ESTRO 2022 - Abstract Book

S588

Abstract book

ESTRO 2022

PD-0653 Reproducibility of deep inspiration breath hold during RT for lung cancer patients.

A.B. Rasmussen 1 , D.S. Møller 1 , L. Hoffmann 1 , M.M. Knap 1 , H. Schmidt 1 , M.L. Ehmsen 1 , T. Ravkilde 1 , E. Worm 1 , M.H. Andersen 1

1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark

Purpose or Objective Deep Inspiration Breath Hold (DIBH) during RT for lung cancer (LC) patients is attractive, as it may reduce dose to the lungs and heart compared to free breathing RT. However, geometric instability during several breath holds may decrease the target coverage. In this study, we investigate the reproducibility of DIBH during treatment planning and delivery. Materials and Methods Twenty-four LC patients were treated with DIBH-RT. The RPM system (Varian, CA) with an external marker (EM) placed caudally on the thoracic cage was used as a surrogate for the DIBH level. The patients were trained in performing DIBH before planning CT (pCT) and at the first three treatments. For all scans and treatments, the DIBH level measured by EM was ≤ 2mm. Four DIBH pCT scans were acquired: one for RT planning and three to assess the positional difference between DIBHs (inter-DIBH) and this uncertainty was included in the margin. To assess inter-DIBH uncertainty during treatment, we acquired and compared pre-treatment CBCT scans for daily setup on the tumour (CBCT1) and post-treatment CBCT scans (CBCT2) at the first three fractions and once a week afterwards. Results In Fig.1 left, the inter-DIBH uncertainty between the four repeated pCTs is shown for one lung cancer patients. Fig. 1 right shows the inter-DIBH uncertainty between CBCT1 and CBCT2 for one fraction for the same patients. A much larger difference is seen for CBCT scans for the selected patient. This is the case for most of the LC patients, as seen in the boxplot for the primary tumour (GTV-T) in Fig. 2. For 67% of LC patients, larger variation was seen at CBCT in the CC direction. For all patients, the median [range] inter-DIBH motion in LR, AP and CC was 1mm [1-5], 2mm [1-8], and 3mm[0- 15] at pCT and 3mm [1-11], 3mm [1-18], and 6mm [1-21] at CBCT, respectively. Our experience is that a proper introduction to DIBH is crucial for a high level of patient compliance. This includes showing the patient the impact of how the EM moves, when the breath is hold multiple times, and what happens when e.g. the EM is brought into position based on lifting the lower back instead of the thoracic wall. Also ,the difference between abdominal and thoracic DIBH should be discussed. Based on this, a checklist was developed for the radiographers in order to document how the patient performs DIBH during pCT and the RTT´s uses the checklist while training the patient before start of treatment. Likewise, it seems to be important that patients know that if the DIBH is becoming exhausting they can just breath out and take a break.

Conclusion Inter-DIBH uncertainties vary a lot between patients. For some patients, the size of the uncertainties will undermine the advantages of DIBH. DIBH-pCTs acquired before treatment do not predict the inter-DIBH uncertainty observed during treatment. Proper training of the patient is important to ensure reproducible DIBH.