Radiation Therapy in Oncology

External beam, stereotactic, particle therapy, brachytherapy, and radioligand therapy — the physics, radiobiology, and clinical decision-making behind modern radiation oncology

Key Points

• Radiation therapy is used in approximately 50% of all cancer treatment courses — as definitive curative therapy, adjuvant therapy after surgery, or for symptom palliation — making it one of the most broadly applied oncologic treatments.
• The linear-quadratic (LQ) model governs fractionation: tissues with low α/β ratios (prostate, breast, CNS) are more sensitive to large doses per fraction, enabling hypofractionated regimens; tissues with high α/β ratios (head-and-neck squamous, lung) favor conventional small-fraction schedules.
• Intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) deliver conformal high-dose volumes that tightly spare adjacent organs at risk (OARs), and image-guided RT (IGRT) uses on-board imaging to account for daily setup variation.
• Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) deliver ablative doses in 1–5 fractions with sub-millimeter precision, enabling curative-intent treatment of early lung cancer, oligometastatic disease, and brain metastases without surgery.
• Proton therapy exploits the Bragg peak — near-zero exit dose — to reduce integral radiation dose to surrounding normal tissue; it is particularly advantageous in pediatric malignancies, ocular tumors, and certain skull-base and paraspinal tumors.
• Radioligand therapy (RLT) delivers targeted radiation by coupling radionuclides to tumor-homing molecules: lutetium-177 DOTATATE (Lutathera) for somatostatin receptor-positive neuroendocrine tumors and lutetium-177 vipivotide tetraxetan (Pluvicto) for PSMA-positive metastatic castration-resistant prostate cancer represent landmark approvals in this rapidly expanding class.

What Is Radiation Therapy?

Radiation therapy (RT) uses ionizing radiation — predominantly high-energy photons (X-rays or gamma rays), electrons, or charged particles — to damage DNA within tumor cells, ultimately triggering cell death. It is one of the three historic pillars of cancer treatment alongside surgery and systemic therapy, and remains central to the management of virtually every solid tumor type. Approximately one in two cancer patients will receive radiation at some point during their illness. In curative settings, RT is used as the definitive treatment (e.g., early-stage larynx cancer, prostate cancer,…

Radiobiology: Fractionation and the Linear-Quadratic Model

Understanding why radiation is delivered in multiple fractions over several weeks — rather than as a single massive dose — requires an appreciation of the underlying biology. The guiding framework is the **linear-quadratic (LQ) model** and the **"Four Rs" of radiobiology**. **The Linear-Quadratic Model:** Cell survival after radiation follows a curve with two components: a linear component (α) proportional to dose, representing direct lethal DNA damage (the α term, or "single-hit kill"), and a quadratic component (β) proportional to dose squared, representing accumulation of sub-lethal…

External Beam Radiation Therapy: 3D-CRT, IMRT, VMAT, and IGRT

The dominant form of radiation delivery for most solid tumors is **external beam radiation therapy (EBRT)**, in which a linear accelerator (linac) generates a beam of high-energy X-rays (typically 6–18 MV photons) that is directed at the patient from outside the body. The past three decades have seen a dramatic evolution in technique. **Three-Dimensional Conformal Radiation Therapy (3D-CRT):** Using CT-based treatment planning, 3D-CRT shapes each radiation beam to conform to the outline of the tumor (the planning target volume, or PTV) in three dimensions using a **multi-leaf collimator…

Stereotactic Techniques: SBRT and SRS

**Stereotactic body radiation therapy (SBRT)** — also called stereotactic ablative radiotherapy (SABR) — and **stereotactic radiosurgery (SRS)** represent the most important paradigm shift in radiation oncology since IMRT: the delivery of ablative doses (typically 6–20 Gy per fraction) in 1–5 fractions to precisely defined targets, with margins as narrow as 1–3 mm and steep dose gradients to protect adjacent critical structures. The biological rationale for extreme hypofractionation in SBRT extends beyond the LQ model: at doses >8–10 Gy per fraction, additional vascular damage (endothelial…

Particle Therapy: Proton and Carbon Ion

Unlike X-ray photons, which deposit dose throughout their path through tissue (entrance, target, and exit), charged particles such as protons and carbon ions exhibit the **Bragg peak** — a sharp, finite dose deposition near the end of their range, beyond which dose drops to nearly zero. This physical property fundamentally changes the dose-volume relationship: proton therapy delivers lower integral radiation dose to normal tissues compared to the best photon techniques, while matching or exceeding coverage of the target. **Proton therapy physics:** Protons are accelerated to 70–250 MeV in…

Brachytherapy: Internal Radiation Sources

**Brachytherapy** (from the Greek βραχύς, "short") places a radioactive source directly within or immediately adjacent to the tumor, achieving an extremely steep dose gradient — dose falls off rapidly with the square of the distance from the source. This proximity enables very high local doses with sparing of distant tissues. Brachytherapy is indispensable in gynecologic, prostate, and breast cancers. **High-dose-rate (HDR) brachytherapy:** An iridium-192 source (high activity, ~10 Ci) on a cable is robotically afterloaded into temporary catheters or applicators positioned in or around the…

Systemic Radiation: Radioiodine, Radium-223, and Radioligand Therapy

A distinct class of radiation treatment uses radionuclides delivered systemically — either as simple ionic solutions exploiting natural tissue uptake, or as molecular conjugates that actively target tumor-expressed receptors. This field, encompassing classical radioiodine therapy and the modern era of radioligand therapy (RLT), blurs the boundary between radiation oncology and nuclear medicine. **Radioactive iodine (I-131) for differentiated thyroid cancer:** The thyroid gland uniquely concentrates iodine through the sodium-iodide symporter (NIS). After thyroidectomy for differentiated…

The Radiation Oncology Workflow: Simulation to Delivery

The journey from a decision to treat with radiation to the first treatment fraction involves a carefully choreographed sequence of steps spanning 1–2 weeks for standard fractionation, or as little as 24–48 hours for palliative single-fraction treatments. **Step 1 — Consultation and multidisciplinary discussion:** The radiation oncologist reviews pathology, staging imaging, surgical and systemic therapy history, and performance status. Goals of treatment (curative vs. adjuvant vs. palliative), treatment technique, and the interaction with concurrent or planned systemic therapy are discussed…

Acute and Late Toxicities

Radiation toxicity is classified as acute (occurring during or within 90 days of treatment), subacute (months after treatment), or late (6+ months after treatment completion). Acute toxicities reflect injury to rapidly proliferating normal tissues that are caught in the high-dose field; late toxicities reflect vascular and stromal damage to slowly turning-over tissues. **Universal acute effects:** - **Fatigue:** The most common complaint across all sites, occurring in 60–90% of patients during treatment. Likely multifactorial — anemia, cytokine release, sleep disruption, psychological…

Radiation with Concurrent Systemic Therapy

The combination of radiation with systemic agents — exploiting radiosensitization, spatial cooperation, and immunomodulation — has established chemoradiation as the standard of care for many locally advanced cancers. **Cisplatin + radiation — the most validated sensitizer:** Cisplatin forms platinum-DNA adducts that interfere with DNA repair and inhibit potentially lethal damage repair, enhancing radiation-induced cell kill in a supra-additive fashion. Concurrent weekly cisplatin (40 mg/m²) with radiation is the backbone of treatment for: - **Locally advanced head-and-neck SCC:** High-dose…

Emerging Technologies in Radiation Oncology

Radiation oncology is undergoing a technological renaissance, with several innovations poised to redefine the therapeutic ratio. **FLASH Radiotherapy:** FLASH RT delivers the entire radiation dose in a fraction at dose rates >40 Gy/s — orders of magnitude faster than conventional linac delivery (~0.05 Gy/s). Preclinical data across multiple animal models consistently show remarkable normal tissue sparing with FLASH compared to conventional RT at the same tumor control dose — the "FLASH effect." The leading mechanistic hypothesis involves rapid oxygen depletion by FLASH (transient hypoxia…