Medical physics is the branch of applied physics devoted to using physics concepts, tools, and techniques in medicine, spanning everything from designing safer imaging scans to ensuring a radiation beam hits a tumor with sub-millimeter precision. The field quietly underpins many of the diagnostic and treatment technologies patients encounter in hospitals, yet most people have never heard of it. Its practitioners, called medical physicists, work alongside oncologists, radiologists, and surgeons to make sure equipment works correctly, radiation doses are accurate, and patients receive the safest possible care.
How the Field Took Shape
The roots of medical physics reach back further than most people assume. The earliest known attempt to apply physics to medicine came in the early seventeenth century, when the Italian physician Sanctorius used a weighing chair to track changes in body mass as a way of monitoring health. Over the following centuries, biomechanics, medical electricity, and physiological physics emerged in pockets across Europe. Medical physics became a recognized academic discipline in France around the time of the Revolution, with Jean Hallé appointed as its first professor.1PubMed. The origins of medical physics
The field’s modern identity, though, really crystallized after Wilhelm Roentgen discovered X-rays in 1895. Early medical physicists focused on the safe handling of radium for cancer treatment and on protecting medical staff from radiation exposure. By the end of World War II, the scope had expanded to include dosimetry, the measurement and calculation of radiation doses delivered to patients, which was essential for evaluating whether a treatment was working.2PubMed. History of medical physics The war’s nuclear technology programs also made large quantities of radioisotopes available for the first time, giving birth to clinical nuclear medicine and adding yet another subspecialty to the profession.2PubMed. History of medical physics
Then came the computer revolution. The invention of computed tomography in 1972 and the development of MRI around the same time transformed diagnostic imaging and, with it, medical physics. Suddenly the field was responsible not just for radiation safety but for understanding image quality, reconstruction algorithms, and the physics of entirely new scanning methods. Three-dimensional treatment planning, stereotactic radiosurgery, and intensity-modulated radiation therapy followed in subsequent decades, reshaping how cancer is treated and further expanding the medical physicist’s job description.3PubMed. Medical physics: some recollections in diagnostic X-ray imaging and therapeutic radiology
Shaping Radiation Beams to Spare Healthy Tissue
When people think of cancer treatment, radiation therapy is one of the first things that come to mind. What they rarely think about is the physicist who makes sure that beam does what it’s supposed to. Intensity-modulated radiation therapy, widely known as IMRT, is one of the most significant advances in this space. The concept of inverse planning was first introduced in 1982, and over three decades of development have made IMRT the most common delivery method in radiation therapy worldwide.4PubMed Central. Intensity-modulated radiation therapy: a review with a physics perspective Rather than bathing a wide area in a uniform beam, IMRT uses computer-controlled devices called multileaf collimators to modulate the beam’s intensity across the treatment field, sculpting the dose so that it conforms tightly to the tumor’s shape.
The medical physicist’s role here is central. They develop and validate the treatment plan, run patient-specific quality checks before the first fraction is delivered, and ensure the hardware performs within tight tolerances. The American Association of Physicists in Medicine (AAPM) has published detailed guidance on setting up a safe IMRT program, encompassing everything from delivery systems to inverse planning to clinical quality assurance.5PubMed. Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee In complex cases where tumors sit right next to vital organs, physicists lead the optimization effort, adjusting plan parameters until the best possible compromise between tumor coverage and organ sparing is achieved.6Journal of Clinical and Scientific Research. The critical role of multidisciplinary teams in health care: Perspectives from radiation oncology
Proton Therapy and the Bragg Peak
Proton therapy represents a different approach to the same problem. Instead of X-ray beams, it uses protons, which behave very differently as they pass through tissue. A proton beam deposits relatively little energy as it enters the body, then releases most of its energy in a narrow spike at a specific depth called the Bragg peak, with essentially no dose deposited beyond that point.7PubMed Central. Image-Guided Proton Therapy: A Comprehensive Review This property makes proton beams attractive for tumors located near sensitive structures like the spinal cord or the brain.
Medical physicists are deeply involved in characterizing and exploiting this behavior. The Bragg peak is the characteristic maximum on the curve that plots energy deposition along the beam’s path, and getting its position right is the difference between treating a tumor and damaging healthy tissue behind it.8Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. Jump stochastic differential equations for the characterisation of the Bragg peak in proton beam radiotherapy Physicists model how the beam interacts with tissue, account for patient anatomy that may shift between sessions, and verify the delivered dose using imaging built into the treatment machine.
Brachytherapy and Internal Radiation Sources
Not all radiation therapy comes from external beams. Brachytherapy involves placing sealed radioactive sources directly inside or next to a tumor, delivering a high dose to a small volume while limiting exposure to surrounding tissue. It is commonly used for cervical, prostate, and certain head-and-neck cancers. The medical physicist’s job here includes calibrating every source before it goes into a patient.
For high-dose-rate brachytherapy sources like iridium-192, calibration involves independently measuring the source’s output using well-type ionization chambers, in-air measurements, or solid phantom setups. These results are compared against the manufacturer’s stated values to confirm accuracy. In practice, variations across methods have been shown to be quite small, typically within about one percent of the manufacturer’s figure.9PubMed Central. Calibration of (192)Ir high dose rate brachytherapy source using different calibration procedures European-wide recommendations from the GEC-ESTRO working group aim to standardize this calibration process across hospitals to maintain high treatment quality.10PubMed. GEC-ESTRO ACROP recommendations on calibration and traceability of HE HDR-PDR photon-emitting brachytherapy sources at the hospital level
Keeping Imaging Equipment Honest
Medical physics is not only about therapy. A large part of the profession is devoted to diagnostic imaging, making sure that every CT scan, ultrasound, and nuclear medicine study produces reliable pictures at the lowest necessary radiation dose.
CT dose optimization is a particularly active area. As the number of CT scans performed worldwide has climbed, concern about cumulative radiation exposure has grown in parallel. Physicists work on dose-saving strategies that adjust scanning parameters to individual patients, using factors like body size to predict the minimum dose needed for diagnostic image quality.11PubMed Central. CT radiation dose optimization and estimation: an update for radiologists Mathematical models can now predict and optimize both image quality and radiation dose on the basis of patient size and the noise level a particular radiologist considers acceptable.12PubMed. System for verifiable CT radiation dose optimization based on image quality. Part I. Optimization model
In nuclear medicine, the physics is different but the goal is similar. PET/CT scanners detect gamma rays emitted by radioactive tracers injected into the body, and medical physicists develop and validate the mathematical algorithms that reconstruct those signals into usable images.13PubMed Central. Image reconstruction for PET/CT scanners: past achievements and future challenges Acceptance testing and ongoing quality assurance for PET/CT systems follow standardized procedures, including eight distinct performance evaluations using common phantoms and freely available software to ensure consistency across vendors and institutions.14PubMed. PET/CT acceptance testing and quality assurance: Executive summary of AAPM Task Group 126 Report
Even ultrasound, which uses no ionizing radiation, requires physics-based quality checks. Test objects and protocols are used to measure parameters like axial resolution, lateral resolution, and geometric uniformity across the ultrasound field, all of which drift over time if equipment isn’t maintained.15PubMed. A digital image analysis method for diagnostic ultrasound calibration
Quality Assurance as a Daily Practice
If there is one thread that runs through everything medical physicists do, it is quality assurance. The terms “quality management,” “quality assurance,” and “quality control” get used loosely in hospitals, but in medical physics they have specific meanings. Quality management is the overarching system of policies and goals. Quality assurance consists of the planned activities that ensure a process meets requirements. Quality control is the operational testing that verifies equipment is performing within accepted limits. A white paper from the AAPM clarified these distinctions for use across radiology, radiation oncology, and nuclear medicine to improve consistency in how clinical teams communicate about safety.16PubMed Central. Quality management, quality assurance, and quality control in medical physics
In radiation therapy, this translates to physicists checking the output of treatment machines against national or international calibration protocols on an annual basis and performing more frequent spot checks in between.17International Journal of Radiology and Radiation Oncology. Timing of Annual Output Calibration of Radiotherapy Linear Accelerators For image-guided radiotherapy systems, the AAPM publishes practice guidelines covering commissioning and ongoing checks to ensure that imaging hardware used to position patients before each treatment session is itself accurate.18PubMed Central. AAPM Medical Physics Practice Guideline 2.a: Commissioning and quality assurance of X-ray-based image-guided radiotherapy systems
Radiopharmaceutical Therapy and Internal Dosimetry
A fast-growing corner of medicine involves injecting or swallowing radioactive drugs that seek out tumor cells, delivering radiation from the inside. You may have heard the term “theranostics,” which combines a diagnostic scan with a matched therapeutic agent using the same targeting molecule. Medical physicists are responsible for calculating the radiation dose each organ absorbs from these internal sources.
Advanced imaging, especially SPECT and PET, can map where the radioactive drug goes in the body over time, allowing physicists to build patient-specific dose estimates.19PubMed Central. Quantitative Imaging for Targeted Radionuclide Therapy Dosimetry – Technical Review The absorbed dose is then calculated using one of several approaches: dose-factor methods like the MIRD formalism, dose point kernel convolution, or full Monte Carlo radiation transport simulations.20Journal of Nuclear Medicine. Dosimetry in Radiopharmaceutical Therapy In practice, patients treated with agents like lutetium-177-labeled peptides undergo whole-body imaging at multiple time points after injection so that physicists can model the drug’s clearance from each organ and calculate personalized doses.21Frontiers in Biomedical Technologies. Internal Dosimetry in Patients Undergoing Peptide Receptor Radionuclide Therapy (PRRT) with 177Lu-[DOTA0-Tyr3] octerotate: A Single-Center Experience
Designing Safe Facilities
Before a treatment machine is ever turned on, medical physicists help design the room it sits in. High-energy radiation beams can penetrate ordinary building materials, so treatment vaults require careful shielding calculations. These calculations follow protocols from bodies like the National Council on Radiation Protection and Measurements and the International Atomic Energy Agency, factoring in beam energy, workload, and the occupancy of surrounding areas.
A recent study of shielding for medical linear accelerators operating at 6 and 10 megavoltage found that strict adherence to instantaneous dose-rate criteria could increase shielding demands by roughly 11% to 46% compared to conventional approaches, while reducing personnel dose to a fraction of dose constraints. The authors cautioned that this level of strictness could amount to overdesign, raising construction costs without a proportional safety benefit.22Advances in Radiotherapy & Nuclear Medicine. Shielding design calculations for a radiotherapy vault of a 6 and 10-megavoltage medical linear accelerator operating with or without a flattening filter Balancing patient and staff safety against practical cost is one of the less glamorous but very real contributions medical physicists make to healthcare infrastructure.
Artificial Intelligence and the Changing Toolkit
AI is beginning to touch almost every aspect of the medical physicist’s work. Machine learning algorithms can now detect and classify abnormalities on images, segment tumors and organs automatically, and assist with image registration and reconstruction. In treatment planning, AI tools are being trained to optimize radiation therapy plans by incorporating patient-specific anatomy, predicted tumor response, and organ-at-risk constraints.23Journal of Cancer Treatment and Research. The Role of Artificial Intelligence in Radiation Therapy Treatment Planning: A Comprehensive Review
Deep learning tools are already being integrated into commercial treatment planning systems for external beam radiotherapy, automating tasks that previously required hours of a physicist’s or dosimetrist’s manual effort.24PubMed. The Emergence of Artificial Intelligence within Radiation Oncology Treatment Planning These tools don’t replace the physicist; instead, they shift the physicist’s role from drawing contours and adjusting plan parameters to reviewing, validating, and catching the errors that automated systems inevitably produce. The quality assurance burden may actually increase as the planning itself becomes faster.
FLASH Radiotherapy and What’s on the Horizon
One of the most intriguing research frontiers in medical physics right now is FLASH radiotherapy, which delivers radiation at ultra-high dose rates, orders of magnitude faster than conventional treatment. Preclinical studies have shown that FLASH markedly reduces damage to normal tissue without compromising its ability to kill tumor cells, a phenomenon researchers call the “FLASH effect.” The proposed mechanisms include transient oxygen depletion in tissues, shifts in how free radicals recombine, and differences in how cells repair DNA damage at extreme dose rates.25PubMed Central. Harnessing the sparing effect of FLASH-RT: From phenomenon observation, radiophysical determinants to molecular mechanisms and synergistic strategies If the effect translates to humans at scale, it could change the fundamental parameters of radiation treatment. Medical physicists are central to this research because characterizing and delivering beams at such extreme rates requires entirely new dosimetry methods and hardware.
Photoacoustic imaging is another area where physics is opening new doors. This technique uses short laser pulses and acoustic detectors, rather than X-rays, to visualize structures like blood vessels and nerves during minimally invasive surgery. It has the potential to help surgeons see critical structures that are otherwise hidden, reducing the risk of accidental injury during procedures.26PubMed Central. Photoacoustic imaging for surgical guidance: Principles, applications, and outlook
Training, Certification, and the Global Workforce
Becoming a medical physicist is not a quick path. In most countries it requires a graduate degree in medical physics or a closely related field, followed by a structured clinical residency and a board certification examination. Over recent decades, training has shifted from informal apprenticeship models to formalized residency programs, guided by policies from organizations like the International Organization for Medical Physics (IOMP) and its regional counterparts.27PubMed. Certification and licensing of clinical medical physicists in AFOMP countries The IOMP now represents over 27,000 medical physicists across 87 national member organizations and maintains official relations with the World Health Organization and the International Atomic Energy Agency.28PubMed Central. Science diplomacy in medical physics – an international perspective
Despite this growing professional infrastructure, the global distribution of medical physicists is starkly uneven. There are roughly 14,000 teletherapy machines worldwide, but low-income and lower-middle-income countries have access to fewer than 10% of them. The deficit in medical physicists is even harder to quantify, but the inequity is clear.29PubMed Central. Access to Radiation Therapy: From Local to Global and Equality to Equity In many parts of Africa, South Asia, and the Pacific, a single medical physicist may serve an entire region, handling equipment that would be supported by a team of specialists at a large Western hospital. This gap matters because without a trained physicist to commission equipment, calibrate sources, and run quality checks, the equipment itself is only as good as the guess behind its settings.
The Cost Side of Physics-Driven Treatment Choices
Advanced technologies don’t always translate to better value. One study comparing IMRT to older three-dimensional conformal radiotherapy for head-and-neck cancers found that the conformal technique produced more quality-adjusted life years at lower cost, saving roughly $7,400 per quality-adjusted life year compared to IMRT in that setting.30PubMed Central. The cost effectiveness of intensity-modulated radiation therapy and three-dimensional conformal radiotherapy in the treatment of head and neck cancers That’s a counterintuitive result, since IMRT is the newer technology, and it underscores the fact that “more advanced” does not automatically mean “better for every patient.”
For whole-breast radiation after surgery, cost-effectiveness varies widely by technique. Simple field-in-field methods showed the lowest incremental cost per quality-adjusted life year, while full IMRT showed the highest.31PLOS ONE. Cost-effectiveness analysis of radiotherapy techniques for whole breast irradiation Economies of scale also play a role: the cost per treatment course drops as a department grows, with the most pronounced savings occurring up to about three treatment machines before diminishing returns set in.32PubMed. Cost evaluation to optimise radiation therapy implementation in different income settings: A time-driven activity-based analysis Medical physicists are increasingly being asked to contribute to these economic conversations, helping departments choose the right technology for their patient population rather than defaulting to the most expensive option available.