The medical abbreviation XRT most commonly stands for External Radiation Therapy. This treatment is a cornerstone of modern oncology, primarily used to manage various types of cancer. XRT uses focused, high-energy radiation beams, generated by specialized machines, to target and destroy malignant cells within the body. The goal of this localized therapy is to deliver a precisely measured dose to the disease site while sparing surrounding healthy tissues.
Defining XRT and Its Clinical Purpose
XRT is broadly synonymous with radiation therapy (RT) but often refers specifically to External Beam Radiation Therapy (EBRT), distinguishing it from internal methods like brachytherapy. This treatment involves directing radiation from a machine outside the patient’s body, typically a linear accelerator, to the targeted area.
The applications of XRT fall into several categories based on the treatment goal. It can be used with curative intent for localized cancers, aiming to eradicate the disease entirely. XRT is a standard treatment for many solid tumors, including those of the prostate, breast, head and neck, and lung.
XRT is frequently used in conjunction with other treatments, serving as adjuvant or neoadjuvant therapy. Adjuvant therapy is given after a primary treatment like surgery to eliminate residual cancer cells and reduce the risk of recurrence. Conversely, neoadjuvant XRT is administered before surgery to shrink a tumor, potentially making it easier to remove.
When a cure is not possible, XRT provides value as palliative care. The radiation is used to control local disease progression and relieve painful symptoms, such as pain from bone metastases or pressure from a tumor mass. This use focuses on improving the patient’s quality of life.
The Biological Mechanism of Action
The power of XRT lies in its ability to damage the genetic material of cancer cells using ionizing radiation. When the high-energy beams penetrate the tissue, they cause ionization, stripping electrons from atoms. This energy deposition leads to damage in the deoxyribonucleic acid (DNA) of the cells.
DNA damage can occur either directly, where the radiation hits the DNA molecule itself, or indirectly, which is the more common pathway. Indirect damage happens when the radiation ionizes water molecules within the cell, producing highly reactive free radicals, notably hydroxyl radicals. These free radicals then break the DNA strands.
Cells possess mechanisms to repair damaged DNA, but the double-strand breaks caused by radiation are difficult to fix. Cancer cells, which are rapidly dividing, are vulnerable because they cannot effectively repair this extensive damage before attempting to divide. This leads to cell death through processes like apoptosis (programmed cell death) or mitotic catastrophe.
The treatment is delivered in a series of smaller doses, a practice known as fractionation, which is a fundamental principle of XRT. This approach allows healthy, normal cells more time to repair radiation damage between sessions. Cancer cells, being less efficient at repair, accumulate damage leading to their demise. Fractionation exploits this biological difference to maximize the therapeutic effect while minimizing side effects to normal tissue.
Key Delivery Techniques
The precision of XRT has advanced through sophisticated delivery techniques. Image-Guided Radiation Therapy (IGRT) is a standard practice that uses imaging, such as on-board computed tomography (CT) scans, immediately before or during each treatment session. This allows the radiation team to verify the exact location of the tumor and patient position in real-time, making adjustments to ensure accurate delivery. IGRT is used because tumors and internal organs can shift slightly due to breathing or other physiological movements.
Intensity-Modulated Radiation Therapy (IMRT) represents a major leap in geometric precision. IMRT allows the radiation beam to be broken into numerous small beamlets, with the intensity of each adjusted. This ability to modulate the intensity creates a highly customized dose distribution, conforming the radiation dose tightly to the tumor’s irregular shape while reducing the dose received by nearby sensitive structures. Volumetric Modulated Arc Therapy (VMAT) is an advanced form of IMRT where the treatment machine rotates continuously around the patient while simultaneously changing the beam shape, intensity, and speed. This technique delivers the complex dose distribution much faster than static IMRT, often in less than two minutes, which is beneficial for patient comfort and accuracy.
Stereotactic Body Radiation Therapy (SBRT) and Stereotactic Radiosurgery (SRS) are specialized high-dose XRT techniques. SBRT is used for tumors outside the brain and delivers a high radiation dose in a small number of fractions, typically one to five sessions. SRS is the term used for similar focused treatments in the brain. These ablative treatments rely on extreme targeting accuracy, using advanced immobilization and IGRT, to achieve a steep dose fall-off outside the target.
The Patient Treatment Experience
The XRT journey begins with a treatment planning phase known as simulation. During this session, the patient is positioned on a table, and immobilization devices, such as customized molds, are created to ensure the exact same position for every daily treatment. A planning CT scan is performed, sometimes with other imaging like MRI or PET scans, to map the tumor and surrounding organs.
Radiation oncologists and medical physicists use this imaging data to design a treatment plan, determining the angles, shapes, and intensities of the radiation beams. The goal is to calculate the dose distribution that maximizes tumor killing while keeping the dose to healthy tissues below safety thresholds. This planning process ensures the treatment is delivered exactly as intended during the daily session.
The actual daily treatment session is typically short, often lasting 10 to 30 minutes. The time spent on the treatment table is mostly dedicated to setting up the patient and performing IGRT checks to verify position. The delivery of the radiation dose itself often takes only a few minutes, with the patient lying still while the machine moves around them.
A common side effect is fatigue, which often begins a few weeks into treatment and can worsen over time. Skin changes in the treatment area are also common, resembling a mild sunburn with redness, dryness, or itching. Other acute side effects are localized to the treatment site, such as a sore throat with head and neck radiation or diarrhea with pelvic radiation. These acute reactions are temporary, resolving within a few weeks after the course of XRT is completed.