In which International Commission on Radiation Units (ICRU) report are Volumetric Modulated Arc Therapy (VMAT) and Intensity-Modulated Radiation Therapy (IMRT) defined?

ICRU Report 83 Defines IMRT and VMAT

IMRT and VMAT are formally defined in ICRU Report No. 83, which provides standardized terminology, dose prescription, and reporting guidelines for intensity-modulated radiation therapy techniques. 1

ICRU Report 83 Overview

ICRU Report No. 83, published by the International Commission on Radiation Units and Measurements, specifically addresses "State of the art on dose prescription, reporting and recording in Intensity-Modulated Radiation Therapy." 1 This report serves as the international standard referenced across multiple clinical practice guidelines for radiation oncology. 2

Definitions from ICRU 83

Intensity-Modulated Radiation Therapy (IMRT)

IMRT is defined as an advanced form of 3D conformal radiation therapy that uses inverse treatment planning with beams from different angles, shaped according to the target form, with intensity adjusted throughout treatment to optimize dose to the target while limiting dose to surrounding normal tissues. 3 The technique allows modulation of radiation beam intensity to decrease doses to normal structures without compromising doses to cancer targets. 2

Key technical characteristics include:

• Uses inverse treatment planning algorithms 3
• Delivers radiation from multiple beam angles with varying intensities 3
• Can be delivered via "step and shoot" or "sliding window" techniques with fixed gantry positions 4
• Permits dose painting with different dose levels to different structures within the same treatment fraction 2

Volumetric Modulated Arc Therapy (VMAT)

VMAT is defined as a form of IMRT where the radiation source rotates around the patient in one or more arcs while continuously delivering radiation. 3 This represents a rotational delivery technique also known as intensity-modulated arc therapy (IMAT). 4

Key technical characteristics include:

• Continuous gantry rotation during treatment delivery 3
• Simultaneous variation of dose rate, gantry speed, and multi-leaf collimator positions 4
• Treatment delivery time typically less than 5 minutes 4
• Tends to yield similar dose distributions to fixed-gantry IMRT but with reduced monitor units 4

ICRU 83 Standardization Framework

The report provides comprehensive guidance on:

• Selection and delineation of target volumes and organs at risk 1
• Refined concepts of dose prescription and dose-volume reporting 1
• Standardized nomenclature for planning target volume (PTV), clinical target volume (CTV), and gross tumor volume (GTV) 2
• Dose reporting parameters including D98, D95, D50, D2, and homogeneity index 5

Clinical Implementation Guidance

Multiple guidelines reference ICRU 83 as the standard for IMRT implementation:

• NCCN guidelines for Hodgkin lymphoma specifically cite ICRU 83 for dose prescription, reporting, and recording 2
• Malignant pleural mesothelioma guidelines recommend ICRU 83 for clinical target volume definition 2
• Head and neck cancer guidelines reference ICRU 83 principles for target delineation 2
• Prostate cancer guidelines utilize ICRU 83 criteria for plan evaluation 2, 6

Common Pitfall

Compliance with ICRU 83 recommendations remains poor in clinical practice, with nearly 95% of treatments deviating from recommended D50 prescription dose delivery and significant variability in target nomenclature. 5 This variability is statistically significant across institutions, tumor sites, and techniques, emphasizing the need for strict adherence to ICRU 83 standardization to reduce dosimetric variability and improve radiation outcomes. 5