SU-F-T-523: Radiobiological Comparison of Helical Tomotherapy and VMAT in the Treatment of Head and Neck Tumors

Purpose:This study aims at comparing the efficacy of Helical Tomotherapy (TOMO) and Volumetric Modulated Arc Therapy (VMAT) in producing highly conformal dose distributions in challenging head u0026 neck cancer patients. Furthermore, to interpret the dosimetric findings into expected tissue response rates in order to estimate their clinical impact.Methods:Seven patients were studied and for each patient two treatment plans (one TOMO and one VMAT) were created. Structures used for optimization were: high risk PTV, standard risk PTV, brainstem, spinal cord, parotid gland, larynx, mandible and surrounding tissue. The prescription varied between 60 – 74.4Gy to the HR. The same dosimetric constraints were used for both pairs of plans. Additionally, the TCP and NTCP values of the different targets and organs at risk (OAR) were calculated and compared together with the P+, which is the probability of achieving tumor control without normal tissue complications.Results:The proposed plan evaluation shows that the VMAT gives better results than TOMO in terms of expected clinical outcome. Specifically, the average difference in P+ = 3.3±1.8%, TCP = 3.3±1.8%, NTCP = 0.0±0.0%. The difference observed mainly stems from the lower TCP with TOMO (average difference of 2.9% for the high risk PTV and 3.1% for the standard risk PTV). The NTCP differences between the two modalities are very small even though their average differences in BEUD can be large (e.g. −21.9 Gy in brainstem, 27.2 Gy in larynx).Conclusion:The findings of the analysis indicate that VMAT achieves better coverage to the targets with lower doses to the OARs compared to TOMO for the patients tested. However, the dosimetric differences appear to have a measurable impact only in the expected TCP rates of the targets. Radiobiological evaluation of treatment plans should provide a closer association of the delivered treatment with the clinical outcome.