Quantification of Skin Tone and Glossiness to Predict False Intrafraction Motion Detected by Surface Imaging During Frameless Stereotactic Radiosurgery

Objectives: : Surface imaging is a common tool to detect intrafraction motion for frameless radiosurgery. Prior studies at our institution have found a higher rate of false positive motion detection at non-zero couch angles, especially among African American patients. In the current study, we hypothesize that commercial dermatologic tools to quantitatively measure skin tone (Fitzpatrick scale) and skin glossiness will better identify patients at higher risk of false positive intrafraction motion.

Methods: : Patients receiving intracranial radiotherapy with HyperArcTM (Varian Medical Systems, Palo Alto, CA) were immobilized with an EncompassTM (Qfix, Avondale, PA) device. After CBCT localization, motion was monitored with the IdentifyTM system. Dermatological instruments, SkinPhotoTyperTM and SkinGlossMeterTM (Delfin, Miami, FL), were applied to the forehead to quantify skin tone and glossiness. Surface imaging logs were compared to quantified measurements of skin glossiness and tone.

Results: : Forty-two patients underwent HyperArcTM radiosurgery with skin measurements taken. 12/42 (29%) had darker skin tones (Fitzpatrick 5 or 6). At non-zero couch angles, these patients had greater absolute deviations from initial imaging when a camera was blocked than lighter skin tones (median 1.3 vs 0.8 mm, p=0.003). The magnitude shift when a camera was blocked vs unblocked was predicted by skin tone, (median 0.1 vs. 0.4mm, p< 0.001). Glossiness data was not variable enough for analysis.

Conclusion(s):: For patients undergoing frameless radiosurgery with real-time surface imaging to monitor intrafraction positioning, Fitzpatrick skin tone 5 or 6 predicts false positive deviations in patient positioning. Blocked cameras and higher Fitzpatrick skin tone increase the chance of observed deviations over our institutional threshold to continue treatment and increase the frequency of unnecessary intrafraction CBCT.