In radiation therapy with conventional fractionation, should the dmean (dose mean) and d2% (near maximum dose) of the Planning Target Volume (PTV) be kept under specific limits?

PTV Dose Limits in Conventional Fractionation Radiation Therapy

Yes, the maximum dose to the PTV should be kept under specific limits in conventional fractionation radiation therapy, with external-beam radiation doses exceeding 72 Gy using conventional fractionation (2.0 Gy/fraction) leading to unacceptable rates of normal tissue injury. 1

Maximum Dose Constraints (D2% or Near-Maximum Dose)

The critical upper limit for conventional fractionation is well-established:

• External-beam radiation doses exceeding 72 Gy at 2.0 Gy/fraction may lead to unacceptable rates of normal tissue injury 1
• For high-risk sites (primary tumor and involved lymph nodes), the maximum dose limits are typically 66-70 Gy (2.0-2.2 Gy/fraction) 1
• When doses greater than 70 Gy are considered necessary, fractionation should be modified to less than 2.0 Gy/fraction for at least some of the treatment to minimize toxicity 1

Mean Dose (Dmean) Considerations

While the guidelines focus primarily on maximum dose limits rather than mean dose to the PTV, the mean dose is implicitly controlled through proper dose prescription:

• The prescribed dose should be delivered to the PTV according to ICRU standards, which typically specify that 95% of the PTV receives the prescribed dose 1
• Dose homogeneity within the PTV is important, though some heterogeneity is acceptable with modern techniques like IMRT 1

Site-Specific Guidance

Head and Neck Cancers

• High-risk sites: 66-70 Gy (2.0-2.2 Gy/fraction) 1
• Intermediate/low-risk sites: 44-63 Gy depending on technique (44-50 Gy for 3D-CRT at 2.0 Gy/fraction; 54-63 Gy for IMRT at 1.6-1.8 Gy/fraction) 1

Postoperative Settings

• Standard postoperative RT: 60-66 Gy 1
• High-risk features (positive margins, extracapsular extension): up to 66 Gy with or without chemotherapy 1

Critical Pitfalls to Avoid

The 72 Gy threshold is a hard limit - exceeding this with conventional fractionation (2.0 Gy/fraction) significantly increases the risk of severe normal tissue complications, including necrosis, fibrosis, and organ dysfunction 1. This applies regardless of the tumor site or clinical circumstances.

Hot spots within the PTV matter - while older techniques prescribed to isocenter, modern practice using dose-volume parameters means the D2% (near-maximum dose, representing dose to 2% of volume) should not exceed these limits 1. The D2% essentially represents the maximum dose accounting for small volume hot spots.

Dose calculation algorithms affect reported values - advanced algorithms (Type B) calculate doses more accurately than older algorithms, and absolute doses cannot be compared between algorithm types 1. Ensure your planning system uses appropriate algorithms and that dose constraints are interpreted in that context.

Practical Implementation

When planning conventional fractionation treatments:

• Set your optimization constraints so that no more than 2% of the PTV receives more than 107-110% of the prescribed dose (this keeps hot spots reasonable while maintaining coverage) 1
• For a 70 Gy prescription, the D2% should not exceed approximately 75-77 Gy (107-110% of prescribed dose)
• If clinical circumstances require doses approaching or exceeding 70 Gy, strongly consider modifying the fractionation schedule to smaller doses per fraction 1

The evidence consistently demonstrates that these limits are based on normal tissue tolerance rather than tumor control considerations, making them non-negotiable safety thresholds for conventional fractionation schedules.