Radiotherapy

Medical Physics | AQA A-Level Physics

External beam radiotherapy (EBRT): a linear acceleratorA machine (LINAC) that accelerates electrons to high energies and directs them at a metal target to produce high-energy X-rays. The beam rotates around the patient to deliver radiation from multiple angles. (LINAC) produces high-energy X-rays directed at the tumour from outside the body. The LINAC rotates around the patient so the beam enters from different angles, concentrating the dose on the tumour while spreading it across surrounding healthy tissue.
Conformal radiotherapyA technique where the X-ray beam is shaped to match the outline of the tumour, minimising exposure to surrounding healthy tissue. Metal filters or multi-leaf collimators shape the beam profile. shapes the beam to match the tumour outline using metal filters or multi-leaf collimators, further reducing damage to healthy tissue.
Metal filters remove low-energy X-rays from the LINAC beam. These low-energy photons would be absorbed by the skin and surface tissues without reaching the tumour, so removing them reduces skin dose.
Radioactive implants (brachytherapy): small radioactive sources are placed directly inside or next to the tumour. These sources emit beta radiationHigh-energy electrons (or positrons) emitted during radioactive decay. Beta particles have a short range in tissue (a few millimetres), which makes them suitable for treating small, localised tumours without damaging distant tissue., which has a short range in tissue (a few millimetres), so the radiation is confined to the tumour region. The sources have short half-lives.
Alpha emitters are not used for implants because alpha particles are too ionising and would cause severe local tissue damage.
Comparing imaging techniques (approximate spatial resolution): CT = 0.5 mm, X-ray = 0.5 mm, MRI = 1 mm, PET = 1 mm, ultrasound = 2 mm, gamma camera = 6 mm.