Half-Value Layer and Depth of Maximum Dose for Cobalt-60 and Megavoltage Linear Accelerators
Half-Value Layer (HVL) Values
The half-value layer represents the thickness of material required to reduce radiation intensity by 50%, serving as a critical beam quality specification parameter 1.
Cobalt-60 and Common MV Energies
| Energy | HVL in Water (cm) | HVL in Lead (mm) |
|---|---|---|
| Cobalt-60 | ~11 cm | ~11-12 mm |
| 4 MV | ~12-13 cm | ~13-14 mm |
| 6 MV | ~13-14 cm | ~14-15 mm |
| 10 MV | ~15-16 cm | ~16-17 mm |
| 15 MV | ~17-18 cm | ~18-19 mm |
| 18 MV | ~18-19 cm | ~19-20 mm |
| 25 MV | ~20-21 cm | ~21-22 mm |
Note: HVL measurements should be performed using narrow-beam transmission methodology in water for accurate photon beam quality specification, as depth-dose measurements can be affected by electron contamination at high energies 1.
Depth of Maximum Dose (Dmax) Values
The depth of maximum dose represents where the absorbed dose reaches its peak along the central axis, occurring due to the buildup of secondary electrons 2.
Dmax for Various Beam Energies
| Energy | Dmax (cm) | Dmax (mm) | Field Size Dependency |
|---|---|---|---|
| Cobalt-60 | 0.5 | 5 | Minimal variation |
| 4 MV | 1.0-1.2 | 10-12 | Slight increase with field size |
| 6 MV | 1.5 | 15 | Increases ~2-3 mm from 5×5 to 25×25 cm² [2] |
| 10 MV | 2.5 | 25 | Increases ~3-4 mm from 5×5 to 25×25 cm² [2] |
| 15 MV | 3.0-3.5 | 30-35 | More pronounced field size effect |
| 18 MV | 3.5-4.0 | 35-40 | Significant field size dependency |
| 25 MV | 4.5-5.0 | 45-50 | Marked field size dependency |
Critical Clinical Considerations
Surface Dose Characteristics
- For single-field treatments: Surface dose ranges from <10% to nearly 100% of prescription dose depending on energy and field size 2.
- For parallel opposed pairs (POP): Surface dose is relatively energy-independent, ranging 30-70% of prescription dose, primarily dependent on field size 2.
Buildup Region Parameters
- The d90 depth (where dose reaches 90% of prescription) is always <22 mm for open fields, and <4 mm for energies ≤6 MV 2.
- Maximum dose (Dmax) values can exceed 300 cGy for 100 cGy prescription in certain single-field configurations, but are significantly reduced in POP treatments 2.
Electron Contamination Effects
- At 10 cm depth: Contaminating electrons contribute negligibly for energies up to 20 MV (TPR₂₀,₁₀ = 0.772), but contribute 2-3% at 50 MV (TPR₂₀,₁₀ = 0.810) 3.
- Reference depth recommendation: 10 cm is recommended as both reference and normalization depth for all megavoltage photon beams from ⁶⁰Co through 50 MV to minimize electron contamination effects 3.
Beam Quality Characterization
- The buildup parameter (n) increases with photon energy: 0.17 (4 MV), 0.208 (6 MV), 0.495 (10 MV), 1.2 (18 MV), representing beam hardening 4.
- The attenuation coefficient (μ) decreases with photon energy: 0.065 (4 MV), 0.0515 (6 MV), 0.0458 (10 MV), 0.0422 (18 MV) 4.
Common Pitfalls to Avoid
- Do not rely on depth-dose measurements alone for beam quality specification at high energies, as electron contamination significantly affects TPR₂₀,₁₀ ratios and can introduce >2% errors 1.
- Do not assume Dmax is independent of field size, particularly at higher energies where field size variations can shift Dmax by several millimeters 2.
- Do not use surface as reference point for dose specification with electron beams, as doses should be specified at 90% of Dmax to account for buildup characteristics 5.
- Do not overlook the clinical significance of reduced incremental benefit in Dmax reduction for POP treatments when increasing beam energy above 10 MV 2.