Clinical impact of DVH uncertainties

Area of Science:

• Medical Physics
• Radiotherapy Planning
• Computational Dosimetry

Background:

• Dose-volume histograms (DVHs) are critical for radiotherapy treatment planning, but variations exist between different treatment planning systems (TPS).
• Discrepancies in DVH calculations can impact clinical plan acceptability and lead to plan check failures.
• Small structures pose particular challenges due to digital representation limitations, affecting dose and volume metric accuracy.

Purpose of the Study:

• To design and implement methods for assessing the accuracy of dose and volume computations in TPS.
• To quantify the impact of calculation differences on the clinical acceptability of radiotherapy plans.
• To provide a detailed DVH analysis for small targets relevant to stereotactic radiosurgery (SRS) and dose assessments.

Main Methods:

• Developed methods to assess the accuracy of dose and volume computations in TPS.
• Generated synthetic spherical structures and Gaussian dose distributions for testing.
• Compared commercial DVH calculations against analytically derived reference DVHs in RayStation, MasterPlan, and ProKnow.
• Evaluated dose metrics like Paddick Conformity Index (PCI) and Modified Gradient Index (MGI) for small structures.
• Compared the D95 metric across multiple TPSs using real clinical plans.

Main Results:

• Differences in TPS calculations can alter the assessment of a plan's clinical acceptability.
• Errors up to 20% in DVHs were observed for volumes less than 1 cc.
• For a 3 mm radius sphere (≈0.1 cc), RayStation showed nearly 10% error in DVH calculations.
• Errors in PCI and MGI reached up to 40% and 75% respectively for volumes ≈0.1 cc.
• Clinical plans showed variations up to 9% in D95 for small volumes, with smaller variations for larger volumes.

Conclusions:

• Quantitative evidence highlights the critical need to test DVH calculator accuracy in TPS before clinical implementation.
• Significant errors in DVHs can occur for volumes less than 1 cc, impacting dose constraint recommendations.
• Deviations in DVH calculations can lead to plan check failures, necessitating appropriate uncertainty tolerances.
• Caution is urged when using DVH metrics for very small volumes; setting appropriate tolerances is recommended.