Volumetric variations and the effects of these differences on dosimetry during the course of volumetric modulated arc therapy for head and neck cancer

Introduction To observe the changes in the patients anatomy and the effects of these differences on dose distribution throughout the treatment in order to design an optimal adaptive plan and to find the optimal time for replanning in head and neck cancer patients who receive primary radiotherapy or chemoradiotherapy. Materials and Methods Fifteen head and neck cancer patients were evaluated prospectively. VMAT plan with simultaneous integrated boost or sequential technique was performed. The last CBCT of the 3th and 5th week were fusioned deformably with planning CT. Initial plans were adapted to these CBCT images. An adaptive plan was generated during the week when normal tissues overdosed more than 5% or the target volume underdosed more than 5%. The volumetric changes and dosimetric differences in target volumes, parotid glands and spinal cord were compared between planning CT and CBCT images. Also adaptive plan doses were compared with delivered doses in terms of target volumes, parotid glands and spinal cord. Results While there was no significance at the comparison of the volumes in GTV and PTV70 between the planning CT and 3rd week CBCT, it became significant at the 5th week. Primary and lymph node GTVs reduced by 44.8% and 70.9% respectively. Parotid glands and spinal cord doses increased in the 3rd week as well as; it rose a significant level for the ipsilateral parotid and spinal cord in the 5th week. Adaptive plan was needed in 10 patients. Adaptive plan provided a 1 Gy dose reduction in contralateral parotid glands, 1.4 Gy in ipsilateral parotid glands and 1 Gy in the spinal cord. Conclusion Significant changes were observed in the volume of target and parotid glands despite it not being reflected much in dosimetry. A new CT scan can be recommended to evaluate for an adaptive plan in 5th week in the absence of clinically usable online correction methods