11.06 - Choice of beam and modifiers

Choice of Beam Energy

There are five typical beam energies to choose from:

  • Superficial/orthovoltage/kilovoltage beams
  • Low megavoltage photon beams (60Co, 4 MV, 6 MV)
  • High megavoltage photon beams (15 MV, 18 MV, 25 MV)
  • Low megavoltage electron beams (6, 9 MeV)
  • High megavoltage electron beams (15, 20 MeV)

Kilovoltage photon beams

Superficial x-rays have their zmax at skin surface, and dose falls of rapidly beyond this point. They are not suited for treatment to depths over 5 mm.
Superficial x-rays are best used for superficial lesions that do not extend into deeper tissues. Examples where superficial x-rays may be useful are:

  • Squamous cell carcinoma in situ (Bowen's Disease)
  • Superficial basal cell carcinoma / squamous cell carcinoma
  • Non-malignant conditions such as keloid scars

If the lesions invade into the subdermal layers, electrons are often a better choice for treatment.
Some regions (cheek, lips, eyelids) require internal shielding to be placed to prevent early / late side effects. Internal shielding requires coating with a tissue equivalent material to prevent backscattering onto the internal mucosal layer.

Megavoltage photon beams

The main differences between the different megavoltage beams are:

  • Skin sparing is more pronounced for beams > 10 MV
  • zmax moves deeper as beam energy increases
  • Increased dose at depth as beam energy increases
  • Widening of penumbra as beam energy increases
    • Note that 60Co beams (the lowest MV energy) have a wider penumbra than linac beams due to the finite source size

Photon beams are ideally suited to treating deeply seated targets. Lower MV energies are more useful in thinner regions (such as the head, neck and limbs) whereas higher energy photons are more easily able to deliver dose to central targets in the abdomen and pelvis.
The lung is a special case, as the widened penumbra seen with higher MV energies are more pronounced in lung inhomogeneity. The buidl up region in tissues surrounded by lung is also longer as photon energy increases. For these reasons, it is often preferable to use lower megavoltage beams (6 MV) in lung treatments which allow field sizes to be smaller.
Photon beams are less suited when a lesion is not as deeply seated, and perhaps located near to critical structures. In these scenarios, an electron beam may be preferable.

Megavoltage electron beams

The main differences between the different electron beams are:

  • Skin sparing becomes lower as beam energy increases (almost non-existant for energies of 16 MeV and above)
  • Therapeutic depth (the depth at which the beam is still useful) increases as beam energy increases
  • zmax initially increases with increasing energy
    • Data at my department suggest that the 20 MeV beam has a shallower zmax than the 16 and 12 MeV beams, but the therapeutic range is still deeper
  • Higher energies show a broadening of penumbra at depth. This is due to increased lateral range of the electrons, which becomes apparent after they have lost there initial energy in the superficial tissues. Lateral isodose lines are constricted above 50% and broaden below 50%.

Electron beams are suited to superficial treatments. They are able to treat to deeper depths than superficial x-rays, but also show a rapid dose fall off with minimal dose to structures beyond the treated area. Like superficial x-rays, electrons penetrate dense tissues (bone, metal) poorly and experience backscattering, making them unsuitable for treatment of tumours within bony cavities (such as the cranial vault).

Summary of beam energies

Kilovoltage photon beams are best suited for very superficial lesions of the skin which demonstrate minimal invasion. They outperform electrons in areas such as the face where there are critical structures in close proximity.
Megavoltage photon beams are chosen when deep targets require treatment. They are minimally affected by bony inhomogeneity. Higher megavoltage energies are better for deeper targets in the abdomen and pelvis. In the thorax, lower megavoltage energies are better due to a thinner penumbra and faster buildup when lung inhomogeneity is present.
Megavoltage electron beams are suited for lesions that are relatively superficial and not blocked by bony inhomogeneity. Lower energies have minimal depth penetration and are suitable for superficial lesions that invade deeply. Higher energies are suitable for treatments that are aimed at deeper structures, but suffer from a widening of the penumbra at depth.

Choice of Field Size

Once a planning target volume has been decided, beams must be chosen to cover the volume adequately. The important step in this process is recognising that all beams have a penumbra, and this penumbra must be accounted for when choosing a field size. The penumbra is dependent on the type of radiation used and the energy.

  • Kilovoltage beams have a relatively small penumbra but a margin of at least 0.5 cm should be used to surround the PTV.
  • Megavoltage beams have a sharp penumbra (about 0.7 cm) which becomes larger with increasing beam energies.
  • Electron beams have sharper penumbras at low energies and shallow depths. High megavoltage beams show significant broadening of the penumbra at depth which must be accounted for if a deep structure is the target.

Choice of Beam Arrangement

Kilovoltage and electron beams are typically used as direct fields. Electron beams are sometimes junctioned with other electron beams or megavoltage beams, leading to junctioning problems described elsewhere.
With megavoltage beams, there are several options for multiple beam arrangements.

  • A single field is rarely used as the dose distribution given is rarely of clinical use. There are some situations where a single field is preferable, for instance in supraclavicular fossa treatments or single palliative spine fields.
  • Parallel opposed fields are commonly used for palliative treatments, as well as in some other circumstances. They deliver a high dose to most structures covered by the fields, making them less suitable when there are organs at risk present. They are best used for:
    • Palliative treatments of the spine, thorax, brain and numerous other sites. This is because the dose delivered is unlikely to result in long term problems due to poor life expectancy.
    • Treatment of limbs. There are few critical structures in the limbs, so long as the entire width of the limb is not treated.
    • Treatment of para-aortic nodal regions. Some tumours (seminoma) metastasise preferentially to the para-aortic nodes. Due to the proximity of the kidneys laterally, it is often best to use parallel opposed fields to avoid dosing the kidneys at all. The relatively low dose used in this treatment assists in limiting toxicity.
  • A wedge pair is a special type of treatment used for irregular contours or in an attempt to treat a relatively superficial lesion without dosing deeper structures.
  • Multiple fields are the treatment of choice when the target volume is located near critical structures. Multiple fields allow the dose to be concentrated on the target volume, with a lower dose spread out to neighbouring tissues. This lower dose field leads to less deterministic side effects, but may increase the risk of stochastic effects in the treated organs.


Bolus is typically tissue equivalent material may serve two purposes:
Compensating bolus is used when there is contour irregularity on the patient's surface. The bolus smooths out the contour, and prevents hot spots and cold spots from developing in undesirable locations.
Build up bolus is used when skin sparing is not desirable. Skin sparing is a feature of megavoltage photon beams as well as electron beams with energies under about 15 MeV. In some situations, such as treatment of skin tumours which are invading deeply, dose is required at skin surface as well as at depth. Alternatively, skin dose may be desirable following the resection of some tumours (such as following mastectmoy for breast cancer, or some sarcoma treatments). Bolus thickness is based on the build up region of the depth dose curve, which is typically larger for lower electron energies and higher photon energies.