ESTRO 37 Abstract book

S1104

ESTRO 37

The intrafractional motion during a hypofractionated prostate radiotherapy treatment was separated in two components (Eq. 5): the patient displacement and the prostate gland motion from pelvic bony structures. The shifts based of the CBCT post-fiducial registration to the planning CT were used for total intrafractional motion determination (Eq. 6). The intrafractional component because of the patient displacement was estimated from the subtraction of the CBCT pre-bones and CBCT post-bones shifts (Eq. 7).

Material and Methods 43 patients who were assessed within our motion management strategy were reviewed. The patients included several abdominal and thoracic sites (predominantly pancreas, liver, lung, oesophagus, and mediastinum). For each patient, PTVs were generated using both ITV (PTV ITV ) and breath-hold (PTV BH ) techniques. For ITV, a 5mm PTV margin was applied. For breath-hold, the breath-hold uncertainty during treatment was assessed and included on top of 5mm in the CTV-PTV margin. The volumes of the PTVs generated by both motion management techniques were compared. Results The difference in the PTV volumes between the two techniques varied, with a mean volume difference for all patients of 51cc (15% relative change) with a standard deviation of 52cc. For pancreas, the PTV BH was smaller in 7/11 patients with a mean reduction of 60cc (28.4%) and maximum of 143cc (35%). For 4/11 patients, the PTV ITV was smaller by a mean of 23cc (17%) and a maximum of 64cc (13%). For liver, the PTV BH was smaller in 6/7 patients with a mean reduction of 93cc (14.9%) and maximum of 189cc (38.7%). For 1/7 patients, the PTV ITV was smaller by a 89cc (23.5%). For lung, the PTV BH was smaller in 4/7 patients with a mean reduction of 15cc (13.9%) and maximum of 38cc (46%). For 3/7 patients, the PTV ITV was smaller by a mean of 27cc (10.8%) and a maximum of 51cc (18.8%). For oesophagus, the PTV BH was smaller in 2/6 patients with a mean reduction of 120cc (19.7%) and maximum of 124cc (23.9%). For 4/6 patients, the PTV ITV was smaller by a mean of 52cc (14.6%) and a maximum of 85cc (10.4%). For mediastinum, the PTV BH was smaller in 1/5 patients with a reduction of 180cc (15.3. For 4/5 patients, the PTV ITV was smaller by a mean of 12cc (4.5%) and a maximum of 14cc (5.6%). Conclusion The results show that the optimal motion management strategy to minimise the irradiated volume is patient- specific. While liver (PTV BH ) and mediastinum (PTV ITV ) both showed a clear trend, both sites had a case where significant volume reduction was achieved by using the alternative technique. Therefore it's important to have a flexible approach to motion management. EP-2020 Deep inspiration breath hold & locally advanced lung cancer: validation of geometrical uncertainties M. Josipovic 1 , M. Aznar 2 , S. Damkjær 1 , J. Thomsen 1 , J. Rydhög 3 , L. Nygård 1 , L. Specht 1 , M. Pøhl 1 , G. Persson 1 1 The Finsen Center - Rigshospitalet, Dept. of Oncology- Section of Radiotherapy, Copenhagen, Denmark 2 University of Manchester, Division of Cancer Sciences- School of Medical Sciences - Faculty of Biology- Medicine and Health, Manchester, United Kingdom 3 Skånes University Hospital Lund, Department of Radiation Physics, Lund, Sweden

Therefore the component because of a prostate intrafractional motion from bony anatomy was calculated from equation 5. Results The average displacement for total intrafraction motion was 3.2 mm (σ=2 mm), for patient motion was 1.7 mm (σ=1.0 mm) and for prostate gland from bone anatomy was 3.2 mm (σ=1.9 mm). In the Table 1 was shown the two components of intrafraction motion distribution in percentage of sessions.

Table 1. Intrafractional motion distribution (in percentage of sessions) during radiotherapy separated in two components: intrafractional motion of the patient and intrafractional motion of prostate gland taking pelvic bony anatomy as a reference. Conclusion The origin of geometric uncertainties during hypofractionated radiotherapy treatment was the prostate gland motion from pelvic bony anatomy, and not the patient displacement. EP-2019 A respiratory motion management strategy for both abdominal and thoracic VMAT radiotherapy. M. Bray-Parry 1 , J. Gesner 1 , S. Stevens 1 , A. Richmond 1 , J. Konieczek 1 , K. Finnegan 1 , I. Ho 1 1 The London Clinic, Medical Physics, London, United Kingdom Purpose or Objective Thoracic and abdominal treatment sites can be affected by respiratory motion. When targeting with radiotherapy, it is important that this respiratory motion is accounted for. This is typically done by creating an ITV, determined by a 4DCT or extreme-phase breath-hold CT (i.e. end- expiration (EEBH) and inspiration (IBH) CT). However, this approach can often lead to large volumes as the ITV needs to cover the entire extent of respiratory motion. Alternative approaches to minimise the respiratory motion include a breath-hold technique. The breath-hold from CT is replicated throughout treatment, and the uncertainty in breath-hold position is incorporated into a CTV–PTV margin. This study investigates a motion management strategy which aims to provide the optimal motion management technique for each individual patient. Figuratively, this strategy is shown in figure 1.