Dosimetry — Radiation Dose Calculation Science

In clinical oncology, dosimetry involves determining the absorbed dose of ionising radiation—expressed in units known as grays (Gy)—delivered to specific anatomical targets. This process is carried out by qualified medical physicists and dosimetrists using sophisticated computer simulations, physical measuring devices, and three-dimensional anatomical data acquired from diagnostic imaging.

Dosimetry applies to external beam radiotherapy, brachytherapy (internal source placement), and radiopharmaceutical therapy (targeted radionuclides). Through iterative algorithms, dosimetry models how radiation beams or radioisotopes penetrate, scatter, and deposit energy within varying biological tissues, establishing safe margins and verifying that critical organs remain well below established toxicity thresholds.

Dosimetry directly underpins the safety and efficacy of your radiotherapy. Because every individual's internal anatomy is unique, bespoke dosimetry ensures that treatment margins account for your specific tumour size, shape, and surrounding vital structures—such as the spinal cord, heart, or bowel. High-calibre dosimetry gives your clinical oncology team confidence that the prescribed radiation will maximise tumour destruction while minimising immediate and long-term adverse effects.

Dosimetry is not a biological blood test, but rather an imaging- and physics-based assessment. It relies on high-resolution computed tomography (CT) simulation scans, often combined with magnetic resonance imaging (MRI) or positron emission tomography (PET), to contour anatomical structures. Physical verification uses specialised detectors—such as ionisation chambers, thermoluminescent dosimeters (TLDs), and electronic portal imaging devices—to measure actual dose outputs on linear accelerators before treating the patient.

Dosimetry is vital for nearly any cancer managed with radiation. It is heavily utilised in head and neck cancers, where tumours sit millimetres away from salivary glands and optical nerves, and in prostate cancer, where the rectum and bladder require strict protection. It is also central to lung cancer, brain tumours (such as glioblastoma), and systemic radioligand therapies for neuroendocrine tumours and metastatic prostate cancer.

The findings from dosimetry calculations shape the entire treatment delivery technique, such as Intensity-Modulated Radiotherapy (IMRT), Volumetric Modulated Arc Therapy (VMAT), or Stereotactic Body Radiotherapy (SBRT). In targeted radionuclide treatments, such as lutetium-177 therapy, post-infusion whole-body scans calculate internal dosimetry to personalise future therapeutic doses, ensuring that kidneys and bone marrow are not overloaded with radioactivity during repeated cycles.