ESTRO 38 Abstract book

S424 ESTRO 38

5.8 – 20 mm). This extension was corrected for each individual determined shrinkage rate (mean: 93%). The presence of microscopic spread of cancer beyond EDTB was only significantly associated with initial tumor length ( p = 0.028). None of the patients had macroscopic tumor spread beyond the EDTB.

M. Machiels 1 , M.L. Van Montfoort 2 , N.B. Thuijs 2 , M.I. Van Berge Henegouwen 3 , J.E. Van Hooft 4 , T. Alderliesten 1 , S.L. Meijer 2 , M.C.C.M. Hulshof 1 1 Amsterdam UMC- locatie AMC, Radiation Oncology, Amsterdam, The Netherlands ; 2 Amsterdam UMC- locatie AMC, Pathology, Amsterdam, The Netherlands ; 3 Amsterdam UMC- locatie AMC, Upper GE Surgery, Amsterdam, The Netherlands ; 4 Amsterdam UMC- locatie AMC, Gastroenterology, Amsterdam, The Netherlands Purpose or Objective The microscopic tumor extension beyond the macroscopic tumor borders of esophageal cancer is scarcely investigated. Nonetheless this information is crucial for determining the clinical target volume (CTV) in radiation therapy [1]. Modern innovations in gross tumor volume (GTV) determination allow delineation with the aid of (echo)endoscopically placed fiducial markers on the tumor borders. The question arises whether the current voluminous clinical target volume (CTV) margins of 3-5 cm around the GTV to account for microscopic extension - based on one study solely - are still accurate [1]. In the present study, we analysed microscopic spread beyond the (echo)endoscopically determined tumor border (EDTB) with the use of fiducial markers at the tumor borders. Furthermore, we validated the use of fiducial markers placed on the EDTB as a surrogate for macroscopic tumor borders. Material and Methods Thirty-two consecutive esophageal cancer patients were included in this study. All patients were treated with neo- adjuvant chemoradiotherapy and fiducial markers were (echo)endoscopically implanted before treatment at the cranial and caudal tumor borders. The surgical specimens of all patients were collected from the operation room, fiducial marker positions were detected under CT guidance and demarcated with beads, and subsequently analysed at the pathology department for macroscopic and microscopic spread beyond the demarcations (EDBT) (Figure 1). Per specimen a shrinkage rate was calculated (i.e., distance between fiducial markers ex-vivo divided by distance between fiducial markers in-vivo). A logistic regression analysis was performed to determine predicting factors for microscopic spread beyond EDTB (significance level α=0.05).

Conclusion No macroscopic tumor was found beyond the EDTB. Only 19% and 25% of respectively the cranial and caudal EDTB, were traversed with a mean of 7 mm - 13.5 mm, and a maximum of 20 mm microscopic tumor. Our results indicate that current recommended CTV margins around the GTV to compensate for microscopic tumor along the esophageal wall can be reduced when the GTV is determined with the aid of fiducial markers. [1]X.-S.Gao,et al. Int.J.Radiat.Oncol.Biol.Phys. 2007;67:389-396. PO-0813 A Phase I/II Study of durvalumab and stereotactic radiotherapy in locally advanced pancreatic cancer R. Tuli 1 , N. Nisen 2 , S. Lo 3 , V. Placencio 1 , G. Gresham 3 , A. Hendifar 3 1 Cedars-Sinai Medical Center, Radiation Oncology, los angeles, USA ; 2 Cedars-Sinai Medical Center, Surgery, Los Angeles, USA ; 3 Cedars-Sinai Medical Center, Medicine, los Angeles, USA Purpose or Objective Previous studies targeting the PD-1/PD-L1 pathway in varying tumor types have shown blockade of this pathway can enhance the efficacy of radiotherapy (RT). However, no such data is available in pancreatic cancer (PC) and no significant responses have been seen in this disease with immune checkpoint blockade. Our group and others have observed the effect of both chemotherapy and RT is enhanced in PC by targeting the PD-1/PD-L1 pathway, and the degree of this response is dependent upon the native expression of PD-1/PD-L1. These data led us to hypothesize that the clinical efficacy of RT in PC can be enhanced with concurrent PD-L1 blockade. Herein, we report interim results of an ongoing phase 1/2 clinical trial in locally advanced (LA) and borderline resectable (BR) PC (NCT03245541). Material and Methods Patients with LA or BR PC treated with standard of care gemcitabine and nab-paclitaxel for 3 to 6 cycles were enrolled. Treatment consisted of durvalumab (750 mg Q14

Results A total of 60 EDTB were examined in 32 patients. As listed in Table 1, 16/32 (50%) of patients had a Mandard 3 or higher (i.e., residual tumor group) and were included for analysis of microscopic tumor spread analysis. In the residual tumor group, only 3/16 (19%) patients had microscopic spread beyond the cranial EDTB, and 4/16 (25%) had microscopic spread beyond the caudal EDTB. The mean microscopic spread beyond EDTB was 7 mm cranially (range: 6 – 9 mm ) and 13.5 mm caudally (range: