Detectability of anatomical changes with prompt-gamma imaging: First systematic evaluation of clinical application during prostate-cancer proton therapy

Purpose: The development of online-adaptive proton therapy (PT) is an essential requirement to overcome limitations encountered by day-to-day variations of the patient anatomy. Range verification could play an essential role in an online feedback loop for the detection of treatment deviations such as anatomical changes. Here, we present results of the first systematic patient study regarding the detectability of anatomical changes by a prompt-gamma imaging (PGI) slit-camera system.

Materials & Methods:  For 15 prostate-cancer patients, PGI measurements were performed during 105 fractions (201 fields) with in-room control CTs. Field-wise doses on control CTs were manually classified whether showing relevant or non-relevant
changes. Spot-wise ground-truth range shift information was retrieved by integrated depth-dose (IDD) analyses serving for a field-wise ground-truth classification. Spot-wise PGI-based range shifts were initially compared to corresponding IDD shifts and then combined in a PGI-model to match the field-wise IDD-based classification. This model was optimized and tested for a sub-cohort of 10 and 5 patients, respectively.

Results:  The correlation between PGI and IDD range shifts was high, ρ_pearson = 0.67 (p<0.01). Field-wise binary PGI-classification resulted in an area under the curve (AUC) of 0.72 and 0.80 for training and test cohort, respectively. The model detected relevant anatomical changes in the independent test cohort with a sensitivity and specificity of 74% and 79%, respectively.

Conclusion:  For the first time, evidence of the detection capability of anatomical changes in prostate-cancer PT from clinically acquired PGI data is shown. This emphasizes the benefit of PGI-based range verification and demonstrates its potential for online-adaptive PT.