Experimental assessment of inter-centre variation and accuracy in SPR prediction within the European Particle Therapy Network

Purpose/Objective:

Experimental evaluation of inter-centre variation and absolute accuracy in stopping-power-ratio (SPR) prediction within the European Particle Therapy Network.

Material/methods:

A head&body phantom with 17 tissue surrogate inserts were scanned consecutively at the participating centres using their individual clinical scan protocol. The SPR calculation was performed using each centre’s CT scan and HLUT (Fig.1). The inter-centre variation and absolute accuracy in SPR prediction were quantified for lung, soft tissues and bones. To evaluate the integral effect on range prediction for typical clinical beams traversing different tissues, for three simplified beam paths the determined SPR deviations were accumulated according to their respective tissue distribution. So far, data from 12 out of 17 participating centres was analysed.

Results:

A 2σ inter-centre variation in SPR prediction of 7.4% and 6.1% relative to water was determined for the bone inserts in the head and body setup, respectively. Comparable results were observed for the lung tissue surrogates (5.8% and 2.8%). In the soft tissues, smaller variations were achieved (1.4% and 1.2%). For the three exemplary beam paths, inter-centre variations in relative range were 2.1% on average. Moreover, absolute range deviations from reference exceeded 2% in specific centres (Fig 2B).

Conclusion:

Large inter-centre variations in SPR prediction were observed in low- and high density tissue surrogates. The differences in deviation for bone between the two setups indicate a strong influence of scanning parameters such as the level of beam hardening correction, potentially resulting in range shifts of clinical relevance. Hence, inter-centre standardisation is highly desirable.