ESTRO 2023 - Abstract Book

S1598

Digital Posters

ESTRO 2023

Patients recruited prospectively to receive whole bladder (CTV) radiotherapy, 55Gy in 20 fractions on the 1.5T MR-linac (Unity Elekta AB, Stockholm, Sweden) system within a single centre clinical research and ethics committee approved study (NCT03727698) were evaluated. All patients were treated with an empty bladder, with on-line recontouring and optimisation at each fraction (Adapt-to-Shape, ATS workflow) based on daily T2W session MRI. For treatment the PTV was defined as CTV with an anisotropic expansion of 1.5cm superiorly and anteriorly, 1cm posteriorly, and 0.5cm laterally and inferiorly. Retrospectively 56 fractions from 7 patients were randomly selected. CTV coverage was assessed on T2W MRI acquired at approximately 20 minutes (CTV MR_20mins) of the clinical workflow with PTV defined as CTV with isotropic 4mm expansion (PTV4mm) and CTV with isotropic 5mm expansion (PTV5mm). CTV MR_20mins coverage was assessed i) visually to determine if would have been deemed clinically acceptable for treatment, and ii) by re-calculating the estimated dose delivered on recontoured CTV (Monaco v5.40.01 Elekta AB, Stockholm, Sweden). Target dose coverage was deemed acceptable if 95% of CTV MR_20mins received >95% of the prescribed dose. Results Mean time between daily T2W session MRI and the second T2W MRI on which assessment was made was 19.1 minutes. PTV4mm encompassed CTV MR_20mins in 22/56 (39%) fractions. PTV5mm encompassed CTV MR_20mins in 37/56 (66%) fractions. Of the 19 fractions where PTV5mm did not encompass CTV MR_20mins, D99 median (range) was 52.3Gy (33.0 - 53.4Gy) and CTV MR_20mins V95% median (range) was 99% (85.3 - 99.9%). 52/56 (93%) fractions met target volume constraints with PTV5mm. Of the fractions not meeting target volume constraints with PTV5mm, Adapt-to-Position (ATP) workflow i.e., virtual couch shift after ATS would have encompassed CTV MR_20mins in 2/4 fractions. Both fractions not meeting target constraints occurred in the same patient. Conclusion Bladder target coverage can be maintained with PTV margin reduction to 5mm when utilising a full online adaptive bladder radiotherapy workflow delivered in 20 minutes. 1 Guy's and St Thomas' NHS Foundation Trust, Department of Medical Physics, London, United Kingdom; 2 Guy's and St Thomas' NHS Foundation Trust, Department of Oncology, London, United Kingdom; 3 Guy's and St Thomas' NHS Foundation Trust, Department of Radiotherapy, London, United Kingdom Purpose or Objective Respiratory gating using end expiration breath hold (EEBH) radiotherapy has the potential to reduce tumour motion during treatment in tumours near the diaphragm. This may obviate the need for an internal target volume (ITV), and reduce the dose to organs at risk (OAR) such as the heart, lungs and liver by repositioning them with respect to the target volume compared to free-breathing (FB) radiotherapy using 4DCT. The aim of our study was to evaluate the dosimetric and volumetric effects of EEBH in patients with oesophageal cancer undergoing (chemo)radiotherapy. Materials and Methods Thus far, 12 lower third oesophageal cancer patients have been planned for, using volumetric modulated arc therapy (VMAT) in both FB and EEBH scans. For patients treated in EEBH, clinicians created a new set of target volumes on the FB scans and vice-versa. New treatment plans were retrospectively generated for the non-clinical CT & structure set. The plans were optimised and evaluated in order to be comparable to the original clinical plan. All plans were reviewed and approved by clinicians using the same criteria. Paired t-tests were performed to examine within-patient volumetric and dosimetric differences. Results Of the 12 patients, 10 were male and two female with a median age of 73.5 years. 10 patients presented at stage T3 and two at T2, with the prevailing histology being adenocarcinoma (10/12). 11 were treated with EEBH and one in FB. Volumetrically, we observed a mean PTV volume reduction of 61.8cc [p=0.002] for EEBH plans compared to FB (Table 1). Heart/PTV overlap volumes were reduced by 1.21% [p=0.024] and Lung/PTV overlap volumes by 0.28% [p=0.010] between EEBH and FB plans. Dosimetrically, mean heart doses reduced by 0.75Gy [p=0.231] with EEBH but not with statistical significance, with lung and liver mean dose reductions of 0.61Gy [p=0.086] and 0.76Gy [p=0.096] respectively. Table 1 – Average volumetric and dosimetric results for comparison between EEBH and FB for lower oesophagus RT Mean PTV Volume /cc (Range) Mean Heart/PTV Overlap Volume as a percentage of heart volume /% Mean Lung/PTV Overlap Volume as a percentage of lung volume /% Mean Liver/PTV Overlap Volume as a percentage of liver volume /% Mean Heart Dose /Gy Mean Lung Dose /Gy Mean Liver Dose /Gy PO-1858 Dosimetric and volumetric effects of end expiration breath hold radiotherapy for oesophageal cancer C. Mayhew 1 , J. Venkatasai 2 , M. Khan 3 , B. Leung 3 , K. Owczarczyk 2 , G. Ntentas 1

381.1 (132.0 821.0) 442.9 (153.8 904.8) 61.8 (0.002)

EEBH

4.20

0.29

1.29

17.85

7.26

10.65

FB

5.42

0.57

1.53

18.61

7.87

11.41

Difference (p-value)

1.21 (0.024)

0.280 (0.010)

0.246 (0.266)

0.751 (0.231)

0.609 (0.086)

0.763 (0.096)

Conclusion