Abstract Book

S755

ESTRO 37

Results Median follow up: 31.13 months (mo) Median OS: 45.7 mo. 3 year OS, LC and DFS rates were 71.9%, 78.3% and 45.2%, respectively. In univariate analysis MTV 2 (HR = 1.030) MTV 2.5 (HR = 1.039) MTV 30 (HR = 1.124) MTV 40 (HR = 1.131) MTV 50 (HR = 1.150) and tumor diameter (TD) (HR = 1.098) were associated with OS. MTV 2 (HR = 1,024) MTV 30 (HR = 1.08) MTV 40 (HR = 1.087) MTV 50 (HR = 1.101) and TD (HR = 1.066) were associated with DFS. No level of TLG was found to be associated with OS nor DFS. LC was no statistically related with any metabolic or clinical factor. We further analyzed the site of tumor recurrence i.e. mediastinal/visceral vs. local: MTV 30 (HR = 1.091) MTV 40 (HR = 1.099) MTV 50 (HR = 1.116) and TD (1.099) were associated with distant failure (DF). In multivariate analysis MTV 2 (HR = 1.385, p = 0.104) MTV 30 (HR = 1.167, p = 0.63) and TD (HR = 1.117, p = 0.59) showed a trend for predicting OS, DFS and DF respectively. Conclusion MTV, but no TLG neither SUVmax, was associated with outcome in univariate analysis. Due to small number of patients, multivariate analysis demonstrated no significant association of MTV neither TLG with any end point. TD was the only non-metabolic tumor factor associated with prognosis. The results of this study suggest that MTV and TD could identify patients with high risk of distant tumor recurrence. EP-1382 use of cone beam CT (CBCT) to evaluate the intrafraction patient movements during SBRT M. Soraya 1 , A. Giraldo 1 , A. Seoane 2 , M. Ramos 1 , D. Santamaria 1 , D. Moreno 1 , J. Giralt 1 1 Vall D'Hebron, radiation oncologist, Barcelona, Spain 2 Vall D'Hebron, medical physics, Barcelona, Spain Purpose or Objective Our standard IGRT protocol for SBRT of pulmonary lesions consists of an initial CBCT (BT-CBCT) to determine the couch shift that adjusts the tumour position to the planning CT. With the objective of evaluating the intrafraction movement, we acquired a CBCT post treatment (PT-CBCT) after every fraction Material and Methods 41 patients (pts) with lung lesions (primary tumour or metastasis) were treated either with IMRT or VMAT techniques. All pts were immobilised with a vacuum customised cushion and immobilised with arms raised above the head. We adapted the motion compensation strategy with abdominal compression in all pts. 4D-PET images were used for internal target volume (ITV) generation, and a 5mm isotropic margin was added to create the planning target volume (PTV). The dose and number of fractions were selected according to our institution protocol, ranging from 3-8 fractions and from 50-60 Gy. Prior to and after each fraction, a CBCT was acquired. Firstly, the pt was positioned and aligned with the lasers on the CT marks. CBCT was obtained and registered to the planning CT in order to obtain and apply the setup corrections (rotation was not allowed) helped by automated soft-tissue registration; the BT-CBCT was used to correct the tumour position, and PT-CBCT to evaluate the intrafraction movements. The tumour position variability in Left/Right(LR), Cranial/Caudal(CC) and Anterior/Superior(AP) was calculated in terms of Group Mean(GM), systematic(∑) and random(σ) variations. After every fraction, with the displacements obtained from the PT-CBCT, and in accordance with our PTV margin, the pts were classified into 2 groups: Low risk group: every axis was ≤4mm: no action was required for the next fraction. High risk group: at least 1 axis was >4mm: for the next fraction, a repositioning CBCT in the middle of the fields/arcs of treatment was acquired.

Results 164 treatment fractions were administered between February 2015 and August 2017. The mean time from localisation of the tumour to the end-fraction CBCT was 23 minutes (range 9-51 ), other characteristics are available on Table 1. The mean systematic error for all pts was in LR -0.2mm (range-3 to 3), in CC -0.6mm (range -4 to 5) and in AP - 1mm (range-6 to 4). The standard deviation of the σ and ∑ setup errors are displayed in Table 2. Of all 41 patients, just 3 (7%) pts were categorised as high risk intrafraction movement group and required improvement actions during most days of treatment.

Conclusion We can conclude that intrafraction movements are not negligible, and probably depend on many factors that are necessary to control in a second analysis. The differences between the intrafraction displacements, observed through treatment using CBCTs, seem to be clinically acceptable and are contemplated in our PTV margin. The implementation of a CBCT after fraction increases the total treatment time, but we do consider it necessary for a qualified treatment. EP-1383 The use of Real World Evidence to audit NTCP-models for acute esophagus toxicity in NSCLC patients M.H. Kwint 1 , I. Walraven 1 , M.S. Marshall 1 , M. Verheij 1 ,

J.J. Sonke 1 , J.S.A. Belderbos 1 , T.M. Janssen 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands

Purpose or Objective Clinical audits of NTCP-models are ideally supported by real world evidence (RWE); real world data derived from observational studies and/or clinical registries. RWE is often criticized due to its lack of validity caused by missing data compared to results of clinical trials. By