Dosimetric influence of pitch in patient positioning for radiotherapy of long treatment volumes; the usefulness of six degree of freedom couch.

OBJECTIVE: Pitch, the rotation around the transversal axis of the patient during radiotherapy has little impact on the dose distribution of small spherical treatment volumes; however it might affect treatment of long volumes requiring a correction with a six degree of freedom couch. METHODS: We included 10 patients each with nasopharyngeal carcinoma (NPC) and esophageal cancer, treated with volumetric modulated arc therapy. Pitch was simulated by tilting the planning CT in ventral and dorsal direction by +/- 1.5 degrees and +/- 3 degrees . Verification plans were calculated on the tilted datasets and were compared to the original plan and the dose constraints of the organs at risk (OAR). RESULTS: The deviation in dose to the planning target volume is increasing with the degree of pitch with mean changes of up to 2% for NPC and 1% for esophageal cancer. The most affected OAR in NPC patients are brainstem (max. dose +6.0%) and spinal cord (max. dose +10.0%) when tilted by 3 degrees dorsally and lenses (max. dose +3.3%), oral mucosa (mean dose +2.6%) and parotid glands (mean dose +4.3%) when tilted by 3 degrees ventrally. For esophageal cancer patients, there was no significant change in dose to any OAR. Whereas for esophageal cancer, all tilted treatment plans were still clinically acceptable regarding OAR, 5 NPC plans would no longer be acceptable with a pitch of 1.5 degrees ventral (N = 1), 3 degrees ventral (N = 2) and 3 degrees dorsal (N = 2). CONCLUSION: Planning target volume coverage in both tumor entities was only slightly affected, but pitch errors could be relevant for OAR in NPC patients. ADVANCES IN KNOWLEDGE: A correction with a six degree of freedom couch is recommended for NPC patients with a pitch mismatch of more than 1.5 degrees to avoid exceeded doses to the OAR.