ESTRO 2022 - Abstract Book

S1664

Abstract book

ESTRO 2022

with 6 MV beams for Varian Truebeam STx using the Eclipse Treatment Planning System (TPS) version 15.06 and all the plans met the plan objectives as per the RTOG guidelines. While creating JTT plans, the jaw tracking function was selected when calculating the leaf motions and volumetric dose. Both the SJT and JTT plans were analyzed and compared based on the Dose Volume histograms (DVH), tumor coverage and OAR doses. Various other dosimetric plan parameters such as Homogeneity Index (HI), Conformation Number (CN) and Dose Gradient index (DGI) were also used for evaluating both the plans. The dose agreement between the Portal dose image prediction (PDIP) and the portal dosimetry measurement was also analyzed for both JJT and STT plans of all patients by using gamma analysis criteria of of 3% dose difference and 3mm distance to agreement (3%/3mm), which was further evaluated with 2%/2mm and 1%/1mm criteria for comparison. Results The dosimetric parameters evaluated for JJT and STT plans showed that most of the parameters under study gave significant P values where D50% showed the most significant P value of 0.0104. Similarly other parameters like mean dose, D2%, D98%, D80%, CI95% and CN95% showed significant P values of 0.0138, 0.0172, 0.0313, 0.0466, 0.0279, 0.0561 respectively. The significant P values obtained among OARs are 0.0224 for brainstem (mean dose), 0.0017 for RT optic nerve (D1%), 0.0001 for LT optic nerve (D1%), 0.0040 for optic chiasm (D1%). Similarly the healthy tissues showed significant P values as 0.0115, 0.0067 and 0.0125. From the plan verification results of JTT and SJT plans with the gamma evaluation method, it was concluded that JTT plans showed better passing results of 99.58±0.5, 98.39±0.8 and 94.54±1.1 with 3mm/3%, 2mm/2%,1mm/1% gamma analysis criteria when compared to the SJT plan values of 99.01±0.8, 97.45±0.8 and 94.52±1.3 respectively. Their P values were significant in the order of 0.0028 and 0.0005 for 3mm/3% and 2mm/2% criteria which in- turn shows the importance of jaw tracking technique in the study. Conclusion The findings in the study emphasizes the importance of using JTT in the radiotherapy treatment plans and the importance of this feature in their units as it lowers the risk of acute/late toxicity and secondary radiogenic cancers in patients by reducing the OAR doses and also it can be concluded that this JTT also contributes to deliver quality treatment plans with better target coverage. The gamma analysis showed that for JTT plans, the dose measurements agreed well with the TPS when compared to that of the SJT plans. Purpose or Objective In the radiotherapy treatment of oesophageal cancer (EC), the lungs and heart are in close proximity of the target volume. Reducing lung dose was historically the primary aim in treatment planning. However, due to increased awareness of the risk of cardiac complications after radiotherapy, the optimisation process nowadays is more focused on reducing heart dose. The result is an increased lateral dose contribution which may lead to less robust treatment plans. In this study, we explored the trade-off between heart and lung dose, and corresponding target robustness. Materials and Methods Two EC patients were selected for this study, who presented diaphragm displacements during treatment (Table 1). Target volumes were delineated on the phases and the average of the planning 4DCT, and of two repeated 4DCTs. For each patient, two volumetric modulated arc therapy (VMAT) plans were created to cover the planning target volume; one with the aim to reduce mean lung dose (MLD) as much as possible (VMAT lung ) and one with the aim to reduce mean heart dose (MHD) as much as possible (VMAT heart ). Both plans had to obey priority 1 and 2 constraints as defined in our clinic (MHD <26 Gy; MLD <16 Gy). First, we investigated the presumed static dose cloud of the plans by shifting the isocenter 8mm in all directions on the planning image. The resulting 14 dose scenarios were summarized in a voxel-wise minimum dose distribution to evaluate target coverage. Additionally, the plan was robustly evaluated (including 2mm shifts to account for residual uncertainties) on the repeated 4DCTs, using the average and using accumulative dose of all the phases. Results For both patients, the optimisation window of heart dose was substantially larger than that of lung dose comparing VMAT lung and VMAT heart (Figure 1). The average MHD reduction was 12.5 Gy comparing VMAT heart to VMAT lung , while on average the MLD increased by 3.3 Gy. Robust evaluation at baseline suggested reduced target robustness for the VMAT heart plans, compared to the VMAT lung plans (Table 1). Variations in the diaphragm position resulted in reduced target coverage on the repeated CT. This was especially true for the VMAT heart plans if the diaphragm moved cranially as observed for patient 2. On the opposite, the diaphragm moved caudally for patient 1 and hotspots were observed in the heart region for VMAT heart . For both patients, the changes in diaphragm position were consistent along the treatment. When all phases were considered, target coverage slightly improved, as the dose averaged out. However, target underdosage was still present for patient 2 (V95: 74-79%). Conclusion In VMAT radiotherapy for EC, the heart can be spared without a substantial increase in MLD. However, this results in increased risk of underdosage if the diaphragm shifts in cranial direction or cardiac hotspots if it shifts in caudal direction. Robustness evaluation of the treatment plan seems to indicate these risks already at baseline. PO-1879 Trade-off in lung and heart dose and the impact on target robustness in VMAT of oesophageal cancer S. Visser 1 , M. Schol 1 , P. Klinker 1 , M. Dieters 1 , V.E. Mul 1 , J.A. Langendijk 1 , E.W. Korevaar 1 , S. Both 1 , C.T. Muijs 1 1 UMCG, Radiotherapy, Groningen, The Netherlands