Normal liver tissue sparing by intensity-modulated proton stereotactic body radiotherapy for solitary liver tumours

Abstract

Background. Stereotactic body radiotherapy (SBRT) is often the preferred treatment for the advanced liver tumours which owing to tumour distribution, size and multi-focality are out of range of surgical resection or radiofrequency ablation. However, only a minority of patients with liver tumours may be candidates for conventional SBRT because of the limited radiation tolerance of normal liver, intestine and other normal tissues. Due to the favourable depth-dose characteristics of protons, intensity-modulated proton therapy (IMPT) may be a superior alternative to photon-based SBRT. The purpose of this treatment planning study was therefore to investigate the potential sparing of normal liver by IMPT compared to photon-based intensity-modulated radiotherapy (IMRT) for solitary liver tumours. Material and methods. Ten patients with solitary liver metastasis treated at our institution with multi-field SBRT were retrospectively re-planned with IMRT and proton pencil beam scanning techniques. For the proton plans, two to three coplanar fields were used in contrast to five to six coplanar and non-coplanar photon fields. The same planning objectives were used for both techniques. A risk adapted dose prescription to the PTV surface of 12.5–16.75 Gy × 3 was used. Results. The spared liver volume for IMPT was higher compared to IMRT in all 10 patients. At the highest prescription dose level, the median liver volume receiving less than 15 Gy was 1411 cm³ for IMPT and 955 cm³ for IMRT (p < 0.005); also the mean liver dose was lower with IMPT compared to IMRT (median 9.1 Gy vs. 20.0 Gy; p < 0.005). All IMPT and IMRT plans met the V_{D < 15 Gy} > 700 cm³ constraint. For the D_mean ≤ 15 Gy constraint, nine of 10 cases could be treated at the highest dose level using IMPT whereas with IMRT, only two cases met this constraint at the highest dose level and six at the lowest dose level. Conclusion. A considerable sparing of normal liver tissue can be obtained using proton-based SBRT for solitary liver tumours.

Acknowledgements

Supported by CIRRO – The Lundbeck Foundation Center for Interventional Research in Radiation Oncology, The Danish Council for Strategic Research, and Varian Medical Systems (Palo Alto, CA, USA). Morten Høyer is supported by a grant from the A.P. Møller og hustru Chastine Mc-Kinney Møllers Fond til almene Formaal.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.