Radiation Therapy

An Overview of Types, Diagnosis, and Treatment

Cancer involves abnormal cells growing uncontrollably, invading nearby tissues, and spreading to other parts of the body through metastasis.

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Radiation Therapy

Radiation therapy, also called RT or XRT, is one of the main treatments for cancer, along with surgery and chemotherapy. It involves carefully using ionizing radiation on body tissues to destroy cancer cells. This treatment uses high-energy particles or waves, such as x-rays, gamma rays, electron beams, or protons, to damage the DNA of cancer cells. The main goal is to stop the tumor from growing and dividing, while protecting the nearby healthy tissues as much as possible.

At the cellular level, radiation therapy works by breaking the chemical bonds in DNA. This can happen directly, when radiation hits the DNA, or indirectly, when radiation creates free radicals from water in the cell that then damage the DNA. If a cell with damaged DNA tries to divide, it cannot do so properly and dies. Cancer cells are especially sensitive to this process because they divide quickly and often cannot repair their DNA as well as normal cells.

From the perspective of regenerative medicine and tissue engineering, radiation therapy is a potent modulator of the local microenvironment. While its primary intent is destructive towards the malignancy, it also initiates a cascade of cytokine releases and vascular changes that alter the landscape of the treated tissue. Modern radiation oncology is no longer a “one-size-fits-all” approach but a highly personalized discipline. It has evolved from 2D planning based on simple X-rays to 4D treatment planning that accounts for the physiological motion of organs, such as breathing or digestion. The definition of radiation therapy now encompasses a spectrum of techniques ranging from curative intent, where high doses are delivered to eliminate the tumor, to palliative intent, where lower doses are used to relieve symptoms like pain or bleeding, and even to stereotactic ablative body radiotherapy (SABR), which delivers surgical-like precision without an incision.

The Physics of Ionization and Cellular Interaction

The efficacy of radiation therapy relies on the transfer of energy. When high-energy photons enter the body, they interact with the atoms of the cells, ejecting electrons from their orbits. This process is called ionization. The biological consequence is determined by the Linear Energy Transfer (LET) of the particle. Traditional X-rays have a low LET, depositing energy sparsely along their track.

Newer treatments like proton or heavy-ion therapy use particles that have mass. These particles release most of their energy at a certain depth, called the Bragg Peak. This lets doctors focus the radiation on the tumor and protect healthy tissues behind it, which is especially helpful for treating children and tumors near important areas like the brainstem or spinal cord.

Radiobiology: Fractionation and the Four Rs

The principles of radiobiology govern the clinical application of radiation, specifically the “Four Rs”: Repair, Reassortment, Repopulation, and Reoxygenation. These principles dictate why radiation is typically delivered in daily small doses, called fractions, over several weeks rather than in a single massive dose.

Repair: Normal cells have robust mechanisms to repair sub-lethal DNA damage between daily treatments, whereas cancer cells are often less efficient at this repair.
Reassortment: Cells are most sensitive to radiation during the G2 and M phases of the cell cycle. Fractionation allows cells to cycle into these sensitive phases between treatments.
Repopulation: If the treatment course is too prolonged, surviving tumor cells may begin to divide rapidly. This necessitates strict adherence to treatment schedules.

Reoxygenation: Hypoxic (oxygen-starved) tumor cells are resistant to radiation. As the outer layers of a tumor are killed, oxygen can penetrate deeper, making the remaining core cells more sensitive to subsequent doses.

Global Biotechnological Integration

Biotechnology and artificial intelligence are now key parts of radiation oncology. Modern linear accelerators (LINACs) can shape the radiation beam very precisely using small tungsten leaves that move during treatment. Some machines now combine MRI with the LINAC, allowing doctors to adjust the treatment plan each day based on changes in the tumor or organs. This approach keeps the treatment focused on the right spot throughout therapy.

The Regenerative Context and Tissue Sparing

Modern radiation therapy aims to protect the body’s ability to heal by reducing damage to healthy tissues. Radiation can harm important stem cell areas in organs like the salivary glands, bone marrow, and the hippocampus. Advanced methods like IMRT and VMAT help shape the radiation dose to avoid these areas. By doing this, doctors try to keep organs working well and maintain the patient’s quality of life after treatment.

Key Physiological Mechanisms Utilized

DNA Double-Strand Breaks: The primary lethal lesion induced by radiation, severing the genetic blueprint and preventing successful mitosis.
Free Radical Generation: The radiolysis of cellular water creates hydroxyl radicals that chemically attack DNA and cell membranes.
Vascular Damage: High-dose radiation can damage the tumor’s blood supply (endothelial apoptosis), effectively starving the malignancy of oxygen and nutrients.
Immunogenic Cell Death: Radiation can cause tumor cells to die in a way that releases antigens, alerting the immune system to the presence of the cancer (the abscopal effect).
Cell Cycle Arrest: Radiation triggers checkpoints that pause the cell cycle, allowing time for repair or, if severe damage occurs, forcing the cell into permanent senescence.

Fibrosis Induction: In the healing phase, radiation induces collagen deposition, which can wall off residual tumor cells but also impairs tissue flexibility.

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FREQUENTLY ASKED QUESTIONS

What is the difference between external beam radiation and brachytherapy?

External beam radiation delivers high-energy rays from a machine (a linear accelerator) outside the body, aiming them at the tumor from various angles. Brachytherapy, or internal radiation, involves placing radioactive sources (seeds, ribbons, or capsules) directly inside or next to the tumor, allowing for a higher dose to the cancer with less exposure to surrounding healthy tissues.

Is radiation therapy radioactive?

Patients undergoing external beam radiation do not become radioactive. The radiation ends the moment the machine is turned off, and it is safe for them to be around others, including children and pregnant women. However, patients receiving systemic radioisotopes or permanent brachytherapy implants may emit low levels of radiation for a short time and require specific safety precautions.

How does radiation therapy differ from chemotherapy?

Chemotherapy is a systemic treatment that uses drugs traveling through the bloodstream to kill rapidly dividing cells throughout the entire body. Radiation therapy is a local treatment that affects only the specific part of the body where the beam is aimed. Therefore, radiation side effects are generally limited to the treated area, whereas chemotherapy side effects are systemic.

What is the “Gray” (Gy) in radiation?

The Gray (Gy) is the unit of absorbed radiation dose. One Gray is defined as the absorption of one joule of radiation energy per kilogram of matter. Oncologists prescribe the total dose in Grays (e.g., 60 Gy) and divide it into smaller daily fractions (e.g., 2 Gy per day) to maximize tumor kill while sparing normal tissue.

Does radiation therapy hurt?

The actual delivery of external beam radiation is painless, similar to having an X-ray taken. Patients do not feel, see, or smell the radiation as it enters the body. However, side effects that develop over time, such as skin irritation (e.g., sunburn) or inflammation of internal linings (mucositis), can cause discomfort and pain as the treatment progresses.