ESTRO 2021 Abstract Book

S439

ESTRO 2021

. As shown in Figure 2, the average MR thermometry accuracy using all patient data was 2.0°C±1.5°C versus 1.0°C±0.8°C for selected data. The selection based on Jaccard coefficient improved MR thermometry accuracy by 50%.

Conclusion This study showed that imaging-based inclusion can be used as a tool for prospective patient selection and as a benchmark in MR thermometry assessments. We presented that air-motion-imaging based selection prior to treatment predicts for satisfactory MR thermometry accuracy during MR guided hyperthermia. PH-0553 Prognostic factors in hyperthermia-based thermoradiochemotherapy for locally advanced rectal cancer Y. Lee 1 , S. Kim 2 , S.H. You 2 1 Wonju Severance Christian Hospital, Radiation Oncology, Wonju, Korea Republic of; 2 Wonju Severance Christian Hospital, Radiation oncology, Wonju, Korea Republic of Purpose or Objective A prospective clinical trial was done and further analysis was performed to evaluate potential prognostic factors of modulated electro-hyperthermia (mEHT) combined concurrently in the neoadjuvant treatment for locally advanced rectal cancer. Materials and Methods Between March 2014 and March 2017, sixty patients with cT3-4 or cN positive rectal cancer were enrolled in single non-inferior trial for mEHT feasibility. The main process of the protocol was as follows; whole pelvis radiotherapy of 40 Gy with 2 Gy daily fraction, 13.56 MHz mEHT boost with twice-a-week schedule during radiotherapy period, and surgical resection 6 – 8 weeks after radiotherapy. Median age at the time of registration was 59 (range, 33 – 83). Median total energy of mEHT was 3902 J (range, 2704 – 4429 J). Tumor response-related indices and survival rates were analyzed in terms of main clinical factors through median 52 months (range, 6 – 77 months) of follow-up period. Results Pathologic complete response was identified in 9 patients (15.0%). Total or near total tumor regression was observed in 20 patients (33.3%). T- and N- downstaging was identified in 40 patients (66.6%) and 53 patients (88.3%), respectively. Positive resection margin was reported in 3 cases (5.0%). Small lymph node volume (<5 mL) resulted in good response rate (p = 0.002). Three-year overall survival (OS), disease-free survival (DFS), locoregional recurrence-free survival (LRRFS), and distant metastasis-free survival (DMFS) were 100.0% and 83.0%, 97.9% and 85.2%, respectively. Positive resection margin tended to act as a negative factor in most survival rates (OS (p = 0.032), DFS (p = 0.009), LRRFS (p = 0.017), and DMFS (p = 0.004)). ypN status was associated with DFS (p = 0.019) and LRRFS (p = 0.007). Tumor location from anal verge (p = 0.252), icT (p = 0.658), icN (p = 0.311), ypN stage (p = 0.521), serum CEA level (>5 ng/mL) (p = 0.434), and total mEHT energy (p = 0.171) had no impact on survival. For multivariable analysis, ypN+ (p = 0.011) and positive resection margin (p = 0.003) led to relatively poor outcome in DFS. Conclusion In relatively low dose irradiation (40 Gy) plus mEHT boost setting, comparable clinical outcome was derived in