Tomotherapy SBRT for liver metastases: recommendations for potential candidates based on tumor size and location

1
Tomotherapy SBRT for liver metastases
recommendations for potential candidates based on
tumor size and location J.M. Baisden, K. Sheng,
J.A. Molloy, J.M. Larner, and P.W.
Read Department of Radiation Oncology,
University of Virginia Health Science Center,
Charlottesville, Virginia, USA
Abstract
Results
Purpose/Objective Stereotactic Body
Radiotherapy (SBRT) has been shown to be
effective for local control of metastatic liver
tumors. Several groups have shown that SBRT is
well tolerated. Published schedules included
doses of 20-40 Gray in 2-4 fractions, 14-26 Gray
in 1 fraction and 30 Gray in 3 fractions. Side
effects were reported as transient, mildly
symptomatic grade 1 and 2 radiation-induced liver
disease only. Phase II trials are currently
examining the clinical effectiveness of this
treatment regimen in terms of both response and
local control. Helical tomotherapy delivery of
coplanar SBRT allows for rapid dose falloff and
improved sparing of normal tissues required for
the delivery of 60 Gray within millimeters of
normal organs. This study sought to develop
general guidelines for liver tumors suitable for
helical tomotherapy SBRT based on tumor size,
liver size and proximity to normal structures.
  Materials/Methods Hypothetical liver lesions
were created using an existing CT planning scan.
These GTV lesions were of sizes from 1 to 6 cm in
diameter and were constructed as cylindrical
structures of the stated diameter and similar
height. A 5mm radial expansion and a 10mm
craniocaudal expansion of the GTV was used to
create a PTV from each GTV. The Tomotherapy
inverse planning system was used to design a
treatment plan for each PTV, with a prescription
goal of 60 Gray in 3 fractions delivered to the
95 isodose line. Normal organ constraints were
as follows 30 Gray maximum dose to the heart,
stomach, and small intestine 18 Gray maximum
dose to the spinal cord 700 cc of normal liver
to receive lt 15 Gray 35 of the kidneys to
receive lt 15 Gray. For plans which did not meet
the constraints, the prescription was decreased
by 3 Gy incrementally until an acceptable dose
over 30 Gy was found. Multiple regression
analysis was performed on the resulting data
using SYSTAT11 software. This software package
was also used to create graphical representation
of the derived mathematical model.   Results
Evaluation of the optimization of treatment plans
indicated the tolerable dose delivered by helical
tomotherapy followed a mathematical model
represented by the equation Dose0.0819(Liver
volume) - 0.159(PTV volume) -26.73. This
relationship was found to be statistically
significant (plt0.005), with the standard error of
prediction being 3 Gy.  
Conclusions
This study provides a guideline for patients with
hepatic metastases who may be suitable for
Tomotherapy SBRT. Included is a predictive
function for likely tolerable SBRT dose based on
both PTV and liver volumes. This equation may be
helpful in the initial screening of potential
SBRT candidates and streamline the helical
tomotherapy SBRT planning process.