maximum dose in electron beams. This study investigated the variation of the ESSD with the depth of measurement. The dose was measured with the range of SSDs 100-130 cm, using a water-equivalent parallel-plate ion chamber in solid water. ESSDs were calculated for electron beams in the energy range 4-20 MeV and were found to vary with depth. The surface ESSD varied from 68 cm for 4 MeV to 82 cm for 16 MeV, but increased with depth to a maximum value, which was found at approximately half the practical range (R(p)), at 0.3R(p) for 4 MeV and at 0.6R(p) for 20 MeV. Beyond this depth the ESSD decreased towards the end of the practical range. Without an electron applicator, the ESSD was higher at the surface. For smaller field sizes, the depth of the maximum ESSD increased towards R(p), and ESSD values increased. The 20 MeV beam in the 6 cm x 6 cm(2) field showed a difference of 31 cm between the surface ESSD and the maximum ESSD. The ESSD calculated at the maximum dose depth (D-max) may be used with reasonable accuracy for calculation of the dose in the therapeutic range, except at larger SSDs or when high-energy beams are used in small fields. Depth-dose distributions under these conditions should be compared with measured results.