(P038) 4π Noncoplanar Stereotactic Body Radiation Therapy for Head and Neck Cancers

Jean-Claude M. Rwigema, MD, Dan Nguyen, BS, Allen M. Chen, MD, John A. Vargo, MD, Daniel A. Low, PhD, Dwight E. Heron, MD, Saiful Huq, PhD, Michael L. Steinberg, MD, Ke Sheng, PhD; Department of Radiation Oncology, University of California, Los Angeles; Department of Radiation Oncology, University of Pittsburgh Cancer Institute

Purpose: Stereotactic body radiation therapy (SBRT) is an emerging treatment strategy with increasing clinical applications in patients with head and neck cancers (HNCs), including mainly those with recurrent, previously irradiated (rHNC), locally advanced unresectable primary, or metastatic HNCs who have shown radioresistance to conventional radiotherapy, amongst others. The aim of this study was to reduce treatment toxicity of SBRT for patients with HNCs using highly noncoplanar 4π RT.

Materials and Methods: Between April 2007 and October 2013, a total of 25 patients (median age 66 yr [range: 42–87 yr]; females = 11) with 26 HNCs (rHNC = 23, primary = 1, metastatic = 2) who were previously treated with SBRT at UCLA (n = 10) and University of Pittsburgh Cancer Institute (UPCI) (n = 15) were included in the study. Patients with rHNC had undergone radiotherapy for primary disease with a median dose 70 Gy (range: 45–131.2 Gy) and reirradiated with a median dose of 44 Gy (range: 35–44 Gy) in five fractions, and the remaining had 40 Gy (n = 2) and 22.5 Gy (n = 1) in five fractions. Patients were treated using CyberKnife (n = 6), Trilogy (n = 5), and TrueBeam (n = 14). Histologies were 56% squamous cell carcinoma, and others were mixed. The median tumor volume was 35.2 cc (range: 2.8–209.4 cc). The novel 4π noncoplanar plans were created on each patient by automatically selecting and optimizing 30 highly noncoplanar intensity-modulated beams to meet the objective of 95% of the planning target volume (PTV) covered by 100% of the prescription dose. Doses to organs-at-risk (OARs) and 50% dose spillage volumes were compared against the delivered clinical SBRT plans.

Results: Using 4π plans, mean/maximum doses to the spinal cord, brainstem, pharyngeal constrictors, parotid glands, larynx, mandible, cochlea, and chiasm were reduced by 60%/51%, 53%/44%, 57%/43%, 64%/59%, 56%/34%, 43%/20%, 84%/71%, and 83%/67%, respectively (P < .02), and for the esophagus, carotid artery, and trachea, the respective reductions were 72%/49%, 13%/2%, and 58%/41% (P > .05). R50 (ie, 50% prescription dose volume divided by PTV) was reduced by 33% (P < .01). PTV coverage was also improved, where D95, D98, and D99 were increased by 3%, 5%, and 7%, respectively (P < .01). Escalated PTV doses of 50 Gy, 54 Gy, and 60 Gy in five fractions were achieved while keeping R50 reduction to > 33%, and doses to OARs significantly improved or were unchanged from clinical plans in all patients, except in four patients with tumors encasing the carotid artery, where only doses to the carotid artery were increased.

Conclusions: 4π plans yielded significantly and consistently improved tumor coverage and critical organ-sparing. Given the risk of late toxicities with SBRT, especially in patients with previously irradiated rHNC, 4π radiotherapy using conventional C-arm linear accelerators (LINACs) may enhance locoregional control by allowing safe dose escalation while potentially reducing toxicity.