Influence of collimator rotation on dose distribution and delivery in intensity modulated radiation therapy for parotid cancer
International Journal of Cancer Therapy and Oncology
Rath GK. Influence of collimator rotation on dose distribution and delivery in intensity modulated radiation therapy for parotid cancer. Int J Cancer Ther Oncol 2015; 3(2):3212. Abstract Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed. Methods: The computed tomography (CT) datasets of 5 patients with parotid
... ients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I'mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation. Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles. Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT. 1(a-d) : The beam's eye view in a representative patient for collimator angle at 0 degree (a), 30 degree (b), 60 degree (c), and 90 degree (d). FIG.