Basics of Radiotherapy: How It Works, Types, and Roles in Cancer Care
In Part 1, we drew an overall map of cancer treatment and looked at why some cancers are easier or harder to cure. In Part 2, we explored how surgery, drug therapy, and immunotherapy work together and why cancer sometimes comes back.
In this Part 3, we focus on another major pillar: radiotherapy (radiation therapy).
When people hear “radiotherapy,” they often wonder:
- “How is it different from surgery?”
- “How much damage does it cause to the body?”
- “If I get radiation once, can I never have it again in that area?”
This article explains the basic principles, types, dose concepts, and side effects of radiotherapy in plain language, and helps you see when and why radiation is chosen as part of cancer treatment.
What you will learn in this article
- How radiotherapy damages cancer cells at the DNA level
- The difference between external beam radiotherapy and brachytherapy
- What “dose” and “fractionation” mean in everyday terms
- The three roles of radiotherapy: curative, adjuvant, and palliative
- The basic ideas behind 3D-CRT, IMRT, SBRT, and IGRT
- Main acute and late side effects of radiotherapy and how they are managed
- Which cancers are often treated with radiotherapy and why
Chapter 1 What is radiotherapy? Invisible beams that damage DNA
Basic principle of radiotherapy
Radiotherapy uses high-energy X-rays, electron beams, or other forms of radiation to damage the DNA of cancer cells.
When radiation passes through cells, it can:
- directly break DNA strands, and/or
- create reactive oxygen species via water molecules, which indirectly damage DNA
If DNA damage is extensive and not repaired, the cell can no longer divide properly and eventually dies.
Cancer cells typically:
- have weaker DNA repair mechanisms
- divide more actively than many normal cells
So for the same radiation dose, cancer cells are often more vulnerable. Radiotherapy aims to exploit this difference: deliver enough damage to cancer cells while keeping damage to surrounding normal tissues as low as possible.
Local, not whole-body, treatment
Unlike systemic drug therapy (chemotherapy), which travels throughout the body via the bloodstream, radiotherapy is mainly a local treatment. It delivers dose to a defined volume called the treatment field or target volume.
Because of this, radiotherapy is particularly useful for:
- treating localized tumors in specific organs or regions
- relieving symptoms such as pain or bleeding from a particular site
It is one of the most important tools for local control of cancer.
Chapter 2 External beam vs. brachytherapy: different ways to deliver radiation
External beam radiotherapy
The most common form is external beam radiotherapy.
- A linear accelerator (“linac”) generates high-energy X-rays or electrons.
- Beams are directed at the tumor from outside the body.
- Multiple beam angles are used so that dose builds up in the tumor while sparing normal tissues as much as possible.
A typical course involves:
- daily treatments on weekdays (about 5 times per week)
- for several weeks, depending on the tumor type and goal
Each session usually takes only a few minutes of actual beam time, although positioning and checks may take a bit longer.
Brachytherapy (internal radiotherapy)
Brachytherapy (from a Greek word meaning “short distance”) involves placing a radiation source inside the body, in or near the tumor.
Examples include:
- sealed radioactive seeds implanted in the prostate for prostate cancer
- intracavitary brachytherapy for cervical cancer
Because the source is very close to the tumor, dose falls off sharply with distance, allowing:
- high doses to the tumor
- relatively lower doses to surrounding normal tissues
This approach is particularly useful when the tumor is accessible and well-defined in a small area.
Chapter 3 Dose and fractionation: why radiation is usually given in small daily portions
What is “dose” in radiotherapy?
Radiation dose in radiotherapy is measured in units called Gray (Gy).
- For example: “60 Gy in 30 fractions” means 2 Gy per treatment, 30 treatments in total.
The total dose and the size of each fraction are both crucial in determining how effective (and how toxic) a treatment will be.
Why not give all the dose at once?
Instead of giving a very high dose in a single session, standard radiotherapy typically uses fractionation:
- about 1.8–2 Gy per day
- 5 days a week
- for several weeks
This schedule takes advantage of differences between cancer cells and normal cells:
- cancer cells have poorer repair capacity and are more vulnerable to repeated damage
- normal tissues have better repair capacity and benefit from the rest periods between treatments
In other words, fractionation seeks to:
- accumulate lethal damage in cancer cells
- allow normal tissues time to repair sublethal damage
thus improving the therapeutic ratio.
Hypofractionation and SBRT: fewer, larger fractions
In selected situations, it can be beneficial to use fewer treatments with larger doses per fraction, especially when the tumor is small and well-defined, and nearby organs can be protected.
This is the concept behind stereotactic body radiotherapy (SBRT) and related techniques:
- larger dose per fraction (for example, 8–20 Gy)
- 3–5 fractions total
Such regimens aim to deliver ablative doses to the tumor in a short timeframe, often with curative intent in appropriately chosen cases.
Chapter 4 Roles of radiotherapy: curative, adjuvant, and palliative
1. Curative radiotherapy
For some cancers and stages, radiotherapy can be a curative treatment, comparable to surgery in terms of long-term control.
- Prostate cancer: external beam radiotherapy (with or without brachytherapy and hormone therapy) can achieve cure rates similar to surgery in many cases.
- Early-stage laryngeal cancer: radiotherapy can eradicate the tumor while preserving voice.
- Early-stage lung cancer: in patients who cannot undergo surgery, SBRT offers a non-surgical curative option.
2. Adjuvant and neoadjuvant roles
Radiotherapy is often combined with surgery and systemic therapy to improve outcomes:
- Adjuvant radiotherapy: given after surgery to lower the risk of local recurrence.
- Neoadjuvant radiotherapy (with or without chemotherapy): given before surgery to shrink the tumor and make surgery easier or more effective.
Examples include:
- Breast cancer: after breast-conserving surgery, radiotherapy to the remaining breast tissue significantly reduces local recurrence.
- Rectal cancer: preoperative chemoradiation can shrink the tumor and increase the chance of preserving the anal sphincter.
- Head and neck cancers: postoperative radiotherapy reduces local recurrence in high-risk cases.
3. Palliative radiotherapy for symptom relief
Even when cure is no longer realistic, radiotherapy can greatly improve quality of life by relieving symptoms such as:
- bone pain from metastases
- neurological symptoms from brain metastases
- bleeding or obstruction (for example, in the airway or digestive tract)
Short courses of palliative radiotherapy can provide meaningful relief with relatively low burden, and are an important part of supportive and palliative care.
Chapter 5 3D-CRT, IMRT, SBRT, IGRT: what “high-precision radiotherapy” means
3D conformal radiotherapy (3D-CRT)
3D-CRT uses CT-based 3D images of the tumor and surrounding anatomy to shape beams around the target volume.
- Compared to older 2D techniques, 3D-CRT better conforms the high-dose region to the tumor.
- It reduces unnecessary dose to normal tissues.
Intensity-modulated radiotherapy (IMRT)
IMRT goes one step further by modulating the intensity of radiation within each beam.
- It allows more complex dose distributions that wrap around tumors with irregular shapes.
- It is especially useful when critical organs (for example, salivary glands, spinal cord, rectum) are close to the tumor.
IMRT has become standard in many settings, such as head and neck cancers and prostate cancer, because it can:
- maintain or improve tumor control
- reduce certain side effects by sparing normal tissues
Stereotactic radiotherapy (SBRT, SRS)
SBRT (stereotactic body radiotherapy) and SRS (stereotactic radiosurgery) deliver very high doses with millimeter precision to small targets.
- Common indications include early-stage lung cancer, limited (oligometastatic) disease, and brain metastases.
- They are sometimes described as “surgery without a scalpel.”
Because doses per fraction are high, proper patient selection and meticulous planning are essential.
Image-guided radiotherapy (IGRT)
IGRT uses imaging (such as cone-beam CT or X-ray) right before or during treatment to verify and adjust patient positioning.
- Organs move with breathing and daily variation; IGRT helps correct for this.
- It allows tighter margins and better sparing of normal tissues.
IGRT is especially important for SBRT and IMRT, where precise alignment is critical.
Chapter 6 Combining radiotherapy with surgery and systemic therapy
When radiotherapy is chosen instead of surgery
Radiotherapy may be used as an alternative to surgery when:
- the patient has high surgical risk due to age or other illnesses
- organ preservation is a priority (voice, sphincter function, etc.)
- the tumor is small and well-localized, and radiotherapy can deliver an adequate curative dose
In early laryngeal cancer, prostate cancer, and some early lung cancers, patients and doctors often discuss and choose between surgery and radiotherapy based on these factors.
Radiotherapy plus systemic therapy
Radiotherapy is frequently combined with chemotherapy, targeted therapy, or immunotherapy.
- Concurrent chemoradiation: chemotherapy given at the same time as radiotherapy can sensitize tumor cells to radiation and improve local control (for example, in esophageal or cervical cancer).
- Sequential therapy: chemotherapy before or after radiotherapy can help control systemic disease while radiation focuses on local control.
The optimal sequence and combination depend on the cancer type, stage, and patient factors, and are continually refined through clinical research.
Chapter 7 Side effects of radiotherapy: acute and late
Acute side effects: during treatment and shortly after
Acute side effects occur during the course of radiotherapy and in the weeks following it. They depend on the area being treated.
- Skin redness, dryness, or irritation in the treated area
- Mouth sores and difficulty swallowing if the head and neck are irradiated
- Diarrhea or abdominal discomfort if the pelvis or abdomen are treated
- Fatigue and a general sense of tiredness
Most acute side effects gradually improve after treatment ends, although supportive care (for example, pain control, mouth rinses, nutritional support) is often important.
Late side effects: months to years later
Late side effects may appear months or years after radiotherapy and are related to long-term changes in the irradiated tissues.
- Fibrosis (hardening or scarring) of skin and subcutaneous tissues
- Changes in blood vessels or nerves
- Organ-specific effects (for example, on lungs, heart, bowel), depending on the field
- A small increased risk of secondary cancers in the irradiated region
Modern radiotherapy techniques aim to minimize these risks by carefully limiting the dose to normal tissues, but they cannot completely eliminate them.
Balancing benefit and risk
It is natural to be concerned about the side effects of radiation. The key is to consider:
- the potential benefit of the treatment (for example, higher chance of cure, better local control, symptom relief)
- the short-term and long-term risks to normal tissues
In some situations, accepting a certain level of late risk is reasonable if it significantly increases the chance of long-term survival or cure. In others, when cure is unlikely, focusing on quality of life and minimizing toxicity becomes more important.
Chapter 8 Cancers where radiotherapy often plays a central role
Prostate, laryngeal, and early-stage lung cancer
In these cancers, radiotherapy can be a primary, potentially curative treatment.
- Prostate cancer: external beam radiotherapy (with or without brachytherapy and hormone therapy) is a well-established alternative to surgery.
- Laryngeal cancer: early-stage cancers of the vocal cords are often treated with radiotherapy to preserve voice.
- Early-stage lung cancer: SBRT provides a non-surgical curative option for patients who cannot undergo surgery.
Head and neck cancer, rectal cancer, and organ preservation
In head and neck cancers and rectal cancer, organ function is closely tied to quality of life:
- eating, speaking, swallowing
- bowel control and continence
Radiotherapy is used to:
- shrink tumors before surgery, allowing less extensive operations
- sometimes avoid surgery altogether and preserve organ function (with careful selection and follow-up)
These strategies aim not only at survival but at preserving key aspects of daily life.
Chapter 9 Summary and looking ahead
In this Part 3, we have:
- explained how radiotherapy damages cancer cells and why fractionation is used
- described external beam radiotherapy and brachytherapy
- outlined the roles of radiotherapy in cure, adjuvant/neoadjuvant therapy, and palliation
- introduced 3D-CRT, IMRT, SBRT, and IGRT as examples of high-precision techniques
- reviewed acute and late side effects and how to think about their risks
- highlighted cancers where radiotherapy often plays a central or organ-preserving role
In short, we can say that:
Radiotherapy is not just a backup to surgery or chemotherapy; it is a major pillar that expands the possibilities of modern cancer treatment.
In Part 4, we will move on to particle therapies:
- basic concepts of proton and heavy-ion therapy
- how their physical properties differ from X-rays (for example, the Bragg peak)
- in which cancers and settings they are being used (at an overview level)
Our goal is to provide an introductory understanding of what “particle therapy” means and how it fits into the broader radiotherapy landscape.
This article was edited by the Morningglorysciences team.
The content is for general informational purposes only and is not a substitute for individual medical advice. For decisions about diagnosis or treatment, please always consult your treating physician.
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