ESTRO 2021 Abstract Book

S1680

ESTRO 2021

Conclusion This study demonstrates that based on liver and external contours, AC is more reproducible in superior liver lesions. However, variations in belt position and inflation level may lead to anatomical variations for inferior lesions. These results will be validated in a larger cohort as part of ongoing work. PO-1974 The use of in-vivo dosimetry to detect head and neck cancer patients needing adaptive radiotherapy Y.A.C. FIAGAN 1 , D. Nevens 2 , E. Bossuyt 3 , M. Machiels 2 , I. Chiairi 2 , I. Joye 2 , M. Paul 2 , T. Gevaert 4 , D. Verellen 2 1 Iridium Netwerk, Faculty of Medicine and Pharmacy, Vrije Universiteit Brussel, Brussels, Radiation Oncology, Antwerp, Belgium; 2 Iridium Netwerk, Faculty of Medicine and Health Sciences, Universiteit Antwerpen, Radiation Oncology, Antwerp, Belgium; 3 Iridium Netwerk, Radiation Oncology, Antwerp, Belgium; 4 Iridium Netwerk, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, Radiation Oncology, Brussels, Belgium Purpose or Objective Patients treated with radiotherapy (RT) for head and neck cancer (HNC) often need adaptive radiotherapy (ART) because of anatomical changes. In this study, we investigated the sensitivity of an electronic portal imaging device (EPID)-based in-vivo dosimetry (IVD) system to detect HNC patients that require ART and to identify limitations of using an IVD system as a trigger. We correlated clinical parameters of replanning with deviations detected by the IVD system, to investigate if it can identify patients that might benefit from ART. Materials and Methods A retrospective study was conducted, including 3 groups of HNC patients treated with RT: laryngeal cancer without elective lymph nodes (ELN) (group A), postoperative RT (group B) and primary RT including ELN (group C). EPID-based integrated transit dose images were acquired on the first 3 days of treatment and weekly thereafter or more if failed fractions (FF) occurred. Results were standardly analyzed using a global gamma analysis with a threshold of 20%, a dose difference of 3%, a distance to agreement of 3 mm and a passing level of 95%. The clinical reasons that led to replanning were identified. We compared dose-volume histograms of the original plan on the original CT scan, with those projected on the rescan CT. The doses D98% and D2% to the clinical target volume (CTV), mean dose to both parotid glands (PG), near-maximum dose to planning target volume (PTV) and to the spinal cord, the change in body contour, volume reduction in PTV and PG, and weight loss were assessed. Results A cohort of 187 HNC patients was analyzed: 22 in group A, 40 in group B and 125 in group C. Forty-two out of 187 patients had at least 1 rescan and replanning: 1/22 in group A, 7/40 in group B and 34/125 in group C. In group A only 1 patient was replanned because he swallowed during CT simulation. It was detected by IVD. The main clinical reasons for replanning in group B and C were weight loss and/or change in body contour. Other reasons for replanning were: new lesions, tracheotomy/feeding tube and swelling. In group B , FF due to anatomy changes occurred in 5/7 cases before replanning. In 2/7 cases no FF occurred. For one of these cases there was a significant increase in D98% of the CTV. This one is considered as a false negative (FN) result. In group C , FF occurred in 25/34 cases before replanning. In 9/34 cases no FF occurred before replanning: in 6 of these cases, there was a significant change in dose to organs at risk and target volumes, considering these as FN results. A summary of these results is shown in Table 1. Most patients needing ART have been detected by IVD. Some reasons for replanning cannot be detected (e.g. mask being too tight or the need to redraw PTV or organs at risk). However, in some cases FN results occurred. Reasons for this will have to be investigated further, to see if other analysis parameters might help to detect them.

Conclusion EPID-based IVD can help us to detect anatomical changes in HNC patients who benefit from ART.