Session Item

The main aim of this study was to show the dosimetric importance of Jaw tracking technique in reducing the doses to organs at risk while achieving the optimal target dose coverage.

A retrospective study of 10 GBM cases where two plans were created for each patient, Static Jaw Technique (SJT) Dynamic Intensity Modulated Radiotherapy (D-IMRT) plan and Jaw Tracking Technique (JTT) D-IMRT plan. Both plans were created 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.

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.

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.