Ten Institutes (INST1-INST10) of the MIKAPOCo consortium set their KB plan prediction models of whole breast irradiation (WBI) delivered with tangential fields (TF), by using RapidPlan (Varian Medical System, Inc.) and follow-ing the same criteria of contouring and plan model building and validation. DVH prediction bands of organs at risk (heart, ipsilateral lung, contralateral lung and contralateral breast) were exported on 20 new patients from the same Institutes (two patients each). SD of mean predicted DVHs among institutes was assessed as inter-institute variability. The estimated Principal Component (PC1) was con-sidered for transferability models evaluation. Transferability cross-validation was further investigated in detail on a larger population for the model showing the poor-est transferability (ModelINST6) against one of the models with high transferability (ModelINST3). Results show a limited inter-institute variability of plan prediction models (1.8% for DVH ipsilateral lung) and a satisfactory inter-institute transferabil-ity, excepting one institute, confirmed by the extended analyses on a larger cohort of test patients of INST6 (vs. INST3). These achievements pave the way for gen-erating benchmarks for plan prediction in WBI with potentially relevant large-scale applications.

PURPOSE:To quantify inter-institute variability of Knowledge-Based (KB) models for right breast cancer patients treated with tangential fields whole breast irradiation (WBI).MATERIALS AND METHODS:Ten institutions set KB models by using RapidPlan (Varian Inc.), following previously shared methodologies. Models were tested on 20 new patients from the same institutes, exporting DVH predictions of heart, ipsilateral lung, contralateral lung, and contralateral breast. Inter-institute variability was quantified by the inter-institute SDint of predicted DVHs/Dmean. Association between lung sparing vs PTV coverage strategy was also investigated. The transferability of models was evaluated by the overlap of each model's geometric Principal Component (PC1) when applied to the test patients of the other 9 institutes.RESULTS:The overall inter-institute variability of DVH/Dmean ipsilateral lung dose prediction, was less than 2% (20%-80% dose range) and 0.55 Gy respectively (1SD) for a 40 Gy in 15 fraction schedule; it was < 0.2 Gy for other OARs. Institute 6 showed the lowest mean dose prediction value and no overlap between PTV and ipsilateral lung. Once excluded, the predicted ipsilateral lung Dmean was correlated with median PTV D99% (R2 = 0.78). PC1 values were always within the range of applicability (90th percentile) for 7 models: for 2 models they were outside in 1/18 cases. For the model of institute 6, it failed in 7/18 cases. The impact of inter-institute variability of dose calculation was tested and found to be almost negligible.CONCLUSIONS:Results show limited inter-institute variability of plan prediction models translating in high inter-institute interchangeability, except for one of ten institutes. These results encourage future investigations in generating benchmarks for plan prediction incorporating inter-institute variability.

To investigate critical aspects and effectiveness of in vivo dosimetry (IVD) tests obtained by an electronic portal imaging device (EPID) in a multicenter and multisystem context. Eight centers with three commercial systems—SoftDiso (SD, Best Medical Italy, Chianciano, Italy), Dosimetry Check (DC, Math Resolution, LCC), and PerFRACTION (PF, Sun Nuclear Corporation, SNC, Melbourne, FL)—collected IVD results for a total of 2002 patients and 32,276 tests. Data are summarized for IVD software, radiotherapy technique, and anatomical site. Every center reported the number of patients and tests analyzed, and the percentage of tests outside of the tolerance level (OTL%). OTL% was categorized as being due to incorrect patient setup, incorrect use of immobilization devices, incorrect dose computation, anatomical variations, and unknown causes. The three systems use different approaches and customized alert indices, based on local protocols. For Volumetric Modulated Arc Therapy (VMAT) treatments OTL% mean values were up to 8.9% for SD, 18.0% for DC, and 16.0% for PF. Errors due to “anatomical variations” for head and neck were up to 9.0% for SD and DC and 8.0% for PF systems, while for abdomen and pelvis/prostate treatments were up to 9%, 17.0%, and 9.0% for SD, DC, and PF, respectively. The comparison among techniques gave 3% for Stereotactic Body Radiation Therapy, 7.0% (range 4.7–8.9%) for VMAT, 10.4% (range 7.0–12.2%) for Intensity Modulated Radiation Therapy, and 13.2% (range 8.8–21.0%) for 3D Conformal Radiation Therapy. The results obtained with different IVD software and among centers were consistent and showed an acceptable homogeneity. EPID IVD was effective in intercepting important errors.