megavoltage

Megavoltage refers to radiation with photon or particle energies in the megavolt range, typically greater than 1 MeV. In medical physics, megavoltage X-rays are produced mainly for external beam radiotherapy.

Production and energy ranges

Megavoltage therapy beams are generated by linear accelerators (linacs). In a linac, electrons are accelerated to high energies (often 4 to 25 MeV) and then directed at a high‑Z target to produce bremsstrahlung photons in the megavoltage range. The resulting X-ray beams are heavily used for treating tumors at various depths.

Applications and dosimetry

Megavoltage beams provide substantial tissue penetration with a skin-sparing effect, meaning the maximum dose occurs beneath the surface instead of at the skin. This property makes MV radiotherapy suitable for treating deep-seated tumors. Dose delivery is characterized by depth-dose curves and buildup regions, with higher energy beams generally delivering dose more uniformly at depth but requiring careful planning to spare nearby organs. In radiotherapy, common MV beam energies include roughly 4–6 MV and 10–18 MV, with newer facilities offering up to about 20 MV. Compton scattering dominates interactions at these energies, offering relatively consistent dose distributions across a range of tissue compositions.

Imaging and safety

Megavoltage beams are also used for treatment verification imaging, such as portal imaging and MV cone-beam CT, though imaging contrast is typically lower than kilovoltage techniques. Shielding requirements for MV beams are substantial due to high penetrability, necessitating thick shielding materials in treatment rooms.

Overview

Megavoltage radiotherapy represents a core modality in cancer treatment, balancing deep tissue penetration with controlled dose deposition through careful planning and QA.