Proton therapy physics explained

Proton therapy uses accelerated protons whose energy and depth can be planned so the main radiation effect reaches a selected tumour region. The beam is shaped and directed using imaging, treatment-planning software, and positioning equipment. Some radiation still reaches tissues before the target, so the plan considers every nearby structure.

Protons are positively charged particles. A treatment system accelerates them and uses magnets to guide the beam. As protons travel through the body, they release increasing energy near the end of their path, often called the Bragg peak. Clinicians select the proton energy to place this high-dose region at a planned depth, then combine beam angles and energies to cover the target.

The body is not perfectly still: breathing, digestion, and small positioning changes can shift anatomy. Imaging, immobilisation, motion management, and safety checks help the team account for these changes. The physical advantage is therefore highly dependent on accurate mapping and delivery, not on the machine alone.

Before treatment, you may have scans and a planning appointment, followed by a custom support to help you hold the same position. At each visit, staff check alignment before the beam is turned on. Tell them if the support feels uncomfortable or if you cannot maintain the position. Keep appointments for imaging and planning because changes in weight, swelling, or breathing can affect the setup.

Contact the treatment team for new or worsening pain, repeated vomiting, marked weakness, breathing changes, or skin reactions that become open, blistered, or increasingly painful. Seek urgent care for severe breathing difficulty, sudden confusion, fainting, or new weakness. The right response depends on the treated body area and your wider cancer care.

Ask how the beam depth was selected, which organs are close to the target, how breathing or movement is managed, and what imaging is used before delivery. Ask what uncertainties the team considered and which symptoms should be reported between visits.