Dosimetric Comparison of Volumetric Modulated Arc Therapy, Tomotherapy and Intensity Modulated Radiation Therapy for Radiation Dose Escalation of Hepatic Malignancies

Abstract

Previously, radiation therapy has had a limited role in the management of hepatic malignancies due to the risk of radiation induced liver disease (RILD). However, recent advancements have allowed for dose escalation within the tumour volume while sparing normal tissues. These techniques have been employed, although comparisons regarding dosimetric integrity and the feasibility of dose escalation using normal tissue complication probability (NTCP) models have yet to be established. The aim of this study is to provide a dosimetric comparison of intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and Tomotherapy (HT) with regards to dose escalation for primary and metastatic hepatic malignancies. Seventeen clinically delivered IMRT plans and two clinically delivered VMAT plans for patients treated for liver cancer were retrospectively reviewed. Each data set was optimized using two alternate delivery techniques, providing each patient with an IMRT, VMAT and HT plan. The gross tumour volume (GTV) was defined by the original treating oncologist, no clinical target volume (CTV) was included and planning target volumes (PTV) were 3mm expansions. The dose prescription was escalated until an NTCP of 5% was exceeded or until dose-volume constraints of all other normal tissues, normalized to 2Gy equivalent biological effective dose (BED), was exceeded. All plans achieved intended normal tissue constraints, including dose to the normal liver. Mean doses for VMAT, HT and IMRT were 85.2±44.6Gy, 79.1±47Gy, and 72.4±41.5Gy, respectively. Analysis revealed statistical significance between escalation levels when comparing the three modalities to one another; VMAT vs. HT (p=<0.001), VMAT vs. IMRT (p<0.001), HT vs. IMRT (p=0.005). In four cases, dose escalations were limited by normal tissues, rather than 5% NTCP. A comparison between treatment times revealed significantly lower times for VMAT (149±42s) than HT (841±317s) (p<0.001). Analysis of monitor units (MU) for VMAT and IMRT depicted mean values of 1479±497MU and 1326±605MU, respectively (p=0.054). The utilization of partial arcs, coupled with the implementation of gating techniques allows VMAT to greatly reduce normal liver dose while achieving a high dose to the target volume. Overall, VMAT is superior in terms of dose escalation maximums and reduced treatment delivery time for hepatic cancer.