ESTRO 2021 Abstract Book

S1024

ESTRO 2021

Conclusion In our experience, increase-dose modulated radiotherapy for esophageal cancer is a safe and effective treatment. The high rate of complete pathological response (55.6%) could be related to better DFS and OS. Controlled randomized trials should be designed to test if dose escalation with modern radiotherapy could increase the clinical outcomes of this disease.

PO-1241 Optimising splenic dose with PBT and VMAT for distal oesophageal cancer O. Nicholas 1 , A. Saplaouros 2 , J. Lambert 2 , G. Fegan 3 , R. Hugtenburg 4 , S. Gwynne 1

1 South West Wales Cancer Centre, Oncology, Swansea, United Kingdom; 2 Rutherford Cancer Centres, Radiotherapy Physics, Newport, United Kingdom; 3 Swansea University, Swansea Trials Unit, Swansea, United Kingdom; 4 Swansea University, Medical Physics, Swansea, United Kingdom Purpose or Objective Higher rates of G4 lymphopenia is predictive of poorer survival in oesophageal cancer (OEC). Previous work has found a dose dependant relationship between mean splenic dose (MSD) and absolute lymphocyte count. In distal OECs, the spleen often receives significant doses due to its proximity to the target volume (TV) but is often not considered during RT planning. PBT’s superior physical properties may reduce splenic dose compared to photon RT. Aims: 1) Ascertain feasibility of reducing dose to the spleen in lower OEC with PBT and VMAT, while meeting dose constraints to other OARs. 2) Test spleen constraints used during optimisation of PBT and VMAT plans, that may be used in future trial protocols and clinical practice. 3) Quantify any dosimetric advantages to the spleen, if present, of PBT over VMAT Materials and Methods Twenty distal OEC cases from the UK NeoSCOPE trial were used. In addition to the quality-assured clinical TVs and OARs, the whole spleen was outlined as per RTOG guidance in each case. Nominal PBT plans were robustly optimised with and without spleen dose constraints. VMAT plans were created with spleen constraints for comparison. All plans were created on Pinnacle ( Philips ; PBT – v14, VMAT – v16.2) to a dose of 45Gy/25#. For TV coverage and OARs, NeoSCOPE trial dose constraints were used. Novel whole spleen constraints used were taken from previous work in pancreatic cancer. These were MSD <4.5GY (optimal), V10Gy <12% (optimal), MSD <10Gy (mandatory) and V15 <20% (mandatory). A 3-beam arrangement with gantry angles of 135°, 180° and 225° were used for all PBT plans. The statistics for PBT plans with and without spleen optimisation were compared. PBT (spleen-optimised) plans were also compared to VMAT (spleen-optimised) plans. Paired t-test (two-tail) were performed for each tested DVH parameter to assess for significance (P < 0.05). Results Full results are detailed in Table 1 and Figure 1.