Proton Therapy — Procedure Guide, Recovery & Risks | MyMedicPlus

Proton therapy delivers ionising radiation using accelerated proton beams that deposit maximum energy at a precise tissue depth (the Bragg peak) with minimal exit dose beyond the target tumour volume. This physical characteristic — the absence of significant exit dose — distinguishes protons from conventional X-ray (photon) radiotherapy and reduces the radiation dose delivered to surrounding normal tissues and critical structures. The clinical benefit is greatest when the tumour is adjacent to radiosensitive structures (brainstem, spinal cord, optic nerves, cochlea, developing growth plates in children, heart) that would otherwise receive substantial dose from conventional radiotherapy. Proton therapy is delivered using a synchrocyclotron or cyclotron accelerating protons to energies of 60–250 MeV, with modern intensity-modulated proton therapy (IMPT) via pencil beam scanning achieving the most conformal dose distributions available in clinical radiotherapy practice. Proton therapy is an advanced form of external beam radiotherapy that uses accelerated protons rather than X-rays (photons) to deliver therapeutic radiation to tumours. The physical advantage of protons lies in their characteristic Bragg peak — a sharp rise in energy deposition at a defined depth followed by negligible exit dose — allowing precise tumour irradiation while sparing surrounding normal tissues. This dosimetric advantage is most clinically meaningful in paediatric cancers (where normal tissue sparing reduces late effects on developing organs), skull base and paraspinal tumours (proximity to brainstem and spinal cord), ocular melanoma (sparing the eye), and head and neck cancers (sparing salivary glands, spinal cord, and swallowing structures). Proton therapy is delivered using cyclotron or synchrotron accelerators in specialised proton therapy centres — facilities costing over $100 million that are available in only around 100 centres worldwide. Pencil beam scanning (PBS) technology allows intensity-modulated proton therapy (IMPT), delivering highly conformal dose distributions with millimetre precision.

Proton therapy is indicated where dosimetric superiority over photon radiotherapy translates into clinically meaningful reduction in late normal-tissue toxicity. Established indications include paediatric CNS tumours (medulloblastoma requiring craniospinal irradiation, ependymoma, low-grade glioma near eloquent structures), skull base chordoma and chondrosarcoma, head and neck cancers adjacent to the optic pathway or brainstem, ocular melanoma (using proton or carbon ion beams), spinal and paraspinal tumours, and re-irradiation of previously treated sites. Localised prostate cancer is a common adult indication, though outcomes data versus photon IMRT remain debated. Proton therapy is not universally available — fewer than 100 proton centres operate worldwide — and cost substantially exceeds that of photon radiotherapy, requiring prioritisation to cases where clinical benefit is expected. Contraindications include metallic implants within the treatment volume that distort dosimetry and tumours in locations where dosimetric gain over photons is marginal.

Proton therapy delivery follows a structured planning and treatment pathway requiring specialised infrastructure — cyclotron accelerators, gantry treatment rooms, and precision immobilisation systems — only available at dedicated proton therapy centres.

Patient immobilisation: High-precision immobilisation is critical because proton range is acutely sensitive to patient positioning. For head and neck and CNS tumours, a rigid thermoplastic mask is custom-fabricated over the patient's face and head in the treatment position, locking to the treatment couch. For body sites, vacuum-cushion body moulds and knee-and-ankle positioning devices ensure reproducibility to within 1-2 mm. Young children receiving craniospinal irradiation are immobilised under daily general anaesthesia.

CT simulation: A planning CT scan is acquired in the treatment position — using CT numbers (Hounsfield Units) that are converted to proton stopping power ratios to calculate precise depth of the Bragg peak in tissue. MRI and PET-CT are co-registered for target delineation. The simulation CT must be of diagnostic quality and extends from above the treatment field to below.

Target volume delineation: The radiation oncologist contours the gross tumour volume (GTV), clinical target volume (CTV), and planning target volume (PTV) with specific beam-path margins accounting for proton range uncertainty — typically 3-5% of the range plus 1-3 mm setup uncertainty. Organs at risk (brainstem, spinal cord, cochleae, optic nerves, kidneys, bowel) are carefully contoured.

Pencil beam scanning (intensity-modulated proton therapy, IMPT): Modern proton centres use active pencil beam scanning — a narrow proton beam (pencil) that is magnetically steered across the target volume layer by layer in a raster pattern. By adjusting the energy (and hence depth) and intensity of each pencil, a highly conformal dose distribution is created in 3D, with the individual Bragg peaks summated to cover the entire target volume. IMPT delivers superior dose conformity to passive scattering techniques and is now standard at most centres.

Daily image guidance: X-ray imaging and/or orthogonal pair X-rays verify patient position before each fraction. Cone-beam CT and proton CT are emerging technologies for direct in-room proton range verification. Bone landmarks or implanted fiducial markers confirm tumour position.

Treatment delivery: Each fraction takes 10-25 minutes including set-up. A rotating gantry (360 degrees) allows beam delivery from multiple angles to optimise dose conformity. Fixed horizontal and vertical beam lines are used for some treatment sites (e.g., horizontal beam for seated head and neck patients). Typical schedules deliver 1.8-2 Gy(RBE) per fraction over 25-38 fractions, or 3-8 Gy(RBE) in hypofractionated prostate or lung protocols.

Proton therapy achieves local control rates of 85–95% for skull base chordoma, 90–95% for uveal melanoma, and 85–90% for localised prostate cancer. In paediatric medulloblastoma, proton craniospinal irradiation reduces neurocognitive decline by 60% versus photon therapy, preserving IQ and academic performance in long-term survivors. For ependymoma, proton therapy achieves local control of 80–90% while significantly sparing cochlear dose, reducing sensorineural hearing loss compared with photon radiotherapy. Long-term cancer control is equivalent to photon radiotherapy in most tumour sites, but with a superior late-effects toxicity profile that is particularly important in patients expected to survive for decades — notably children and young adults. Proton therapy reduces integral radiation dose to normal tissues by 50–60% compared to IMRT for many indications, translating to clinically meaningful reduction in acute and late toxicities. For paediatric CNS tumours (medulloblastoma, ependymoma), proton CSI reduces neurocognitive toxicity, endocrine dysfunction, and secondary malignancy risk compared to photon CSI. For ocular melanoma, proton therapy achieves 5-year local control of over 95% with eye retention in over 80% of cases. For skull base chordoma and chondrosarcoma, proton therapy — particularly with carbon ion therapy available at specialist centres — achieves 5-year local control of 70–85% with acceptable brainstem toxicity.

Acute side effects of proton therapy depend on the treatment site. Common acute effects include fatigue, localised skin erythema (usually mild — protons have low skin dose), mucositis and dysphagia (head and neck sites), nausea and vomiting (craniospinal, abdominal), and urinary frequency and urgency (prostate). Alopecia is limited to the radiation field, unlike whole-brain photon therapy. Late effects depend on treated site but are generally less severe than with photon therapy: proton craniospinal irradiation significantly reduces dose to the cochlea (hearing loss), hippocampus (memory), pituitary-hypothalamus axis (endocrine dysfunction), and growth plates (spinal growth retardation) in children. Second malignancy risk from scattered radiation is lower than with photon radiotherapy. Proton range uncertainty — the possibility of the Bragg peak depositing dose slightly beyond the intended position — is a technical consideration managed through careful treatment planning and image-guided delivery.

Side effects during and after proton therapy are similar in timing but often milder in severity than those following photon radiotherapy. Fatigue typically peaks 2–4 weeks into treatment and persists for 4–6 weeks after completion. Mucositis and skin reactions in head and neck patients resolve over 4–8 weeks with supportive care (analgesics, dietary modification, oral care). Paediatric patients under general anaesthesia are monitored by paediatric anaesthesia teams and recover rapidly between fractions. Long-term endocrine surveillance is required in children receiving craniospinal irradiation: growth hormone axis, thyroid function, adrenal function, and pubertal development are assessed annually by a paediatric endocrinologist. Audiological monitoring detects cochlear toxicity for patients at risk of sensorineural hearing loss. Annual late-effects follow-up by a specialist team for at least 10 years is recommended for all paediatric proton therapy patients.