Purpose: Studies using split field IMRT to spare dysphagia/aspiration related structures (DARS) have raised concern regarding dose uncertainty at matchline. This study explores the utility of hybrid VMAT in sparing the DARS and assesses matchline dose uncertainty in postoperative oral cavity cancer patients and compares it with VMAT.
Methods & materials: Ten postoperative oral cavity cancer patients were planned with h-VMAT and VMAT using the same planning CT dataset. PTV and DARS were contoured using standard delineation guidelines. In h-VMAT 80% of the neck dose was planned using AP/PA technique and then VMAT optimization was done for the total PTV by keeping the corresponding AP/PA plan as the base dose. Planning goal for PTV was V95%≥95% and for DARS, adequate sparing. Plans and dose volume histograms were analyzed using dosimetric indices. Absolute point and portal dose measurements were done for h-VMAT plans to verify dose at the matchline.
Results: Coverage in both the techniques was comparable. Significant differences were observed in mean doses to DARS (Larynx: 24.36±2.51 versus 16.88±2.41Gy; p<0.0006, Pharyngeal constrictors: 25.16±2.41 versus 21.2±2.1Gy; p<0.005, Esophageal inlet: 18.71±2 versus 12.06±0.79Gy; p<0.0002) favoring h-VMAT. Total MU in both the techniques was comparable. Average percentage variations in point dose measurements in h-VMAT done at +3.5 and -3.5 positions were (1.47±1.48 and 2.28±1.35%) respectively. Average gamma agreement for portal dose measured was 97.07%.
Conclusion: h-VMAT achieves better sparing of DARS with no matchline dose uncertainty. Since these patients have swallowing dysfunction post-operatively, attempts should be made to spare these critical structures as much as possible.
Keywords: DARS; Hybrid VMAT; Matchline dose; Oral cavity cancer.
Copyright © 2017 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.