This editorial aims to simplify the understanding of percentage depth doses for photons and electrons, important for FRCR trainees and increase their confidence in attempting these questions.
- Skin surface dose:
Even though the electrons are liberated by photon interactions that deposit most doses within the tissue, some are still left at the patient’s surface, which is attributed to the radiation backscattered within the patient and the photon beam’s contamination with electrons and low energy photons from the treatment head and air.
A useful rule of thumb is that an electron beam’s mean energy decreases by 2 MeV per 1 cm of tissue and per 1 mm of lead. Therefore, the thickness of patient shielding required for a given electron beam is MeV/2. The lead shield should be faced with wax on the beam entry side to prevent high local doses due to backscatter.
In summary, the learning outcomes of this article are:
- As energy increases, the depth of dmax increases, and the surface dose decreases for photon beams. PDDs at depths beyond dmax increase with increasing energy, field size, and SSD.
- As energy increases, the depth of dmax, the therapeutic range, the practical range, and the surface dose increases for electron beams—the slope of fall-off beyond dmax decreases.