Comparison of helical tomotherapy and mixed beam treatment plans for superficial head and neck cancers

Abstract

Purpose: To compare helical tomotherapy (HT) with mixed beam therapy (electron and IMRT) plans for superficial parotid gland and nasal cavity tumors. Methods: Mixed beam and HT dose plans were developed for five patients with superficial tumors (planning target volume or PTV < 5.5 cm depth), three with parotid gland tumors and two with nasal cavity tumors. Seven mixed beam plans included a 5 or 7-field photon IMRT plan optimized on top of a single en-face 16 or 20 MeV electron beam dose distribution. The ratio of photon to electron beam weights (at depth R100) were 1:0 (IMRT only), 2:1, 1:1, 1:2, 1:3, 1:4, and 0:1 (electrons only). Planning objectives for HT plans were set as closely as possible to those in the mixed beam plans, and were determined using our clinical planning protocol for head and neck cancers. The resulting dose distribution from each plan was evaluated using dose-volume quantities, tumor control probability (TCP), normal tissue complication probability (NTCP), and a clinical evaluation by a radiation oncologist. Results: In general, the HT plans showed better target coverage and dose homogeneity index (DHI) than the mixed beam plans. For the parotid patients, the DHI improved an average of 0.056 and 0.035 for the nasal cavity patients compared to the mixed beam plan. TCP was comparable in all patients. NTCP for the mixed beam plan was generally lower or comparable to HT with the largest improvements seen in the contralateral parotid, eye, and lens. Also, the mixed beam plans yielded more favorable PTV and normal tissue results for a single shallow uniform PTV using a heavier weighted electron to IMRT ratio (1:3 or 1:4 ratio of electron to IMRT). Conclusions: The study showed that while HT plans had better target coverage and dose homogeneity, the mixed beam plans (electron and IMRT) had comparable tumor control probability and have the potential for improving NTCP for distal normal tissue for superficial uniform PTVs.