ESTRO 2021 Abstract Book

S379

ESTRO 2021

A retrospective review on treatment records of patients with CRC OM who underwent SABR at our institution between February 2015 and December 2018 was carried out. SABR was performed with daily image guided radiotherapy (IGRT) using a dedicated robotic stereotactic RT machine. OS and PFS was calculated using Kaplan-Meier statistics and the post RT toxicity data was scored following the CTCAE v4.0 protocol. A subgroup analysis of prognostic factors on OS and PFS was performed based on patients’ age, site of primary cancer, type of treatments to primary cancer, SABR site and number of OM, biological equivalent dose (BED10), cumulative gross tumour volume (GTV) and planning target volume (PTV). A Mantel-Cox log-rank analysis was used to assess any statistically significant differences in OS and PFS between these groups. Results 75 CRC patients with 86 OM were involved in this retrospective review. The median follow-up was 23 months (range 3-49 months). Estimated overall median OS and PFS were 36 months (4-49 months) and 14 months (range 1-45 months) respectively. Majority of the cases were lymph node OM (61.3%), followed by liver (26.7%), lung (6.7%) and bone (5.3%). The most common treatment to the primary cancer was surgery followed by chemotherapy (53.3%). The only statistically significant prognostic factors on OS (p<0.05) were the cumulative GTV (≤17.6 cc vs >17.6cc) and cumulative PTV (≤43.4cc vs >43.4cc) volumes; the treatment management of the primary tumour prior to SABR was suggested to influence PFS (p<0.05).

The vast majority of patient tolerated the treatment well with the most common acute side effects of grade 1 fatigue reported (31.0%), followed by genitourinary (8.0%), back pain (5.3%) and gastrointestinal (4%). Only 2 patients reported grade 2 late toxicities and no grade 3 or higher toxicities were reported at any time points. Conclusion This study showed modest OS, PFS and post-treatment toxicities comparable to the current literature on the treatment of CRC OM with SABR. It suggested that the cumulative tumour volume may be a better prognostic factor on OS than the total number of OM.

Poster highlights: Poster Highlights 19: Upper GI

PH-0495 A multivariable prediction model for severe toxicity after chemoradiation for esophageal cancer C. Muijs 1 , E. Oldehinkel 2 , A. Pattipeiluhu 2 , A.G. Niezink 2 , V.E. Mul 2 , M. Dieters 2 , B. Etten van 3 , A. Schaaf van de 2 , J.A. Langendijk 2 1 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands; 2 University Medical Center Groningen / University of Groningen, Radiation Oncology, Groningen, The Netherlands; 3 University Medical Center Groningen / University of Groningen, Surgical Oncology, Groningen, The Netherlands Purpose or Objective In esophageal cancer, a broad spectrum of complications may develop after neo-adjuvant chemoradiation (nCRT) followed by a surgical resection. Hobbs et al composed a toxicity sum score, the total toxicity burden (TTB) 1 , which considers also low incidence complications as well as the severity of complications. The aim of the current study was to develop an normal tissue complication probability (NTCP) model for TTB in order to predict the risk of severe toxicity in esophageal cancer patients treated with nCRT. Materials and Methods We selected all EC patients that were treated with neo-adjuvant chemoradiotherapy (nCRT) between October 2014 and September 2018 from our prospective data registry (n=224). Complications were translated into a TTB score. The primary endpoint was defined as a TTB ≥ 60, which corresponds to at least one CTCAE grade ≥ III or 2 grade ≥ II complications. Candidate predictors checked for transformations in univariable analysis and to have Spearman’s rank correlation < 0.8. The nominal NTCP model was developed using multivariable logistic regression with forward variable selection. In addition, we analyzed the impact of a TTB ≥60 on hospitalization, Intensive Care Unit (ICU) stay and overall survival (OS), using a Mann Whitney U and Log Rank test. Results At the time of analysis, follow up (FU) of all patients was at least 12 months. The median FU time was 26 months (95% CI 23.4-28.6). Thirty-one percent of the patients had a TTB ≥ 60. Most frequently reported complications were atrial fibrillation (AF) and pneumonitis. Patients with a TTB ≥ 60 stayed at the ICU for a median duration of 9.1 days, compared to 2.6 days in patients with a TTB <60 (p<0.01). Time of