Dose Outside the Treatment Volume in External Beam Therapy

The algorithms in commercial treatment planning systems accurately calculate the high dose regions and areas within the primary beam path. "Out-of-field nontarget dose" is not only outside of the planning target volume but also outside of any primary field edge; dose deposited by leakage, scatter, or secondary radiation, that is, stray radiation. Knowing the dose outside the treatment volume can be important for several reasons. High linear energy transfer (LET) particles have been shown to be particularly damaging to these implantable devices, although relatively high cumulative doses from leakage and scatter can also induce malfunctions, and noise or oversensing events can be induced by specific dose rates. Proton or carbon ion beam therapies also produce secondary radiation outside of the treatment field. Diodes can be suitable for out-of-field dosimetry, but some caution is warranted. Ion chambers have been used to measure out-of-field doses, and have advantages in terms of having relatively little energy dependence.