Abstract Book

S163

ESTRO 37

OC-0310 Contrast enhanced oesophageal avoidance for stereotactic body radiotherapy: Barium vs Gastrografin K. Woodford 1 , V. Panettieri 1 , J. Ruben 1 , S. Davis 1 , E. Sim 1 , T. Tran Le 1 , S. Senthi 1 1 The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia Purpose or Objective Stereotactic body radiotherapy may facilitate treatment in a greater proportion of locally-advanced non-small cell lung cancers (NSCLC) patients, just as it has for early stage disease. The oesophagus is the key dose-limiting organ and visualisation during image-guided radiotherapy (IGRT) would better ensure toxicity is avoided. As the oesophagus is poorly seen on cone-beam computed tomography (CBCT), we assessed the extent to which this is improved using two oral contrast agents administered prior to treatment. Material and Methods Patients receiving contrast were enrolled into a prospective trial and were receiving radiotherapy for Stage I-III NSCLC. The first 6 patients were randomly assigned to 50mL Gastrografin or 50mL barium sulphate to determine which agent would be used going forward. Contrast was administered just prior to planning CT(PCT) and each daily/weekly pre-treatment CBCT. Three previously treated patients with matched baseline characteristics, who did not receive contrast were included as a control group. The ability to visualise the oesophagus was determined by assessing how similarly 6 observers (4 radiation oncologists and 2 dosimetrists) were able to contour the organ. Using all contours, a Simultaneous Truth and Performance Level Estimation (STAPLE) contour was created (CERR software) along with a generated Kappa statistic for each dataset. Individual contours were then compared to the STAPLE contour. Descriptive statistics were used and a Dice Coefficient(DC) & Hausdorff distance (HD) (95% CI) were calculated using 3DSlicer and compared using an independent t-test or Mann-Whitney U test. Each observer also scored the influence of any contrast- induced artefact on their ability to visualise the tumour and other critical organs. Results A total of 36 datasets were analysed and results summarised in table 1. Oesophagus contours were more similar between observers in the contrast patients compared to non-contrast patients, as evidenced by significantly higher Kappa, DC and HDs. Significantly higher concordance was seen with barium compared to Gastrografin. Compared to Gastrografin, barium enabled observers to identify the oesophagus with a smaller variation (p=0.002) with there being only 1.6mm between observers 95% of the time. Patient compliance as determined by the % of intended CBCTs given with contrast, was higher with the barium than with the Gastrografin (92% vs 52%), largely due to the occurrence of diarrhoea in 2 of the 3 Gastrografin patients. Some contrast-induced artefact was seen on the CBCTs with both agents, but the observers rated this as minimally affecting their ability to visualise the tumour and other organs.

Conclusion Barium produced lower delineation uncertainties, compared to Gastrografin and no contrast and can therefore be considered superior at highlighting the oesophagus on CBCT images. It is feasible to use oral contrast as a tool in IGRT to help guide clinicians and therapists with online matching and monitoring of the oesophageal position. OC-0311 Optimal bladder filling using CBCT based “virtual bladder” volume L. Igidbashian 1 , F. Girard 1 , M. Hinse 1 , E. Plouffe 1 , J. Tremblay 1 1 Hôpital de la Cité de la Santé de Laval - CSSSL, Radiation Oncology, Laval, Canada Purpose or Objective To develop a dosimetry based evaluation volume that could ensure an optimal bladder filling during Most published data evaluating bladder filling use ultrasound based pre-treatment evaluation with a pre- defined volume cut-off. Several variables including initial bladder filling and dosimetric constraints are often not taken into account . At our center, all prostate cancer patients are treated by arc based IMRT (VMAT) radiotherapy. The fractionation regimen used is either 78Gy in 39 fractions, 60Gy in 20 prostate radiotherapy. Material and Methods