What are the main types of radiation therapy?

External beam (photons, protons, heavy ions), brachytherapy (low- and high-dose-rate), intraoperative radiation, and systemic radiopharmaceuticals.

How is radiation dose chosen?

By tumor type, size, location, surrounding organ tolerances, intent (curative vs palliative), biologic factors, and patient-specific considerations using dose–constraint models.

Is proton therapy always better than photon therapy?

Not always. Proton therapy can spare normal tissues due to the Bragg peak but benefits depend on anatomy, motion, and clinical evidence for the specific cancer.

Can radiation be repeated?

Sometimes. Re-irradiation depends on prior dose to critical organs, time since treatment, and potential benefits; it requires careful planning and risk assessment.

What is radiosurgery?

Stereotactic radiosurgery is a high-precision, high-dose radiation delivered in 1–5 sessions to small targets, commonly used in brain and selected body sites.

From Photons to Protons: Your Guide to Modern Radiation Therapy

Credentials: Board-certified radiation oncologist and academic researcher with clinical experience across photon, proton, brachytherapy, and radiosurgery platforms.

Editorial integrity: Independent, evidence-informed overview designed to help patients and families have high-value conversations with care teams.

Disclaimer: Informational only, not medical advice. Individual treatments must be personalized by a qualified oncology team.

Key Takeaways

Radiation therapy includes external beam (photons, protons, ions), brachytherapy, intraoperative radiation, and systemic radiopharmaceuticals.
Technique choice depends on tumor location, size, motion, nearby organs, and clinical goals (cure, organ preservation, palliation).
Dose is tailored using tumor biology, normal tissue tolerances, and fractionation models to balance tumor control and safety.
Modern planning integrates imaging, motion management, and constraints to protect critical organs while hitting the target.

Direct Answer: What Are the Different Types—and When Are They Used?

Radiation treatments span four main categories: external beam radiation therapy (EBRT) using photons, protons, or heavy ions; brachytherapy, which places radiation sources inside or near the tumor; intraoperative radiation delivered during surgery; and systemic radiopharmaceuticals that carry radiation to specific tissues. Clinicians select among these by considering the disease site, tumor extent, motion, proximity to sensitive organs, and treatment intent, then choose a dose and fractionation schedule that maximizes cancer control while respecting organ tolerances.

Definitions and Context

Radiation therapy uses high-energy particles or waves to damage cancer cell DNA, preventing reproduction and leading to cell death or permanent growth arrest. While normal tissues can also be affected, modern techniques focus dose tightly on targets, using advanced imaging and planning to reduce collateral injury.

Treatments are broadly curative (definitive), adjuvant (after surgery to lower recurrence), neoadjuvant (before surgery to shrink tumors), or palliative (relieve symptoms). Each aim dictates trade-offs in dose, coverage, and time.

Mechanisms and How It Works

Photons (X-rays) deposit energy along their path, causing DNA damage directly and indirectly via free radicals. Protons and heavy ions deposit most energy at a controllable depth (Bragg peak), reducing exit dose. Brachytherapy delivers a steep dose falloff from the source outward. Radiopharmaceuticals deliver radiation selectively based on molecular targeting.

Biologically, fractionation—spreading dose over multiple sessions—exploits normal tissue repair, tumor reoxygenation, and cell cycle effects to improve the therapeutic ratio.

Types of Radiation Therapy

External Beam Photon Therapy

3D-CRT (Three-Dimensional Conformal RT): Uses shaped beams to conform dose to the tumor; foundational for many sites with moderate complexity.

IMRT (Intensity-Modulated RT): Varies beam intensity to sculpt dose around critical organs; standard for head and neck, prostate, pelvic, and many complex thoracic/abdominal tumors.

VMAT/RapidArc: Delivers IMRT while the gantry rotates, improving efficiency and sometimes sparing.

IGRT (Image-Guided RT): Uses frequent imaging (cone-beam CT, stereoscopic X-ray) to adjust for day-to-day setup and anatomical changes.

Stereotactic Techniques

SRS (Stereotactic Radiosurgery): Ultra-precise, high dose in 1–5 fractions, typically for brain metastases, benign brain tumors (e.g., meningioma), and arteriovenous malformations.

SBRT/SABR (Stereotactic Body RT): Similar precision for small targets outside the brain, e.g., early-stage lung cancer in medically inoperable patients, limited liver metastases, spinal lesions, and selected adrenal or pancreatic lesions.

Particle Therapy

Proton Therapy: Uses charged particles with a Bragg peak to reduce exit dose; helpful near critical structures (pediatric CNS, base of skull, some head and neck, select thoracic and pelvic tumors).

Heavy Ions (e.g., Carbon Ions): Offer high linear energy transfer (LET) and potentially greater biological effectiveness; used in select centers for radioresistant tumors (e.g., chordoma, certain sarcomas) and challenging locations.

Brachytherapy

LDR (Low-Dose-Rate): Permanent seeds (e.g., prostate) or temporary implants deliver continuous low-intensity radiation.

HDR (High-Dose-Rate): Temporary catheters guide a high-activity source to specific positions for minutes at a time; used in gynecologic cancers, breast, skin, airway, and select GI/GU sites.

Intraluminal/Interstitial/Surface: Applicators or catheters placed in cavities, tissues, or on skin for localized high-dose treatment with rapid falloff.

Intraoperative Radiation Therapy (IORT)

Delivers a single high dose to the tumor bed during surgery when organs can be retracted or shielded, commonly in select breast, GI, sarcoma, or pelvic recurrences.

Systemic Radiopharmaceuticals

Radiation-emitting drugs home to targets: bone-seeking agents for metastatic prostate or other cancers, and targeted agents for specific receptors (e.g., neuroendocrine tumors, prostate-specific membrane antigen). These are coordinated with nuclear medicine and medical oncology.

Dose and Fractionation: How Dosage Is Selected

Dosage is measured in Gray (Gy). The selection considers tumor type and size, sensitivity, proximity to critical organs, motion, patient health, and treatment goal. Standard fractionation typically uses 1.8–2.0 Gy per session over weeks, while hypofractionation uses larger doses per fraction over fewer sessions.

For small, well-defined targets (e.g., early lung tumors), SBRT delivers high doses in 1–5 fractions to maximize local control. For head and neck or pelvic cancers near sensitive tissues, IMRT with standard fractionation helps spare salivary glands, bowel, or bladder.

When using protons, dose selection also accounts for range uncertainty and tissue heterogeneity. For brachytherapy, dose depends on source strength, dwell times, and geometry of applicators to achieve a conformal implant.

Cancer-Specific Applications (Illustrative)

Breast Cancer

Whole-breast radiation after lumpectomy reduces recurrence; regional nodal irradiation is added for higher-risk features. Hypofractionation is common for convenience and equivalence. Partial-breast irradiation and intraoperative options are considered in select early-stage cases.

Prostate Cancer

Choices include IMRT/VMAT, SBRT for favorable-risk disease, proton therapy in select scenarios, and brachytherapy (LDR or HDR) for dose intensification. Risk group, prostate size, urinary function, and patient preference guide selection.

Head and Neck Cancers

IMRT is standard to protect salivary glands, spinal cord, and swallowing structures while delivering curative doses. Proton therapy can be considered for certain nasopharyngeal or skull base tumors.

Lung Cancer

Early-stage inoperable disease is often treated with SBRT for high local control. Locally advanced disease combines chemoradiation with photon IMRT; protons may reduce heart and lung dose in select cases.

Central Nervous System

SRS treats small brain metastases and selected benign tumors. Proton therapy reduces integral dose for pediatric and skull base tumors. Conventional fractionation is used post-operatively for high-grade gliomas.

Gynecologic Cancers

External beam with brachytherapy boost (HDR) is standard in cervical cancer. Endometrial cancers may receive vaginal cuff brachytherapy, with or without external beam, based on risk.

Gastrointestinal and Hepatobiliary

IMRT or proton therapy for esophageal and pancreatic cancers to spare heart, lungs, and kidneys; SBRT or proton therapy may be used for liver tumors with motion management and strict constraints.

Sarcomas

Pre- or post-operative radiation with photon IMRT or proton therapy to spare joints and neurovascular bundles; heavy ions in select centers for radioresistant histologies like chordoma.

Benefits vs. Trade-offs

Benefits: Noninvasive local control, organ preservation, and symptom relief with modern precision and sparing. Flexibility to combine with surgery and systemic therapy for curative intent.

Trade-offs: Acute effects (fatigue, skin reaction, mucositis) and late risks (fibrosis, xerostomia, bowel or bladder changes). Particle therapy access and cost vary. Planning and delivery require time and multiple visits.

Safety and Contraindications

Relative contraindications include connective tissue disorders with heightened radiosensitivity, prior high-dose to the same region, and inability to comply with immobilization or breath-hold when required. Pacemakers or implanted devices need coordination. Pregnancy requires special precautions.

Implementation Frameworks: How Planning Happens

Planning typically includes simulation CT with immobilization, sometimes MRI/PET fusion, and motion assessment (4D-CT). The team defines targets (GTV/CTV/PTV) and organs at risk (OARs), sets dose prescriptions and constraints, and iteratively optimizes plans. Image guidance ensures accurate daily delivery. For brachytherapy, applicators are placed and imaged to guide dwell positions and times.

Case Scenarios (Examples)

Case A: Early-Stage Lung Cancer, Inoperable

SBRT in 3–5 fractions with motion management (abdominal compression or breath-hold). Tight constraints to chest wall, bronchial tree, and spinal cord. Follow-up with CT scans.

Case B: Prostate Cancer, Intermediate Risk

Options include IMRT/VMAT over 4–8 weeks, SBRT in 5 fractions, or brachytherapy boost. Selection depends on urinary function, anatomy, and preference. Rectal and bladder constraints protect function.

Case C: Cervical Cancer, Locally Advanced

Concurrent chemoradiation with IMRT followed by HDR brachytherapy boost. Image-guided brachytherapy enables customized dose to residual disease while limiting bladder and rectum exposure.

Case D: Brain Metastases, Oligometastatic

SRS to each lesion in 1–5 fractions, sparing whole brain to reduce cognitive effects. Close MRI surveillance and salvage SRS as needed.

Common Pitfalls and How to Avoid Them

Ignoring motion—use 4D-CT, gating, or breath-hold for thoracic/abdominal targets.
Underestimating organs at risk—apply validated dose constraints and review cumulative exposure, especially in re-irradiation.
Inadequate image guidance—daily imaging reduces geographic miss.
Not updating plans during therapy—adapt when anatomy changes (weight loss, tumor shrinkage, effusions).
Poor supportive care—manage skin, nutrition, and symptom control proactively.

What To Do Next

Build a structured plan with the care team to match the right technology and dose to the clinical goal while minimizing risk. Come prepared with questions and priorities, and make sure supportive care is integrated from day one.

Clarify goals: cure, organ preservation, or symptom control, and ask which technique best aligns with each.
Review imaging and motion: ask how motion will be managed and which organs are most at risk.
Discuss dose and schedule: understand the total dose, number of treatments, and rationale for fractionation.
Ask about alternatives: whether protons, brachytherapy, or SBRT offer advantages for the specific case.
Plan supportive care: skin care, nutrition, dental prophylaxis (head/neck), and exercise recommendations.
Align follow-up: schedule post-treatment scans and toxicity checks; know red flags and contact pathways.