On correlations in IMRT planning aims

The purpose was to study correlations amongst IMRT DVH evaluation points and how their relaxation impacts the overall plan. 100 head-and-neck cancer cases, using the Eclipse treatment planning system with the same protocol, are statistically analyzed for PTV, brainstem, and spinal cord. To measure variations amongst the plans, we use (i) interquartile range (IQR) of volume as a function of dose, (ii) interquartile range of dose as a function of volume, and (iii) dose falloff. To determine correlations for institutional and ICRU goals, conditional probabilities and medians are computed. We observe that most plans exceed the median PTV dose (average D₅₀ = 104% prescribed dose). Furthermore, satisfying D₅₀ reduced the probability of also satisfying D₉₈, constituting a negative correlation of these goals. On the other hand, satisfying D₅₀ increased the probability of satisfying D₂, suggesting a positive correlation. A positive correlation is also observed between the PTV V₁₀₅ and V₁₁₀. Similarly, a positive correlation between the brainstem V₄₅ and V₅₀ is measured by an increase in the conditional median of V₄₅, when V₅₀ is violated. Despite the imposed institutional and international recommendations, significant variations amongst DVH points can occur. Even though DVH aims are evaluated independently, sizable correlations amongst them are possible, indicating that some goals cannot be satisfied concurrently, calling for unbiased plan criteria.