What are the guidelines for radiation therapy dosimetry as per International Commission on Radiation Units and Measurements (ICRU) reports 50, 62, and 83?

ICRU Reports 50,62, and 83: Radiation Therapy Dosimetry Guidelines

Radiation target volumes must be defined using pretreatment PET and CT scans following ICRU Reports 50 and 62, with dose prescriptions and reporting adhering to international ICRU standards for all external beam radiotherapy. 1, 2, 3

Core Volume Definitions (ICRU 50 & 62)

The ICRU established a hierarchical framework for defining treatment volumes that remains the international standard:

• Gross Tumor Volume (GTV): All visible and palpable tumor based on clinical examination, imaging, and pathologic findings 3
• Clinical Target Volume (CTV): GTV plus margin for microscopic disease extension, typically 5 mm for lymph nodes 3
• Planning Target Volume (PTV): CTV plus margin accounting for setup uncertainties and internal organ motion 3, 4
• Internal Target Volume (ITV): Used specifically when accounting for respiratory motion in thoracic malignancies 3

Manual PTV adjustments are not permitted, as the PTV inherently accounts for setup errors and breathing motion. 1

Margin Calculation Methodology (ICRU 62)

The dosimetric consequences of systematic versus random geometrical uncertainties differ fundamentally and require specific margin calculations 4:

• Standard margin formula: M = 2Σ(tot) + 0.7σ(tot), where Σ(tot) represents systematic errors and σ(tot) represents random errors 4
• For uncorrelated internal and external uncertainties: Σ(tot) = √(Σ(int)² + Σ(ext)²) and σ(tot) = √(σ(int)² + σ(ext)²) 4
• The ICRU-62 recommendation to quadratically add standard deviations for systematic and random errors is generally not valid 4

Dose Prescription Standards (ICRU 50)

Dose specification must follow ICRU reference point methodology for consistency across institutions 1, 5:

• Doses should be prescribed to the ICRU reference point rather than to isodose lines encompassing the target volume 5
• Dose homogeneity within the PTV significantly affects treatment technique and field sizes 5
• Small deviations in dose prescription particularly influence late-responding normal tissues due to simultaneous adjustment of single and total doses 5

ICRU 83 Guidelines for Advanced Techniques

ICRU Report 83 provides specific guidance for intensity-modulated radiotherapy (IMRT) and modern conformal techniques 1:

• IMRT allows more conformal high-dose delivery with improved hemithorax coverage while minimizing contralateral organ exposure 1
• Strict adherence to dose constraints is mandatory, particularly for contralateral lung (mean dose <8.5 Gy) to avoid fatal pneumonitis 1, 6
• The ICRU 83 guidelines are particularly useful for complex anatomical sites requiring advanced dose calculation 1

Disease-Specific Applications

Lung Cancer

• Target volumes defined using pretreatment PET and CT scans per ICRU 50 and 62 definitions 1
• Radiation doses calculated with inhomogeneity corrections using advanced algorithms (type B) 1
• Three-dimensional conformal techniques preferred, with IMRT considered in selected patients 1

Other Malignancies

• Hodgkin disease: ICRU 50 dose prescription standards apply with 40 Gy to involved areas as sole treatment, 36 Gy after complete remission post-chemotherapy 1
• Breast cancer: ICRU standardized dose prescription mandatory for all radiotherapy 1
• Non-small cell lung cancer: Dose prescriptions and reporting must follow international ICRU standards 1

Quality Assurance Requirements

Radiotherapy must conform to ICRU Reports 29,50, and 62 guidelines, along with quality assurance program recommendations. 1

• Treatment charts should fulfill ICRU 50 Level 2/3 recommendations for clarity in reporting 7
• Documentation must include treatment planning geometry, console parameters, dosimetric data, and treatment data 7
• The ICRU's role encompasses development of internationally acceptable recommendations for quantities, units, measurement procedures, physical data, and terminology 8

Common Pitfalls

• Failure to account for respiratory motion leads to systematic errors in thoracic radiotherapy; 4D-CT scanning is strongly preferred 3
• Inadequate margins for microscopic disease extension result in marginal recurrences 3
• Deliberately reducing PTV margins to spare normal tissues means the planned PTV dose no longer represents the CTV dose; in such cases, report minimum dose to the Representative Target Volume (RTV) 4
• Inconsistent application of ICRU definitions across institutions compromises treatment outcome comparisons 5, 9