Room: Exhibit Hall
Purpose: Pre-IMRT, patients were set up in a neck-extended position to spare important structures from traditional 3-field planning. However, in the IMRT era, many patients are treated in the neck-neutral position. The purpose of this study is to determine the theoretical dosimetric impact between neck-neutral versus the neck-extended positions.
Methods: We prospectively accrued 28 patients with non-larynx/hypopharynx cancers undergoing IMRT. We evaluated 22 patients after excluding 6 receiving post-operative treatment. Each patient had two CT simulations; one in the neck-extended position, and one in the neck-neutral position. Target and organ-at-risk (OAR) contours were transferred from the neck extended scan to the neck-neutral scan utilizing deformable image registration with MIM v6.6. Both image/contour sets were used to generate Feasibility DVHs (FDVH) using PlanIQ software using 70Gy and 54Gy as the high and low risk PTV prescriptions. FDVH computes the idealized achievable DVH for each OAR given patient geometry. FDVH were generated for the neck-neutral and neck-extended positions. The FDVH max dose to the brainstem and spinal cord alongside the FDVH mean doses to the contralateral parotid, larynx and both cochlea were compared using paired T-tests.
Results: The FDVH maximum spinal cord dose along with the FDVH mean dose to the contralateral parotid and both cochlea all differed by less than 30cGy between neck positions (p>0.05). Extending the neck led to a statistically significant reduction in the FDVH mean larynx dose of 1.1Gy and an increase in the maximum brainstem dose of 1.8Gy (p<0.001).
Conclusion: Extending the neck led a theoretical geometric benefit in the potential sparing of the larynx, however this was accompanied by an increase in the ideal maximum brainstem dose. While statistically significant, these idealized differences may not be clinically impactful and patient comfort and setup reproducibility need to be considered.