A small number of centers operate experimental and pilot programs employing beams of heavier particles, particularly protons, owing to the rapid decrease in absorbed dose beneath the depth of the target.

[2] Typically, higher-energy megavoltage X-rays are chosen when it is desirable to maximize "skin-sparing" (since the relative dose to the skin is lower for such high-energy beams).

Such photon beams, derived from radioactive decay, are approximately monochromatic, in contrast to the continuous bremsstrahlung spectrum from a linac.

Orthovoltage units have essentially the same design as diagnostic X-ray machines and are generally limited to photon energies less than 600 keV.

[4][5] Nonetheless, cobalt treatment still retains some applications, such as the Gamma Knife, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator.

Bremsstrahlung X-rays are produced by bombarding energetic cathode rays (electrons) onto a target made of a material with high atomic number, such as tungsten.

For instance, a 100 kVp beam is produced by a 100 kV voltage applied to an X-ray tube and will have a maximum photon energy of 100 keV.

These factors are reflected in the beam's half-value layer (HVL), measured in-air under conditions of "good geometry".

Electron beams are useful for treating superficial lesions, because the maximum dose deposition occurs near the surface and thereafter decreases rapidly with depth, sparing underlying tissue.

Although the X-ray target is removed in electron mode, the beam must be fanned out by sets of thin scattering foils in order to achieve flat and symmetric dose profiles in the treated tissue.

Hadron therapy involves the therapeutic use of protons, neutrons, and heavier ions (fully ionized atomic nuclei).

Of these, proton therapy is by far the most common, though still rare compared to other forms of external beam radiotherapy, since it requires large and expensive equipment.

This technology allows radiotherapy treatment planners great flexibility in shielding organs-at-risk (OARSs), while ensuring that the prescribed dose is delivered to the target organs.

Intensity modulated radiation therapy (IMRT) is an advanced radiotherapy technique used to minimize the amount of normal tissue being irradiated in the treatment field.

Doctors have found that this sometimes allows them to safely give a higher dose of radiation to the tumor, potentially increasing the chance of successful treatment.

[10] Volumetric modulated arc therapy (VMAT) is an extension of IMRT characterized by a linear accelerator rotating around the patient.

[13] Image-guided radiation therapy (IGRT) augments radiotherapy with imaging to increase the accuracy and precision of target localization, thereby reducing the amount of healthy tissue in the treatment field.

Therefore, methods such as stereoscopic digital kilovoltage imaging-based patient position verification (PPVS),[14] and alignment estimation based on in-situ cone-beam computed tomography (CT), enrich the range of modern IGRT approaches.