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. Introduction During the utilization of IMRT, sharp dose decline may allow dose reduction to target volume and dose escalation to surrounding normal tissues1,2,3,4. Modifications of daily anatomy and changes in the shape and position of the target volume may cause significant differences between the planned and delivered dose. Therefore, adaptive radiotherapy (ART) requirement which means creation of a new plan during radiotherapy occurs. Head and neck cancers can benefit from ART because of weight loss, organ deformation, volume reduction, shrinkage of the tumour and/or involved lymph nodes depending on dose response in the course of the treatment. This study aimed 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 in head and neck cancer patients who receive primary radiotherapy or chemoradiotherapy. In addition, to find the optimal time for an adaptive plan. Materials and Methods Istanbul University Faculty of Medicine Clinical Research Ethics Review Board approval was obtained before the study. This prospective study included 15 nonmetastatic head and neck cancer patients who received radical radiotherapy or chemoradiotherapy between January 2012 and September 2012. Inclusion and exclusion criteria are shown in Table 1. Pretreatment evaluation included medical history, physical examination, panendoscopy, complete blood count and serum chemistry panel. GFR was measured in patients who received chemoradiotherapy. All cases were asked to undergo PET-CT in treatment position with treatment mask and a contrast enhanced MRI of the head and neck. Patients were evaluated by a dentist, if there was a need for dental treatment, it was performed before radiotherapy. *Corresponding author Email: [email protected] 1 Istanbul University, Cerrahpasa Medical Faculty, Department of Radiation Oncology, Istanbul, Turkey Table 1: Inclusion / Exclusion criteria. Inclusion criteria Exclusion criteria · Newly diagnosed ·Between 18-70 years old · Patients with radiologically visible mass . Karnofsky Performance Scale (KPS) ≥70 · Radiotherapy and/or chemotherapy received previously · < 18 years old · Patients with severe malnutrition · KPS <70 · T1 ve T2 glottic tumor ·Patients with recurrence, metastasis