Postoperative Radiation Therapy (RT) for Salivary Gland Carcinoma: A Comparison of Intensity Modulated Radiation Therapy (IMRT) and Proton Treatment Techniques

Abstract

Retrospective evidence and consensus guidelines support the use of post-operative RT for selected patients with resected salivary gland carcinoma and unfavorable pathologic features. However, head and neck RT may be associated with significant treatment related toxicity. Protons may offer an improved therapeutic index in this setting by reducing dose to adjacent organs at risk (OAR). This study compares IMRT with pencil beam scanning proton therapy (PBS) in a series of patients receiving postoperative RT for parotid gland carcinoma. We identified 8 patients with parotid gland carcinoma who received post-operative RT at our institution between December 2009 and July 2011. We retrospectively reviewed each patient's treatment plan and developed IMRT and PBS treatment plans for each patient with a prescription target dose of 60 Gy/GyE. PBS treatment plans were optimized using customized bolus to reduce spot size and resultant penumbra. We measured dose-volume relationships for target volumes, and OAR (cochlea, temporal lobe, brainstem, and contralateral parotid gland) with each treatment technique. Using Wilcoxon Signed Rank test, we performed statistical comparison of the two treatment techniques. Statistical significance was set at 0.05, and all tests were 2-tailed. Both PBS and IMRT provided adequate target coverage with at least 95% of the target volume receiving 95% of the prescription dose. In comparison to IMRT, PBS significantly reduced OAR dose with respect to all measured dosimetric parameters. PBS significantly reduced mean dose to the ipsilateral cochlea when compared with IMRT (8.4 Gy vs. 3.5 GyE; p = 0.04). Mean dose to the ipsilateral temporal lobe was significantly improved with PBS (5.2 Gy vs. 0.3 GyE; p = 0.01). PBS also resulted in lower maximum dose to the brainstem (22.9 Gy vs. 1.6 GyE; p = 0.01), as well as lower mean dose to the contralateral parotid gland (9.4 Gy vs. 0.003 GyE; p = 0.01). We found that post-operative RT with PBS resulted in significantly improved normal tissue sparing relative to IMRT while still maintaining excellent target coverage for patients with resected parotid gland carcinoma. Specifically, treatment plans using PBS technique resulted in a proportional reduction of mean cochlear dose by > 50% relative to IMRT, and doses to other relevant OAR were largely eliminated with PBS. Our findings suggest that RT with PBS may be associated with a clinically improved therapeutic index for these patients. Clinical outcomes with PBS treatment in this setting should be further evaluated in the context of prospective clinical trials, focusing on maintenance or improvement of current locoregional control rates, as well as treatment related toxicities and patient quality of life.