Purpose
The severe reduction of salivary function (xerostomia) is a common complication following radiation therapy for head and neck cancer. Consequently, guidelines to ensure adequate function based on parotid gland tolerance dose-volume parameters have been suggested by the QUANTEC group (1) and by Ortholan et al. (2). We perform a validation test of these guidelines against a prospectively collected dataset and compared to a previously published dataset.
Method and Materials
Whole-mouth stimulated salivary flow data from 66 head and neck cancer patients treated with radiotherapy at the British Columbia Cancer Agency (BCCA) were measured, and treatment planning data were abstracted. Flow measurements were collected from 50 patients at 3 months, and 60 patients at 12 month follow-up. Previously published data from a second institution (WUSTL) were used for comparison. A logistic model was used to describe the incidence of grade 4 xerostomia as a function of the mean dose of the spared parotid gland. The rate of correctly predicting the lack of xerostomia (negative predictive value, NPV) was computed for both the QUANTEC constraints and Ortholan et al. (2) recommendation to constrain the total volume of both glands receiving more than 40 Gy to less than 33%.
Results
Both data sets showed a rate of xerostomia < 20 % when the mean dose to the least-irradiated parotid gland is kept below 20 Gy. Logistic model parameters for the incidence of xerostomia at 12 months after therapy, based on the least-irradiated gland, were D50=32.4 Gy and and γ=0.97. NPVs for QUANTEC guideline were 94% (BCCA data), 90% (WUSTL data). For Ortholan et al. (2) guideline NPVs were 85% (BCCA), and 86% (WUSTL).
Conclusion
This confirms that the QUANTEC guideline effectively avoids xerostomia, and this is somewhat more effective than constraints on the volume receiving more than 40 Gy.
The use of functional imaging in radiotherapy treatment (RT) planning requires accurate co-registration of functional imaging scans to CT scans. We evaluated six methods of image registration for use in SPECT-guided radiotherapy treatment planning. Methods varied in complexity from 3D affine transform based on control points to diffeomorphic demons and level set non-rigid registration. Ten lung cancer patients underwent perfusion SPECT-scans prior to their radiotherapy. CT images from a hybrid SPECT/CT scanner were registered to a planning CT, and then the same transformation was applied to the SPECT images. According to registration evaluation measures computed based on the intensity difference between the registered CT images or based on target registration error, non-rigid registrations provided a higher degree of accuracy than rigid methods. However, due to the irregularities in some of the obtained deformation fields, warping the SPECT using these fields may result in unacceptable changes to the SPECT intensity distribution that would preclude use in RT planning. Moreover, the differences between intensity histograms in the original and registered SPECT image sets were the largest for diffeomorphic demons and level set methods. In conclusion, the use of intensity-based validation measures alone is not sufficient for SPECT/CT registration for RTTP. It was also found that the proper evaluation of image registration requires the use of several accuracy metrics.
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