29. Ultra-hypofractionated prostate cancer radiotherapy: A global Unified Dosimetry Index (gUDI) for treatment plans evaluation (AIRC, grant: IG-13218)

Purpose Formulation of a global Unified Dosimetry Index (gUDI) for the evaluation of ultra-hypofractionated prostate Volumetric Modulated Arc Therapy (VMAT by RapidArc) treatment plans. Methods Radiotherapy treatment plan quality can be assessed in terms of dose coverage, conformity, homogeneity and dose gradient index. All these parameters could be included in the Unified Dosimetry Index (UDI). We developed a global UDI (gUDI) to evaluate treatment plans containing volumes irradiated with different dose prescriptions: Intensity Modulated Radiation Therapy with simultaneous integrated boost IMRT-SIB with 2 dose levels (36.25 Gy/5 fz for the whole prostate gland and 37.5 Gy/5 fz for Dominant Intraprostatic Lesion (DIL)). gUDI score equals to one for an ideal plan. To validate gUDI scoring system, 65 prostate cancer patients treated with ultra-hypofractionated IMRT-SIB using VMAT technique were evaluated. The computation of gUDI index was performed separately for plans with single or double DIL. Mean ( μ ) and standard deviations ( σ ) were calculated for all dosimetry indices and gUDI. Furthermore, gUDI μ and σ were analyzed to compare and classify treatment plans: under this ranking system, plans can be ranked as “excellent”, “good”, “average” or “poor”. Results Dose Coverage and Homogeneity index scores did not get away from the ideal values, both for the prostate and DIL. No statistical differences were founded for plans with single or double DIL. The gUDI index classification showed only 6 cases in the “poor” region and most of the plans scored as “average” and “good”. Conclusions gUDI score can be a useful tool to quantify treatment plans quality also when different dose prescriptions volumes are treated. Furthermore, gUDI allows to establish a standard on which dosimetry plans can be ranked.