Considerations for the implementation of target volume protocols in radiation therapy

Purpose: Uncertainties in patient repositioning and organ motion are accoun ted for by defining a planning target volume (PTV). We make recommendations on issues not explicitly discussed in existing protocols for PTV design. Methods: A quantity called "coverage" is defined to quantify how effectivel y a PTV encompasses the clinical target volume, and is applied to examine t he impact of several factors, A stochastic simulation is used to determine the coverage required for a desirable balance between tumor control probabi lity (TCP) and the irradiated volume. Using a sample anatomy, we assess the importance of the method used to add uncertainties, the shape of the uncer tainty distribution, the effect of systematic uncertainties, and the use of nonuniform margins. Additionally, we examine the benefit of patient immobi lization techniques. Results: Our example indicates that 95% coverage is a reasonable goal for t reatment planning. Using this as a comparison value, our example indicates quadrature addition of uncertainties predicts smaller margins (7 mm) than l inear addition (11 mm), Gaussian distribution of uncertainties (7 mm) requi re the same margin as a uniform distribution (7 mm), systematic uncertainti es have a small effect on TCP below a threshold value (4 mm), and nonunifor m margins allow only a slight reduction of irradiated volume. Conclusion: We recommend that uncertainties should generally be added in qu adrature, the exact shape of the uncertainty distribution is not critical, systematic uncertainties should be maintained below some threshold value, a nd nonuniform margins may be effective when uncertainties are anisotropic, (C) 2001 Elsevier Science Inc.