An evaluation of rescanning technique for liver tumour treatments using a commercial PBS proton therapy system

Background and purpose: The treatment quality of pencil beam scanned (PBS) proton therapy to mobile tumour treatments can be compromised due to interplay effects. The aim of this work is to systematically evaluate the effectiveness of rescanning for liver tumour treatments for a commercial PBS delivery system.

Materials and methods: Plans were calculated to patient specific ITV's (2Gy_RBE), using spot spacings of 4 and 8mm for 1- and 3-field plans. 4D dose calculations were performed using regular and irregular motion extracted from nine 4DCT(MRI) liver datasets with 4 different starting phases. Up to 19 times adaptive-scaled layered and volumetric rescanning were simulated using beam profiles and delivery dynamics of a commercial proton therapy system.

Results: For small (∼10mm) motions, 3-field plans achieved CTV HI's (D5-D95) to within 8.5% (80th percentile) of the static case without rescanning. For larger motions, volumetric rescanning resulted in 4.5% improved HI in comparison to layered, but requires 5 times longer treatment times and is more sensitive to detailed plan characteristics and delivery dynamics. Increased spot spacings were found to reduce sensitivity to interplay and reduce delivery times by 60%, whilst reduced energy switching times decreased treatment time by up to 75% for volumetric rescanning without however improving plan quality.

Conclusion: For the investigated proton therapy system, rescanning can help recover dose homogeneity under conditions of motion but, particularly for motions over 10mm, should be combined with additional motion mitigation techniques.