ESTRO 36 Abstract Book

S454 ESTRO 36 2017 _______________________________________________________________________________________________

Material and Methods 42 patients with inoperable, stage II-III NSCLC were treated with (chemo)radiotherapy as part of the (NCT01024829) PET-boost trial. Patients received escalated doses (≥ 72 Gy) in 24 fractions consisting of either a homogenous boost to the PTV, or an inhomogeneous boost to FDG avid (≥50% SUV max ) areas. Patients whom could not be boosted received 66 Gy in 24 fractions. All patients received an FDG PET/CT scan 3 months post-treatment, which was registered to the planning CT. The lung contours minus the GTV were compared between the SUV on FDG PET and the planning CT dose. The planning CT dose was adjusted to equivalent doses in 2 Gy fractions, assuming an α/β = 3 Gy, and then binned per 5 Gy increments. The SUV was averaged over all patients per dose bin and a dose response sigmoid was fit (SUV vs Gy ) to determine upper and lower asymptotes, as well as the EC50. The linear portion of the sigmoid fit (17.6%-82.4% asymptotes difference) was applied to individual patients for linear fitting, yielding the correlation coefficient and the SUV response to an increase in dose (slope). All values were reported as median[interquartile range (IQR)]. All fits were considered significant with an alpha of 0.05. Results A positive relationship was found between SUV and post- treatment dose. A sample patient with post-treatment grade 1 pneumonitis is shown in figure 1. The sigmoidal fit (figure 2) over all patients was significant (chi-squared = 0.01) with an EC50 at 39 Gy, and lower and upper asymptotes at 0.60 SUV and 1.31 SUV, respectively. The linear portion of the sigmoidal fit (15- 60 Gy) was found to be significantly (p ≤ 0.05), highly linear in individual patients with a median correlation coefficient of 0.93[0.79-0.97]. Four patients did not have significant linear fits (p-values ranging from 0.05 to 0.13). The median SUV response per increase in dose (slope) was heterogeneous with a median of 0.0083[0.005-0.018] SUV/Gy, implying a 55 Gy to 200 Gy increase needed per SUV increase within the IQR.

Conclusion The DVH shape of bowel loops is highly correlated with the risk of acute patient-reported loose stools due to WPIMRT for prostate cancer. Older age was found to be a protective factor. PO-0847 The dose-response curve of post-treatment FDG-uptake in lung tissue of irradiated NSCLC patients M. La Fontaine 1 , W.V. Vogel 1 , G. Persson 2 , G. Westman 2 , B. Reymen 3 , D. De Ruysscher 3 , J.S. Belderbos 1 , J.J. Sonke 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands 2 Copenhagen University Hospital- Rigshospitalet, Department of Radiation Oncology, Copenhagen, Denmark 3 Maastricht University Medical Centre, Department of Radiation Oncology, Maastricht, The Netherlands Purpose or Objective In NSCLC patients undergoing chemoradiotherapy, pneumonitis is a common occurrence. While its exact relationship with radiation dose is unknown, there is evidence that increasing lung dose leads to increased levels of pneumonitis. Using inflammation on post- treatment FDG PET scans as a measure of damage to normal lung tissue, the aim of this study was to investigate the relationship between pneumonitis and planned dose in a radiation dose-escalation trial.

Conclusion Strong linear and sigmoidal relationships were found between post-treatment SUV and planned dose. These results suggest that increasing dose leads to a highly linear