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Recently, Tomotherapy planning software is equipped with automatic daily dose calculation function, which facilitates the assessment of the impact of anatomical changes on treatment plans. This pilot study aimed to investigate the correlation between geometric offsets in patients’ anatomy and actual daily as well as total delivered doses and to establish a quick and easy quantitative strategy for determining the need of adaptive radiotherapy in NPC cases.  

Retrospective data of 10 locally advanced NPC cases with nodal involvement treated with Tomotherapy were recruited. After each treatment fraction, the dedicated software performed fractional dose calculation on the acquired MVCT.  It also performed dose accumulation for the whole treatment course. For this study, the MVCTs were re-aligned to planning CT image twice, focusing on the brainstem and the spinal cord, and two shifts were recorded. The neck separation and the extent of the parotid glands (Fig.1) on MVCTs were also measured. Fractional and accumulative doses were extracted from the software generated report. Statistical correlation between the actual delivered dose and geometric offsets were investigated.

In general, the organ shift in both brainstem and spinal cord increased towards later course of the treatment, contributing to the increase in their total accumulated doses. The correlation analysis showed that every 1mm shift difference in either lateral, longitudinal, vertical or rotational direction could lead to 4.66cGy-7.10cGy increase in brainstem maximum dose. For spinal cord, every 1mm shift difference in any direction could lead to approximate 3.6cGy increase in maximum dose. For a standard NPC radiotherapy with 35fractions, 1mm shift in every single fraction may increase 163.1-248.5cGy in brainstem and 126cGy in spinal cord in total accumulated maximum dose at the end of treatment course.

The correlation between neck separation changes and nodal target volume dose was not obvious, indicating neck separation might not be a good indicator to reflect the dose change of target volumes in the neck node region. For parotid glands, both the extent and volume decreased steadily during the treatment course, in which every 1mm decrease of the parotid extent could reflect 2.12cm³ decrease of parotid volume and 2.7cGy increase in parotid gland mean dose. 

The correlation between organ shifts and dose changes provided a quick check for alerting oncologist the needs of replanning. By simply measuring the organ shift in daily MVCT, one can predict the possible actual delivered dose change and take action when departmental tolerance is reached. The proposed quick check is not limited to Tomotherapy case but can also apply to any image guided radiotherapy for NPC cases.