SU-E-J-13: Dosimetric Discrepancy Investigation with Cone-Beam CT for Adaptive RT of Head-and-Neck Cancer

Abstract

Purpose: To investigate the discrepancy between planned and delivereddoses due to tumor regression and anatomic changes using weekly Cone‐ Beam CT(CBCT) for patients with head and neck cancertreated with intensity modulated radiation therapy(IMRT). Methods: Nine patients each with eight weekly CBCTimages who received IMRT for head and neck cancer were analyzed. For each patient, weekly CBCTimage was converted into “CT of the week” using deformable image registration and fused with the planning CT. The weekly dose was calculated on the fused CBCTimages, and the calculated weekly doses were registered back into planning CT space using deformable image registration. The accumulated dose was then calculated in planning CT space. For the gross tumor volume (GTV) and lymph nodes, the minimum dose and dose range were compared with the treatment plan; for the parotids, the mean dose was evaluated. Results: The minimum dosedelivered to the GTV and nodes was reduced from the treatment plan with a mean discrepancy of 1.0Gy (1.6%) and 2.5Gy (5.3%), respectively. The range of dosesdelivered increased by 2.1Gy (39.3%) for the GTV and 4.0Gy (16.2%) for the lymph nodes compared to the planned dose. For the parotid, the mean dosedelivered increased by 2.0Gy (4.5%) from the treatment plan. Conclusions: Dosimetric discrepancy from treatment plans of patients with head and neck cancer resulted from tumor regression and anatomic changes during IMRT. Such discrepancy can be monitored and investigated with on‐board CBCTimaging. Further studies are needed to determine how best to intervene to provide more accurate radiation therapy.