Radiation therapy

• 1. Introduction

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  Radiation therapy has been used to fight cancer for more than a hundred years, and today roughly half of the United States's 1.2 million invasive cancer cases diagnosed in a single year receive it as part of their treatment. That figure alone raises a cascade of questions. How does a treatment rooted in the discovery of X-rays in 1895 remain central to modern medicine? What does it actually do inside a patient's body? And why, given its proven power against tumors, does it still carry real risks that doctors must weigh carefully against the benefits? This documentary follows radiation therapy from its earliest scientific origins through the intricate techniques now deployed in hospitals worldwide, examining the biology it exploits, the side effects it can cause, and the engineering breakthroughs that have made it steadily more precise.

• 2. How It Damages Cancer

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  Wilhelm Rontgen discovered X-rays in 1895, and within a year Emil Grubbe of Chicago was possibly the first American physician to use them against cancer. The underlying biology, though, took decades to fully map. Radiation works by damaging the DNA of cancerous tissue, leading to cellular death. It does this through two routes: direct ionization of the atoms that make up the DNA chain, and indirect ionization through the formation of free radicals, most notably hydroxyl radicals produced when radiation strikes water inside the cell.

  Double-stranded DNA breaks are the more lethal outcome. Cells can repair single-strand damage relatively well, but double-stranded breaks lead to dramatic chromosomal abnormalities and genetic deletions. Cancer cells are generally less differentiated and more stem cell-like than healthy tissue, and they have a diminished ability to repair even sub-lethal damage. Each dose of radiation therefore accumulates harm that healthy cells can partially recover from but cancer cells cannot.

  One complication is oxygen. Solid tumors often outgrow their blood supply, creating low-oxygen pockets known as hypoxia. Because oxygen is a potent radiosensitizer, tumor cells in a hypoxic environment may be as much as 2 to 3 times more resistant to radiation damage than cells bathed in a normal oxygen environment. Researchers have pursued many strategies to counter this, including high-pressure oxygen tanks, heat therapy to dilate blood vessels to the tumor site, and radiosensitizing drugs such as misonidazole and metronidazole. A newer area of investigation involves an oxygen diffusion-enhancing compound called trans sodium crocetinate.

• 3. Fractions and Timing

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  A course of radiation therapy is rarely delivered as a single massive dose. The standard approach in North America, Australia, and Europe for adult patients is to give 1.8 to 2 gray per day, five days a week, spreading the total dose across many sessions. This strategy, called fractionation, rests on a straightforward biological advantage: normal cells recover between sessions more effectively than tumor cells do.

  Fractionation also exploits the cell cycle. Tumor cells that happened to be in a relatively radiation-resistant phase when one dose arrived will cycle into a more sensitive phase before the next fraction is delivered. Cells that were hypoxic and therefore harder to kill may reoxygenate in the interval, making subsequent doses more effective. For children, typical fraction sizes are slightly smaller, at 1.5 to 1.8 gray per day, because smaller fractions are associated with reduced incidence and severity of late-onset side effects in developing tissue.

  Not every tumor benefits from stretching treatment over weeks. Cancers in the head and neck, and cervical squamous cell cancers, can begin repopulating if fractionation runs too long, so those treatments are designed to finish within a defined window. For painful bone metastases in patients with limited life expectancy, a single fraction gives comparable pain relief and morbidity outcomes to multiple-fraction treatments, sparing the patient repeated hospital visits. A growing schedule called hypofractionation delivers larger doses per session, with stereotactic treatments reaching as high as 20 gray per fraction for certain lesions.

• 4. Precision and Delivery

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  Before a patient receives a single dose, a radiation oncologist must build a treatment plan. Modern planning relies on CT scans to identify the tumor and surrounding normal structures, and the patient receives small skin marks to guide beam placement at each session. Getting the geometry right matters because shaped radiation beams are aimed from several angles to intersect at the tumor, concentrating dose there while limiting what the surrounding healthy tissue absorbs.

  Godfrey Hounsfield's invention of computed tomography in 1971 transformed this process, shifting planning from two-dimensional guesswork to genuine three-dimensional modeling. The subsequent arrival of MRI in the 1970s and positron emission tomography in the 1980s pushed the field further, enabling intensity-modulated radiation therapy, or IMRT, where computer-controlled accelerators distribute precise doses shaped to the tumor's three-dimensional profile. IMRT has only been used commercially since the late 1990s, even at the most advanced centers, meaning some radiation oncologists who trained before that era have had to seek additional education before implementing it.

  A more recent technique, volumetric modulated arc therapy, introduced in 2007, rotates the radiation source in arcs around the patient while simultaneously adjusting beam shape, speed, and dose rate. For certain head and neck cancers such as nasopharyngeal, oropharyngeal, and hypopharyngeal carcinomas, this approach provides equivalent or better protection of nearby organs compared with static-field IMRT. Brachytherapy takes an entirely different approach: a radioactive source is placed inside or next to the area requiring treatment, keeping high doses intensely local and reducing exposure to healthy tissue further away. A course of brachytherapy can often be completed in less time than external-beam techniques.

• 5. Side Effects and Their Causes

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  Serious radiation complications occur in approximately 5% of cases. The main side effects most patients report are fatigue and skin irritation resembling a mild to moderate sunburn. Fatigue typically sets in during the middle of a treatment course and can last for weeks after treatment ends. Acute or sub-acute side effects may begin appearing after 50 gray of cumulative dosing; late or delayed injury, which can emerge anywhere from six months to decades after treatment, may develop after 65 gray.

  Late effects arise largely from damage to blood vessels and connective tissue. The salivary glands and tear glands have a radiation tolerance of about 30 gray in 2-gray fractions, a threshold exceeded by most radical head and neck cancer treatments, which is why dry mouth and dry eyes can become persistent problems that severely reduce a patient's quality of life. The gonads are also very sensitive; they may be unable to produce gametes following direct exposure to most standard treatment doses. Treatment planning is designed to minimize, and where possible completely exclude, dose to the gonads when they are not the primary target.

  Cardiovascular risk is a recognized long-term consequence. Therapeutic radiation increases the risk of a subsequent heart attack or stroke by 1.5 to 4 times a person's normal rate, and the occurrence rate of radiation-induced cardiovascular disease is estimated between 10 and 30%. Most of these events occur ten or more years after treatment, complicating the attribution of cause. In children between the ages of 5 and 11, studies found that IQ declined each year after head radiation by several IQ points. The risk of a radiation-induced glioblastoma or astrocytoma within 15 years of initial radiotherapy is estimated at 0.5-2.7%, though in the vast majority of cases that risk is greatly outweighed by the reduction in risk achieved by treating the primary cancer.

• 6. Safety and Accident History

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  Rigorous procedures exist to minimize accidental overexposure, but mistakes have occurred. The radiation therapy machine Therac-25 was responsible for at least six accidents between 1985 and 1987, in which patients received up to one hundred times the intended dose; two people were killed directly by the overdoses. Years later, from 2005 to 2010, a hospital in Missouri overexposed 76 patients, most of them with brain cancer, during a five-year period because new radiation equipment had been set up incorrectly.

  In response to such incidents, in 2010 the American Society for Radiation Oncology, known as ASTRO, launched a safety initiative called Target Safely. Its goals included recording errors nationwide so that clinicians could learn from each mistake and work to prevent recurrence. ASTRO also publishes questions for patients to ask their care teams about radiation safety at each stage of treatment.

  The society's parallel work on improving equipment and planning software reflects a broader push across the field. Image-guided radiation therapy uses imaging at each session to correct for positional errors before the beam is activated. Daily MR-guided adaptive radiation therapy, building on that foundation, can reshape the high-dose region to match the tumor as anatomy changes throughout the entire course of treatment, a capability that static planning cannot match. These advances trace a line back to the moment in 1953 when the first medical linear accelerator was used at the Hammersmith Hospital in London, replacing the crude X-ray tubes and radium sources that had defined the field for half a century.

• 7. Charged Particles and New Frontiers

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  Protons and heavier ions such as carbon behave differently from X-rays in tissue. As a charged particle penetrates, the dose it deposits increases with depth, peaking sharply at a point called the Bragg peak near the end of the particle's range, then dropping to almost zero. This profile means less energy is deposited in healthy tissue between the radiation source and the tumor, and virtually none beyond it.

  Because protons and heavier ions carry large mass, they scatter very little sideways through tissue; the beam stays tightly focused on the tumor's shape. They also cause direct double-stranded DNA breaks through high linear energy transfer, an antitumor effect that does not depend on the tumor's oxygen supply in the way that photon therapy does. That independence from oxygen makes particle therapy particularly attractive for treating hypoxic tumors that resist conventional X-rays.

  Children stand to benefit most from these improvements. Because their growing bodies are around 10 times more sensitive to developing secondary malignancies after radiotherapy compared to adults, reducing stray radiation dose is especially important in pediatric oncology. Radionuclide therapy, which delivers radiation through targeted molecules infused into the bloodstream, represents another direction. In April 2024, the FDA approved Lutathera, invented by Novartis, for pediatric patients aged 12 or older with certain gastroenteropancreatic neuroendocrine tumors, having already approved the same drug for adults in 2018. Meanwhile, as of 2017, more than half of patients in low and middle income countries still lacked access to radiotherapy of any kind, a gap that frames the next chapter of the field's development.