ESTRO 38 Abstract book

S107 ESTRO 38

1 Istanbul University Oncology Institute, Radiation Oncology, Istanbul, Turkey ; 2 Maryland Proton Treatment Center, Radiation Oncology, Baltimore, USA Purpose or Objective To determine the impact of anatomical changes in target volume, sinus filling and weight on dose distribution in patients undergoing pencil beam scanning proton therapy (PBS-PT) for head and neck cancers (HNC) involving the skull base. This information was used in order to assess the need for adaptive replanning. Material and Methods We analyzed 94 HNC patients treated with PBS-PT between 2016 and 2018 (Fig 1). The structures defined on initial planning CT were used as a reference to merge images on QA-CT scans to evaluate changes in anatomy and dose distribution during the course of treatment. For image registration we used a three-dimensional rigid registration algorithm. Patients’ weekly QA-CT scans and/or daily cone beam CT (CBCT) was overlaid with the initial planning CT to evaluate target coverage and organs at risk (OAR) dose to assess the need for replanning as a result of anatomic changes. To evaluate target coverage, we looked at the dose difference for 95% of the volume (V95) for CTV and PTV, and for critical dose limiting OAR we looked at the absolute increase in maximum dose between initial planning CT and QA-CT scans.

Conclusion Despite the robustness of the initial treatment plan, replanning was necessary in 20 % of patients due to anatomical changes during the treatment. While factors influencing plan robustness is important in HNC treatment, the importance of vigilance to anatomical changes and continuous monitoring during treatment is necessary for a safe and effective outcome. PV-0204 Mortality after radiotherapy or surgery in early stage NSCLC: a population based study C. Ostheimer 1 , F. Palm 1 , E. Christoph 1 , M. Katharina 1 , M. Rafael 2 , D. Vordermark 1 , D. Medenwald 1 1 Martin Luther University Halle-Wittenberg, 1\tDepartment of Radiation Oncology, Halle/Saale, Germany ; 2 Martin-Luther University Halle-Wittenberg, Institute of Medical Epidemiology- Biostatistics and Informatics, Halle Saale, Germany Purpose or Objective Stereotactic body radiotherapy (SBRT) can achieve high tumour control with limited toxicity for inoperable early stage non-small-cell lung cancer (NSCLC) patients. The objective of this study is to evaluate the impact of the introduction of SBRT on survival of stage I lung cancer patients in the general population. Material and Methods The German Epidemiologic Cancer Registries from the Robert Koch Institute were assessed in three time periods according to availability of SBRT: (1) 2000-2003 (pre- SBRT), (2) 2004-2007 (interim) and (3) 2007-2014 (broad availability of SBRT). To assess the association of cancer related parameters with mortality, hazard ratios (HR) from Cox proportional hazards models were computed. To evaluate the change of treatment related mortality, we performed interaction analyses and the relative excess risk due to interaction (RERI, additive scale) was computed. For the tumor-specific survival, subdistributional hazard ratios from Fine-Gray models were estimated. Results A total of 16.292 patients with UICC stage I NSCLC diagnosed between 2000 and 2014 were analysed. Radiotherapy utilization increased from 5% in pre-SBRT era to 8.8% after 2007. Tumor-specific mortality was more favourable in all treatment groups in the years after 2007 compared to 2000-2003 (strongest for the radiotherapy group: HR=0.63, 95% CI: 0.5- 0.79). Comparing surgery and radiotherapy the interaction analysis revealed a stronger improvement for radiotherapy (multiplicative scale for 2000-2003 vs. >2007: 0.78, 95% CI: 0.62-0.98). On an additive scale, treatment*period interaction revealed a RERI for 2000-2003 vs. >2007 of -1.18 (95% CI: -1.84, - 0.52). Conclusion Using population-based data, we observed a survival advantage in stage I lung cancer. With an increasing utilization of radiotherapy, a stronger improvement occurred in patients treated with radiotherapy when compared to surgery. PV-0205 Quantification of Changes in Lung Cancer during Radiotherapy: a comparison between CT and MRI E. Huang 1 , S. Kumar 1 , D. Moses 2 , L. Holloway 3 , S. Vinod 1

Results Weekly QA-CT scans ± daily CBCT were performed for 70 patients. 24 patients did not have any CBCT since it became available only in late 2016. Daily CBCT alone was used to evaluate changes for 18 patients who had no gross disease or where disease site was not extending into sinus cavity and when the beams were not passing through sinus air cavity. Replanning was deemed necessary in 14 patients (20%) due to weight loss (40%), sinus filling (33%) and changes in tumor or in the post operative bed (27%). Replanning was necessary for the following tumor sites: Sinonasal undifferentiated carcinoma, skull base chordoma extending to cervical region, nasopharynx and maxillary sinus. Replanning was performed once in 13 patients and twice in one patient during the treatment course. Median differences for prescription dose coverage for 95 % of the target volume between the initial planning CT and QA-CT scans were found to be -1 (range -26,+1) for CTV, and zero (range -9, +2.5) for PTV. Although we found median of maximum dose differences for all OARs to be <5%, we found our plans were robust to tolerate these changes. Only in two patients should we have continued to treat without replanning, the cumulative doses would have exceed the tolerance dose (Fig 2).