Generalization of the Conformity Index for Multi-Target Radiotherapy Plans

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• Current conformity index (CI) formulas exhibit distortions in multi-target radiotherapy plans.
• The VTV parameter in existing CI formulas requires refinement for accurate multi-target plan evaluation.

Purpose of the Study:

• To redefine the VTV parameter for a more accurate CI calculation in multi-target plans.
• To provide a clinically applicable reference for evaluating treatment conformity.

Main Methods:

• A novel VTV calculation formula was developed, addressing limitations of the van't Riet and Paddick CI.
• 15 breast cancer (BC) and 15 nasopharyngeal carcinoma (NPC) plans were analyzed.
• Statistical comparisons (Wilcoxon signed-rank test, Pearson's correlation) were performed between new and traditional CI calculations.

Main Results:

• The new VTV calculation yielded significantly lower values than traditional methods for BC and most NPC targets (p < 0.05).
• Consequently, new CI values were significantly higher for BC and most NPC targets, indicating improved conformity.
• Identical results were observed for PTV1 in NPC, suggesting target-specific formula performance.

Conclusions:

• The proposed VTV formula accurately reflects target volume conformity by excluding dose spillage from adjacent targets.
• This refined CI calculation provides a more precise measure of treatment conformity compared to traditional methods.
• The new formula is recommended for calculating CI in multi-target plans, including BC and NPC treatments.