Comparison of Pencil Beam Scanning Proton- and Photon-Based Techniques for Carcinoma of the Parotid

To report the dosimetric advantages of a comparison between pencil beam scanning (PBS) proton therapy versus intensity-modulated radiation therapy (IMRT) for parotid gland cancers.

This was a retrospective, dosimetric comparison of 8 patients who received external beam radiation therapy at our institution between 2009 and 2011. Two separate plans were generated for each patient: 1 IMRT and 1 PBS plan. The prescription dose for each plan was 60 Gy for IMRT and 60 Gy (RBE) for PBS. We measured dose-volume relationships for target volumes and organs at risk with each treatment technique. Dosimetric comparisons for each organ at risk were made by using the Wilcoxon signed rank test. All tests were 2-tailed, with P values < .05 considered statistically significant.

The mean patient planning target volume was 160.9 cm3 (SD 74.6). Pencil beam scanning, compared to IMRT, significantly reduced the mean dose to the following structures: ipsilateral temporal lobe (2.86 versus 9.59 Gy (RBE), P = .01), oral cavity (0.58 versus 13.48 Gy (RBE), P = .01), mandible (V50: 7.4% versus 12.8%, P = .01), contralateral parotid gland (0.003 versus 4.64 Gy (RBE), P = .01), ipsilateral submandibular gland (16.59 versus 38.94 Gy (RBE), P = .03), and contralateral submandibular gland (0.02 versus 5.34 Gy (RBE), P = .01). Pencil beam scanning also significantly reduced the maximum dose delivered to the brainstem (7.1 versus 30.9 Gy (RBE), P = .01).

Pencil beam scanning allows for superior normal tissue sparing while still maintaining excellent target coverage in patients with resected parotid gland cancers. These findings suggest that PBS may allow for an improved therapeutic index for these patients. Clinical outcomes with PBS should be evaluated prospectively, with a focus on disease outcomes as well as treatment-related toxicities and patient quality of life.