purpose of multileaf collimator in radiotherapy

Purpose Of Multileaf Collimator In Radiotherapy Skin And Subcutaneous

B, Electron beam isodose curves Note the constriction of 90 isodose at depth which necessitates addition of wider margins to beam width for adequate coverage of tumour at depth.

Papanicolaou, Roozbeh Rezaie, Shalini Narayana, Marina Kilintari, Asim F.

At least half of the patients with cancer require radiotherapy at some time in their illness of which about 60 are treated with curative intent, often in combination with surgery and chemotherapy.

Radiotherapy involves use of various types of ionizing radiation, and X-ray is the commonest type of radiation used.

Other forms of radiation include electrons, protons, neutrons and gamma radiations from radioactive isotopes.

This chapter intends to review the principles of practical radiotherapy ( Box 3.1 ) and a detailed discussion on radiobiology and mathematical modelling are beyond the scope of this chapter.

There are already adequate data on these various indications for radiotherapy for different neoplasms.

With current understanding of radiation biology and improved techniques, more indications are evolving, particularly for tumours previously thought to be radioresistant (e.g.

Curative treatment involves either radical radiotherapy (radiotherapy given either alone or in combination with chemotherapy or biological agents) or adjuvant radiotherapy (given after definitive treatment of tumour, usually surgery).

Palliative radiotherapy is intended to improve symptoms (e.g.

Depending on the site and type of cancer, radiotherapy is delivered by either one of these techniques or a combination.

Sometimes radiotherapy is delivered concurrently with chemotherapy or biological agents to improve the chances of cure (see below).

Before the 1950s, radiotherapy units were kilovoltage machines which produced X-rays with limited penetrability.

These machines, which are still used in some centres, can be one of the following.

In many centres, these machines are being replaced by electron beam treatment.

Megavoltage X-rays are produced by artificial acceleration of electrons through a vacuum to impact on a target in machines called linear accelerators (LINACs) ( Box 3.2 ).

The energy of the X-rays is proportional to the speed of electrons.

Electrons are produced when the target in a linear accelerator is removed from the path of the electron beam ( Box 3.2 ).

Modern LINACs have facilities to produce both photons and electrons.

Electrons have a predictable penetrability and hence are useful when it is important to limit the radiation dose to a deeper organ.

Characteristics of photons that are useful in their clinical use are.

Purpose Of Multileaf Collimator In Radiotherapy Skin And Subcutaneous

This helps to deliver higher dose to depth without causing significant damage to skin and subcutaneous tissue ( Box 3.3 ).

The X-rays produced from the target are collimated by primary collimators in the direction of patient.

This beam is further modified by flattening filter, secondary collimation and multileaf collimators before reaching patients surface.

With increasing energy, the surface dose decreases and hence if higher dose is needed at the surface, a bolus is used ( p.

Bolus is also used to bring up high dose region to surface to avoid radiation to underlying critical structure.

Higher energy electron beams exhibit a lateral constriction of 90 isodose line ( Figure 3.2B ) which need to be taken into consideration while deciding the field size at surface.

Dose at a depth increases with energy of photos.

MV is generally used, however in large patients with very deep seated tumours 10 MV yields a better dose distribution.