Room: Exhibit Hall
Purpose: Rapid dose fall off from planning target volume (PTV) is required to spare organs at risk (OAR) that is hallmark of advanced treatment techniques (IMRT, VMAT, and SBRT). Along with conformity index (CI) and homogeneity index (HI) recently, gradient index (GI) was introduced to provide a measurable quality index for dose fall off from PTV. However, it is not clear if GI can be used in IMRT and if there are consistent differences between the two techniques.
Methods: Dose volume histogram data for 700 patients equally divided between SBRT and IMRT were retrospectively analyzed. GI was calculated for each patient which is ratio of the volume of at half the prescription (PIV50%) to prescription isodose volume (PIV100%) which was calculated treatment plan and from PTV and CI. Physical distance ratio between OAR and PTV coverage was determined based on the cube root of the GI as an additional tool along with gradient measure distance (rPIV50%-rPIV100%) for the evaluation of SBRT and IMRT plans.
Results: The GI varied widely between IMRT and SBRT patients due to inherent differences in techniques. The GIs are nearly constant 4.3Â±1.1 and 12.8Â±4.4 for SBRT and IMRT, respectively. The size of PTV is inversely related to GI with data more consistent in SBRT compared to IMRT. The gradient measure increases with PTV size in a well-defined way (0.5-1.5 cm) compared to IMRT where data is widely spread. This can be used as a surrogate for distance between PTV and OAR in IMRT and SBRT.
Conclusion: Gradient index and measure are effective parameters in evaluating dose gradient that show consistent differences between treatment techniques. These tools could provide an opportunity for plan evaluation especially the distance from the PTV to OAR to optimize the dose to reduce complications.