Acceptable Coverage for Palliative Spine Irradiation per ICRU
For palliative spinal irradiation, the ICRU Report 50 recommends that the planning target volume (PTV) should receive between 90% and 110% of the prescribed dose to achieve acceptable coverage. 1
ICRU Dose Distribution Requirements
The ICRU Report 50 establishes specific dose homogeneity criteria for acceptable treatment planning:
- Minimum PTV dose: Should be at least 90% of the prescribed dose 1
- Maximum PTV dose: Should not exceed 110% of the prescribed dose 1
- Mean PTV dose: Should approximate 100% of the prescribed dose 1
These parameters ensure homogeneous dose distribution throughout the target volume while minimizing hot and cold spots that could compromise treatment efficacy or increase toxicity. 1
Treatment Planning Approaches and ICRU Compliance
Two-Field Technique (AP-PA)
Opposed anterior-posterior/posterior-anterior (AP-PA) field plans successfully achieve ICRU Report 50 recommendations, with studies demonstrating:
- Mean minimum PTV dose: 91% (±1.3%) 1
- Mean maximum PTV dose: 108.8% (±1.3%) 1
- Mean PTV dose: 99.7% (±1.3%) 1
This technique provides homogeneous dose distribution while maintaining reasonable doses to critical structures including the spinal cord, esophagus, and intestines. 1
Single Posterior Field Limitations
Conventional single posterior field radiotherapy fails to meet ICRU Report 50 recommendations, regardless of reference point selection:
- Using ICRU reference points: minimum doses average only 77.3% and maximum doses reach 122.2% 1
- Using IBMC reference points: minimum doses average 83.7% while maximum doses reach 133.9% 1
- Maximum spinal cord doses exceeded 120% of prescribed dose in 49% of cases with IBMC reference points 1
These findings demonstrate unacceptable dose inhomogeneity that violates ICRU standards for safe and effective treatment. 1
Clinical Context and Modern Techniques
Target Volume Definition
For conventional palliative radiotherapy, the gross tumor volume (GTV) should be defined using pre- and post-operative imaging, with the clinical target volume (CTV) typically expanded by 2-3 cm to account for subclinical disease. 2
Important caveat: Historical practice of including one uninvolved vertebral body above and below the involved level may be unnecessary with modern image-guided techniques, as isolated adjacent-level failures occur in less than 5% of patients. 3
Dose Prescriptions
Standard palliative regimens include:
- 20-37.5 Gy in 5-15 fractions over 1-3 weeks 2
- 30 Gy in 10 fractions (most common) 4
- 8 Gy in single fraction (for poor performance status, though associated with higher reirradiation rates) 5
Critical Safety Considerations
The spinal cord must be carefully protected, with maximum doses maintained within tolerance limits even if this requires adjustment of the prescribed dose to the PTV. 6 For AP-PA field plans, mean spinal cord doses are significantly lower compared to single posterior field techniques (p < 0.001). 1
When ICRU dose distribution criteria cannot be met due to spinal cord constraints, multidisciplinary discussion should occur to consider alternative approaches such as stereotactic body radiotherapy (SBRT), which allows for more conformal dose distributions while maintaining cord safety. 2