ESTRO 2022 - Abstract Book

S1506

Abstract book

ESTRO 2022

1 Australian Radiation Protection and Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne, Australia; 2 Peter MacCallum Cancer Centre, Radiation Oncology - Physical Sciences, Melbourne, Australia; 3 Olivia Newton John Cancer and Wellness Centre, Radiation Oncology ONJ Centre, Melbourne, Australia; 4 Australian Radiation Protection and Nuclear Safety Agency, Medical Imaging, Melbourne, Australia; 5 RMIT University, School of Science, Melbourne, Australia Purpose or Objective The Australian Clinical Dosimetry Service (ACDS) plans to implement an end-to-end dosimetric audit encompassing respiratory motion management (MM). The audit design will be informed by a survey of motion management techniques used at Australian and New Zealand radiation therapy (RT) facilities. Materials and Methods The survey was distributed via REDCap (v10.8) to all ACDS-subscribed facilities. The survey aimed to capture the extent to which different MM techniques were utilised, as well as how each of them were implemented practically in the clinic. Five techniques were considered (breath-hold gating (BHG), internal target volume (ITV), free-breathing gating (FBG), mid- ventilation (MidV), and tumour tracking (TT)), across three main stereotactic ablative body radiation (SABR) treatment locations (lung/thorax, liver/upper abdomen, and kidney/lower abdomen). For each combination of MM technique and site, participants described specific practices including use of motion limitation (compression or ventilation), types of imaging available for motion assessment, and dataset used for dosimetric calculation. Responses were extracted from REDCap, anonymised and analysed with Python (v3.8). Results The survey was completed by 78% of facilities in the region, with 98% of respondents indicating that they use at least one form of MM. The ITV approach was common to all MM-users, used for thoracic treatments by 89% of respondents, upper abdominal treatments by 38%, and lower abdominal treatments by 38%. BHG was the next most prevalent (41% MM users), with applications in upper abdominal and thoracic treatment sites (28% vs 25% MM users respectively), but minimal use in the lower abdomen (9%). FBG and TT were utilised sparingly (17%, 7% of MM users respectively), and MidV was not selected at all. Motion limitation used in upper abdominal treatments in more than 50% of facilities, but was used sparingly outside this. Upper abdominal SABR sites also saw a more prevalent use of external imaging modalities including contrast CT and MRI for pre-treatment motion assessment. Choice of planning dataset for dose calculation was largely determined by choice of MM technique – breath-hold CT for BHG, 4DCT average intensity projection for ITV, rather than by treatment site. Conclusion The survey showed that some MM techniques (ITV, BHG) are widely used for the thorax and upper abdomen, but are implemented in different ways. A MM audit thus must include ITV and BHG applied to these anatomical treatment sites but ideally be adaptable to different combinations of treatment site and MM techniques. Further, the audit will not be prescriptive in details critical to accurate treatments incorporating MM such as target delineation and choice of dose calculation dataset. 1 Léon Bérard Cancer Center & CREATIS laboratoires , Université de Lyon, Lyon, France; 2 Léon Bérard Cancer Center, Université de Lyon, Lyon, France Purpose or Objective To investigate two different methods for predicting minimum non-isotropic and asymmetric (NI-AS) treatment margins required for taking into account prostate intrafraction motion occurring during moderate hypofractionated treatments. Materials and Methods Prostate intrafraction 3D translations were recorded using Clarity transperineal ultrasound probe (TP-US) (Elekta AB, Stockholm) in 46 prostate cancer patients (876 sessions) treated by moderate hypofractionated radiotherapy at our institution. All patients underwent volumetric modulated arc therapy. The prescribed dose was 60 Gy in 20 fractions to the Clinical-Target-Volume (CTV) prostate. The treatment goal was that 100% of the prescribed dose must cover 99% of the CTV-prostate. For 18 patients (346 sessions) randomly selected from the cohort, treatment plans were recomputed increasing CTV-to-PTV margins from 0 to 6mm with an auto-planning optimization algorithm. Then, the voxel shifting method (VSM) was used to move the CTV-prostate structure every five seconds of treatment, according to the movements retrieved by the TP-US, and to calculate time-dependent margins. The obtained results were compared to those obtained using van Herk’s margin formula. Results Mean intra-fraction prostate displacements observed were -0.02±0.52mm, 0.27±0.78mm and -0.43±1.06mm in left-right (LR), supero-infero (SI) and antero-posterior (AP) directions, respectively. On average, the largest displacements were observed in inferior and posterior directions. The CTV dosimetric coverage increased with increasing CTV-to-PTV margins and decreased with time. Using van Herk’s formula, after 7min of treatment, a margin of 0.4 and 0.5mm was needed in LR, 0.7 and 1.5mm in SI, 1.1 and 3.2mm in AP directions, respectively, for considering prostate intrafraction motion. On the other hand, margins of 0mm in LR direction, 2mm in superior direction, 3mm in inferior and anterior directions, and 5mm in posterior direction were obtained with the VSM. When applying the intrafraction shifts on the PTVs generated with the VSM margins, the impact of motion on the CTV coverage was the same as using 5mm homogeneous margin. Finally, after 7min of treatment, intrafraction motion triggered an increased in the average dose (D mean ) of 2 and 1.5Gy to the rectum and bladder walls, respectively, with respect to the initial treatment plan, using a 5mm CTV-to-PTV margin. Conversely, a reduction in rectum and bladder walls D mean of 2Gy was reported using the margins obtained with the VSM. PO-1707 Time-dependent margins for prostate intrafraction motion during hypofractionated radiotherapy F. di Franco 1 , T. Baudier 1 , F. Gassa 2 , P. Pommier 2 , D. Sarrut 1 , M.C. Biston 1