P273] Establishing useful clinical guiding values for conformity indexes in pelvic VMAT-treatment using retrospective clinical data

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Purpose To establish clinical treatment site specific Conformity Index criteria for four different types of pelvic volumetric modulated arc therapy (VMAT) treatments. Methods A retrospective database retrieval via SQL was performed in our clinic to find VMAT patients that were treated between 2012-05-31 and 2017-12-31. The ICD code toggles used was C20, C21, C53, C54 and C61 corresponding to malignant neoplasm of rectum, anal canal, cervix uteri, corpus uteri and prostate, respectively. Prostate plans was separated into irradiation of prostate only or prostate plus lymph nodes. Using an in-house developed script (Eclipse scripting API, Varian Medical Systems) approved treatment plans and dose distributions were retrieved and the Paddick Conformity Index (PCI) was calculated. In cases of treatments with different dose levels, the lowest level was analyzed with its corresponding target volume. The PCI was evaluated for each year. Further we established clinical guiding values for use in the daily clinical workflow at our clinic for simple/complex target. Results In total 2235 VMAT treatment plans were analyzed. The global mean PCI was 0.86 for all plans for the years analyzed. Looking on all diagnoses the average PCI per year were 0.87, 0.82, 0.83, 0.85, 0.87 and 0.89 for 2012 to 2017, indicating a yearly increase of 0.02, with the exception for the first year due to mostly simple spherical/circular target volumes (22/28 plans). Separating out treatment plans to simple spherical/circular targets (prostate only plus rectal vs others) over the last two years we found mean PCI = 0.89 for prostate only and rectal treatments. For other more complex treatment the mean PCI was 0.86. Conclusions Evaluating the clinical specific PCI, we decided on a guiding value of 0.92 for rectum and prostate only patients and 0.90 for other, more complex pelvic treatments. A simple script has been implemented in our clinical workflow for guidance during the treatment planning process where treatment planners can quickly evaluate the conformity of their treatment plan and compare it to the set guiding value of PCI.