Session Item

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 

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).

Full results are detailed in Table 1 and Figure 1.

This study demonstrates that, at a dose level of 45Gy/25#, it is feasible to incorporate spleen constraints while meeting other planning goals for distal OEC. Spleen optimised cases had higher cord, heart and liver doses although differences are small and may not be clinically significant. PBT plans resulted in comparable MSD compared to VMAT. However, there was wider variation in splenic dose for PBT suggesting a greater sensitivity to individual patient anatomy. In most cases (PBT and VMAT) mandatory constraints were met but for most plans, optimal constraints were exceeded. A greater proportion of VMAT plans met all constraints. 

VMAT may achieve greater spleen sparing in distal OEC compared to PBT. However, a lower integral dose with PBT may still result in lower rates of G4 lymphopenia. MSD of <10Gy is a feasible optimal constraint for clinical use. Prospective studies are required to ascertain the impact on splenic irradiation on G4 lymphopenia rates and survival outcomes.