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Pencil beam scanned proton therapy (PBS-PT) treatment quality might be compromised by interplay and motion effects. Via fraction-wise reconstruction of 4D dose distributions and dose accumulation, we assess the clinical relevance of motion related target dose degradation in thoracic cancer patients. Methods and materials: For the ten thoracic patients (Hodgkin lymphoma and non-small cell lung cancer) treated at our proton therapy facility, daily breathing pattern records, treatment delivery log-files and weekly repeated 4DCTs were collected. Patients exhibited point-max target motion of up to 20 mm. They received robustly optimized treatment plans, delivered with five-times rescanning in fractionated regimen. Treatment delivery records were used to reconstruct 4D dose distributions and the accumulated treatment course dose per patient. Fraction-wise target dose degradations were analyzed and the accumulated treatment course dose, representing an estimation of the delivered dose, was compared with the prescribed dose. Results: No clinically relevant loss of target dose homogeneity was found in the fraction-wise reconstructed 4D dose distributions. Overall, in 97% of all reconstructed fraction doses, D98 remained within 5% from the prescription dose. The V95 of accumulated treatment course doses was higher than 99.7% for all ten patients. Conclusions: 4D dose reconstruction and accumulation enables the clinical estimation of actual exhibited interplay and motion effects. In the patients considered here, the loss of homogeneity caused by interplay and organ motion did not show systematic pattern and smeared out throughout the course of fractionated PBS-PT treatment. Dose degradation due to anatomical changes showed to be more severe and triggered treatment adaptations for five patients.

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Local treatment for pulmonary oligometastases (one to five lesions) using metastasectomy or stereotactic ablative radiotherapy (SABR) was investigated in a cohort that received multidisciplinary tumor board-based treatment decisions. The first choice of treatment was surgery; SABR was recommended in cases of adverse clinical factors. Propensity score-adjusted and unadjusted overall survival was the primary end point; local control and time to failure of a local-only treatment strategy were also analyzed. With a minimum follow-up time of 5.8 years, the 5-year overall survival rate was 41% for surgery (n = 68) and 45% for SABR (n = 42). Again not different for the two modalities, 40% of patients were free from failure of a local-only treatment strategy, and 20% were free from any progression at 5 years. The 5-year local control rate was 83% for SABR and 81% for surgery. Despite treatment selection clearly disadvantaging SABR against surgery, even unadjusted outcome was not better when pulmonary oligometastases were surgically removed rather than irradiated.