Program Information
The Definite Target Volume
J Siebers*, W Watkins , University of Virginia Health System, Charlottesville, VA
Presentations
SU-I-GPD-T-432 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To define and describe the implications of targeting a new radiation therapy volume, the definite target volume (DTV).
Methods: The ever common planning target volume (PTV) is designed to encompass the physician identified clinical target volume (CTV) when spatial and other uncertainties are considered. The occupancy probability of the CTV within PTV is spatially non-uniform. We define the definite target volume (DTV) as the PTV sub-region where the CTV occupancy probability is approximately unity. Consequently, the probability of an organ at risk (OAR) or planning risk volume (PRV) overlapping with the DTV is zero. To first-order, the DTV can be defined by contracting the CTV by the same margin M as that which is used for CTV-to-PTV target expansion.
Results: Planning implications of the DTV include a) strict compliance to minimum dose constraints within the DTV (DTV cold spot avoidance) and b) the relaxation of maximum dose constraints within the DTV (DTV hot spot permissive). For CTVs which contain a high-risk avoidance volume, such as the urethra OAR through the prostate CTV, DTV hot-spot permissiveness should be excluded for the associated PRV to avoid hot-spot risks. For spherical volumes with radius R>M, the PTV/CTV volume ratio is (1+M/R)³ while the DTV/CTV ratio is (1-M/R)³. The DTV volume fraction is large for small uncertainties (M<
Conclusion: The DTV provides a helper volume to assist in treatment plan design. The DTV and its associated benefits will be greatest for large target volumes and small uncertainties. For small CTVs and large uncertainties, the DTV and its associated advantages diminish until the DTV volume is zero. Clinical utilization of treatment plans that enforce DTV cold-spot avoidance and permit DTV hot-spot spots may have the potential to improve the therapeutic ratio
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