(P140) Comparison of Split-Field IMRT With Whole-Field VMAT and IMRT for Locally Advanced Head and Neck Cancer

Sean Quinlan-Davidson, MD, CM, Adam S. Garden, MD, G. Brandon Gunn, MD, Clifton D. Fuller, MD, PhD, David I. Rosenthal, MD, Jinzhong Yang, PhD, Xiaoqiang Li, PhD, Samuel Tung, MSc, William Morrison, MD, Jack Phan, MD, PhD; UT MD Anderson Cancer Center

Background: Advances in intensity-modulated radiation therapy (IMRT) head and neck target delineation and treatment planning have led to improved sparing of organs at risk (OARs). The selection of an optimal IMRT technique is an ongoing debate. Split-field IMRT (HB-IMRT) and whole-field IMRT (WF-IMRT) represent two common techniques employed for the treatment of oropharyngeal cancers. The advent of volumetric modulated arc therapy (VMAT) offers the potential for fewer monitor units and shorter delivery time. It is unclear whether dose to normal critical structures, particularly organs involved in swallow function, are affected with VMAT compared with HB-IMRT and WF-IMRT. In this study, we compare three IMRT techniques for the treatment of common cases of oropharyngeal squamous cell carcinoma.

Methods: CT of 10 patients with locally advanced oropharynx cancer treated at MD Anderson Cancer Center using HB-IMRT were replanned with WF-IMRT and VMAT. They included five base of tongue and five tonsil squamous cell carcinoma patients. The treatment plans were reviewed by a panel of radiation oncologists, and planning was performed by physicists with head and neck expertise. For each case, the planning target volume (PTV) and critical OARs were compared among the three techniques. In addition, target delineation of the pharyngeal constrictors was included. OARs were delineated according to Radiation Therapy Oncology Group (RTOG) 1016 guidelines. The larynx volume was divided into two subvolumes (supra and infra), separated at the base of the superior horn of the thyroid. For patients treated with the HB-IMRT technique, the isocenter was placed 3 mm above the arytenoids. The paired t-test was used to assess for significant differences of means.

Results: For the 10 bilateral plans, the mean dose (Gy) to the larynx was 23.2 for VMAT (range: 18.9–26.4 Gy), 22.1 for WF-IMRT (range: 17.5–28.2 Gy), and 25.4 for HB-IMRT (range: 15.4–30.8 Gy). The mean supralarynx dose (Gy) was 40.7 (range: 27.7–63.7 Gy), 41.3 (range: 27.9–64.2 Gy), and 53.7 (range: 30.2–68.0 Gy) for whole VMAT, IMRT, and split-IMRT, respectively. The mean infralarynx dose (Gy) was 17.7 (range: 11.9–23.9 Gy) for VMAT, 16.0 (range: 9.7–22.4 Gy) for IMRT, and 15.9 (range: 7.6–25.8 Gy) for HB-IMRT. The upper pharyngeal constrictors received a mean dose (Gy) of 60.1 (range: 53.1–69.7 Gy), 60.1 (range: 54.5–69.2 Gy), and 62.2 (range: 55.6–70.0 Gy), for whole VMAT, IMRT, and HB-IMRT, respectively. The middle pharyngeal constrictors received a mean dose (Gy) of 46.4 (range: 20.3–70.1 Gy) for VMAT, 47.7 (range: 20.1–70.3 Gy) for IMRT, and 57.9 (range: 31.2–70.3 Gy) for HB-IMRT. The PTV receiving > 110% of the volume was 0% for all three techniques. In all comparisons, no statistical differences were observed.

Conclusions: These preliminary data suggest that similar sparing of critical swallow structures may be achieved with VMAT as compared with traditional IMRT techniques. Further analysis is ongoing, as well as recruitment of additional patients to validate these findings and assess whether there is significant compromise of tumor coverage, dose homogeneity, or nonlaryngeal critical structures.