Assessment of the optimal timing for projected dose evaluation in head and neck cancer patients treated with helical tomotherapy

Background: This study aimed to determine the optimal time point at which the projected dose most accurately corresponds to the accumulated dose in head and neck cancer patients treated with helical tomotherapy. Dose metric differences between the International Commission on Radiation Units and Measurements report no 83 (ICRU83) (D50%) and Radiation Therapy Oncology Group (RTOG) (D95%) protocols were also evaluated for the planning target volume and critical organs, including the parotid glands, spinal cord, and brainstem. Materials and methods: This methods-oriented study analyzed data from 61 head and neck cancer patients treated with helical tomotherapy. It compared projected and accumulated doses across the treatment course, assessing dose discrepancies at fractions 1, 5, 10, 15, 20, 25, and 30. A ±3% threshold was defined as the acceptable deviation for both target volumes and critical organs. Results: Among the 61 patients analyzed, 70.5% had T3–T4 tumors and 68.8% were planned using the ICRU83 protocol. No deviations exceeding ±3% were observed in planning target volume dose 70 Gy (PTV70) (D50%) under ICRU83 protocol, whereas 35.0% of RTOG protocol (D95%) cases showed dose underestimation during fractions 1, 5, and 10. Deviations exceeding ±3% occurred most frequently in the left parotid gland (up to 75.4% of cases), followed by the right parotid gland, spinal cord and brainstem, particularly during early fractions. Conclusions: Projected dose proved to be a reliable metric for evaluating dose consistency, with D50% under ICRU83 protocol stable from early treatment and D95% under RTOG protocol consistent after fraction 15. However, variations in projected doses for organs at risk emphasize the need for cautious interpretation and individualized adaptive planning.