Converging small-field electron therapy using 20–25 MeV electrons: a Monte Carlo feasibility study for deep-seated tumors

<i>Objective</i>. In photon beam radiotherapy, modern delivery techniques have allowed to substantially reduce the beam energy needed for the safe and efficient irradiation of deep-seated targets, with even Co-60 beams being now able to irradiate targets at any depth. The purpose of this work is to determine if for electron radiotherapy, advanced beam delivery techniques allow the use of beam energies currently available in the clinic to treat target sites usually reserved for photons or very high energy charged particles.<i>Methods</i>. Segmented computed tomography images from three sites, brain, lung and prostate, were used to model radiotherapy treatments in two modalities: conformal 3D and converging small field. Monte Carlo simulation was used to calculate the absorbed dose distribution in each patient for conformal 3D very-high energy plans and converging small-field, low energy (< 50 MeV) electrons. For comparison, converging small field plans for 6 MV x-ray beams were also calculated.<i>Main results</i>. It is shown that , for the three test cases simulated in this work, electrons with energies in the 20-25 MeV range delivered via the converging small-field modality can produce treatment plans that rival those obtained via conformal very high energy electrons in terms of target dose homogeneity and sparing of the organs at risk. Furthermore, such electron plans also compare well to those obtained with the photon beams.<i>Significance</i>. While the consensus has always been that to reach deeper tumors, higher electron energies, in the order of 150-200 MeV are needed, this work shows that this is not the case and, when small, circular electron fields are delivered in a converging manner, energies below 30 MeV are enough to properly irradiate tumors located at relevant radiological depths for a variety of treatment sites.