Dosimetric Importance of Implementing Jaw Tracking Technique in Radiotherapy Treatment Plan Execution.

OBJECTIVE: To study the dosimetric importance of Jaw tracking technique in reducing the doses to organs at risk (OAR) while achieving the optimal dose coverage for the target. METHODS: We retrospectively selected ten Glioblastoma cases and for each patient, two plans were created namely Static Jaw Technique Dynamic Intensity Modulated Radiotherapy plan and Jaw Tracking Technique D-IMRT plan with 6 MV for Varian Truebeam™ STx machine using Eclipse Treatment planning system. Both plans were analyzed and compared based on various dosimetric parameters for Planning Target Volume (PTV) and OARs. The dose agreement between the Portal dose image prediction and the portal dosimetry measurement was also analysed  using gamma analysis criteria of 3%/3mm, 2%/2mm and 1%/1mm of dose distance/distance-to-agreement. RESULTS: The dosimetric parameters evaluated for both plans showed that most of the parameters gave significant P values, where D50% of PTV showed a mean difference (Δ) of 0.45 with significant P value, 0.0104. Similarly mean dose, D2%, D98%, D80% to PTV, Conformity Index and Conformation number showed Δ values of 0.45, 0.51, 0.41, 0.40, 0.02 and 0.01 with their significant P values as 0.0138, 0.0172, 0.0313, 0.0466, 0.0279, 0.0561 respectively. The Δ values and significant P values obtained among OARs are 0.54;0.0224 for brainstem, 0.54;0.0017 for RT optic nerve, 0.52;0.0001 for LT optic nerve, 0.59;0.0040 for optic chiasm and for the healthy tissues it showed the values with their mean dose, V5 and V30 parameters as 0.19;0.0115, 0.59;0.0067 and 0.25;0.0125 respectively. The JTT plans showed better passing results of gamma analysis criteria when compared to SJT plans. CONCLUSION: The findings in the studies emphasize the importance of using JTT technique in the radiotherapy treatment plans as it lowers the risk of acute or late toxicity and secondary radiogenic cancers in patients by reducing the OAR doses and achieves better tumor control.

Dosimetric Importance of the Implementation of Daily Image Guidance in Radiotherapy Practice.

BACKGROUND: The purpose of this study is to show the dosimetric importance of using daily image guidance in radiotherapy treatment. METHODS: A sample of 30 patients with various head and neck cancers were retrospectively selected for the studies. The prescribed dose, 66 Gy/33 fractions was used for all patients who received 7 to 9 beams, 6 MV Intensity Modulated Radiation Therapy (IMRT) plan delivered by Varian Truebeam STx. Before the first fraction of the treatment, the patient's shift corresponds to isocenter was noted and corrected. Subsequently, the images were taken daily for all 30 patients and the maximum, mode, median and mean of all shifts were recorded and applied to the base plan and recalculated for quantitative analysis of tumour coverage and Organ at Risk's (OARs) doses using various dosimetric parameters such as Homogeneity Index (HI), Conformation Number (CN), Conformity Index (CI), Coverage Index (COVI), Dose Gradient Index (DGI) and Unified Dosimetry Index (UDI) of shift plan. RESULTS: The results showed the Planning Target Volume (PTV) and the OARs values deteriorated from its base plan values in the various shift plans created by applying the patient setup errors analyzed using image guidance. Mean dose of maximum shift plan with a significant P value of 0.002, D2% of maximum shift plan with a significant P value 0.028, the D98% Values of maximum and mode plans with the significant P value 0.004 each, the D50% and D80% values of the maximum shift plans with their significant P values 0.001 and 0.002 respectively. Also, HI95%, CN95% and COVI values of the maximum shift plan showed much variation with significant P values of 0.004, 0.040 and 0.0004 respectively from their base plan values. There were significant changes observed in OARs values between base plan and shift plans. CONCLUSION: Implementation of daily image guidance in radiotherapy is mandatory taking into account of its dosimetric importance and to achieve the goal of radiotherapy practice of sparing the critical organs without compromising the target coverage.

Evaluation of Dosimetric Effect of Millennium and High Definition Multi Leaf Collimator Using 6 MV Photon Beams.

PURPOSE: The present study aims to compare different dosimetric parameters from field sizes defined by secondary and tertiary collimators. A comparison has been drawn between two types of Multi Leaf Collimator (MLC) designs. MATERIALS AND METHODS: The measurements were obtained using Millennium MLC (Mi-MLC) from Varian Unique™ linear accelerator (LINAC-1) and compared with measurements from Varian Truebeam™ linear accelerator (LINAC-2) using High Definition MLC (HD-MLC). Dosimetric analysis included percentage depth dose (PDD), cross profile, dosimetric leaf gap (DLG) and scatter factor (SF) that were taken for different field sizes defined by both the MLC design and jaw. For beam data measurement PTW Radiation field analyse (RFA) was utilized. RESULTS: When the surface dose for MLC field for linac 1 and linac 2 were compared with jaws they were found to be on the higher side that is 2.8% to 4.9% and 2.2% to 3.6% respectively. The SF was found to vary from -3.2% to 0.73% for LINAC-1 with Mi-MLC when compared with jaws. Similarly, the SF variation from -2.4% to 1.1% was observed for LINAC-2 with HD-MLC as compared with jaw. Larger field sizes gave increased SF while smaller field sizes showed the opposite for HD-MLC. The penumbra was found to be less in HD-MLC as compared to Mi-MLC. Similarly, DLG was found to reduce by 0.056 mm in Mi-MLC when compared with HD-MLC. The results of symmetry and flatness were seen within the limits for both MLC designs. CONCLUSION: It can be concluded from the results that both the MLC designs have merits and demerits that are based on their effectiveness and clinical use. However, higher surface dose was found in HD-MLC in contrast to Mi-MLC.