The formal adoption of SI units like the Gray (Gy) and Sievert (Sv). Key Quantities Defined
Report 33 retained Exposure as the ionization produced in air by photons. It essentially serves as a stepping stone: primary calibration labs measure Exposure (or air kerma), which is then converted to Absorbed Dose to water for clinical use. The report clarified the limitations of this quantity, specifically noting its inapplicability to energies above a few MeV or for particle beams other than photons. icru report 33
However, a dangerous inconsistency plagued the field. Different clinics used different phantoms, different ionization chamber calibration protocols, and different methods for specifying "depth of treatment." A prescribed dose of 2000 cGy for a 9 MeV electron beam meant very different biological effects from one institution to another. This lack of standardization led to unpredictable outcomes, ranging from local recurrence (underdose) to severe normal tissue necrosis (overdose). The formal adoption of SI units like the
The ICRU, established in 1925, had long sought to unify these definitions. However, as radiation physics evolved from simple X-ray tubes to high-energy linear accelerators and complex nuclear reactors, the older reports (such as Report 19) became insufficient. The interaction of radiation with matter required definitions that could withstand the scrutiny of modern Monte Carlo simulations and advanced dosimetry. The report clarified the limitations of this quantity,
The unit is the , equivalent to one joule per kilogram ($J \cdot kg^-1$). This definition shifted the focus from the radiation field itself (how much ionization occurs in air) to the effect on the patient (how much energy is deposited in tissue).
In the complex world of medical physics and radiation oncology, certain documents transcend their publication date to become timeless standards. Among these, holds a unique and hallowed place. Published in 1980 by the International Commission on Radiation Units and Measurements (ICRU), this slim but powerful report—titled "Radiation Dosimetry: Electron Beams with Energies Between 1 and 50 MeV" —did far more than its title suggests. It laid the definitive groundwork for how the global medical physics community measures, specifies, and reports radiation doses delivered by electron beam therapy.
Critically, ICRU Report 33 specified that the reference depth for calibration should be at the depth of dose maximum (dmax) for the given beam energy, and secondarily at a practical depth (e.g., R80 or R50) for routine QA.