The M.D. Anderson Cancer Center (MDACC) has developed an anthropomorphic head
phantom for remote monitoring of stereotactic radiosurgery (SRS) treatments. The
phantom contains a 1.9 cm imageable target as well as TLD and radiochromic film. The
TLD allow us to estimate dose to the center of the target with an accuracy of ± 3%. The
radiochromic film allow us to measure thelocation of the dose gradients (field edges)
within ± 1 mm along three orthogonal axis. This phantom has been provided to the
community through the Radiological Physics Center (RPC) and the Radiation Dosimetry
Services (RDS) since 1995. The design of the phantom is presented as well as a
summary of the data obtained through the last four years of monitoring. The data
includes dose to the centerof the target, alignment of isodose distributions, and
comparisons of radiation treatment area with that determined by the institution’s
treatment planning system. We have also been able to compile data to compare dosimetry
(dose and localization precision) between Gamma-Knife and Linac based stereotactic
treatments. We have also collected demographic information on the modalities forlocalization, planning and delivering SRS treatments in our monitored population.
Materials and Methods
The remote monitoring system, shown in figure 1, consists of a water filled head shaped
plastic shell (The Phantom Laboratories, Salem, NY), this shell has been fitted to accept
inserts through the neck, these inserts are 7.5 cm diameter cylinders that reproducibly
align to a high degree ofprecision (0.1 mm). One insert, the imaging insert, is used to
establish a target in stereotactic space. This insert is water filled with a 1.9 cm (3/4”)
nylon ball suspended at a know location. This combination of nylon and water allows
localization with either CT or MRI. A second insert, the dosimetry insert, locates two
TLD capsules and two pieces of radiochromic film at a know location withrespect to the
location of the target. The film sheets are placed orthogonal to one another in the insert.
The films are pierced to indicate position and orientation while in the dosimetry insert.
The remote monitoring system is completed by the addition of software that utilizes
alignment marks on the film to locate the dose distributions in stereotatic space with
respect to the center of thetarget (figure 2.A, 2.b). This system has been found to work
with fixed and relocatable localization systems, with CT and MRI imaging and with
Linac and Gamma Knife treatment machines.
Table 1: Imaging Modality vs. Treatment Modality: We did not explicitly collect this data
so we havea large number of unspecified entries in this table.
Figure 1: The SRS Monitoring System: Shown in the figure is the water filled outer shell, the Imaging insert, partially
inserted into the outer shell, and on the right, the dosimetry insert, with the top removed to show the radiochromic film
used for localization.
Figure 2.A, 2.B: These are radiographic images of the phantom with theimaging insert in-place, the principle axis of
the phantom are labeled and these definitions are used through out this presentation. One of the pieces of photochromic
film lies in the plane of figure 2.A, the other lies in the Coronal plane, which is not shown.
Figure 3.A: SRS Analysis Software: This is screen capture from software used to localize the dose distribution with
respect to thetarget. Figure 3.B: Film Localization System: This demonstrates the system used to translate the two
alignment marks on the films to the location of the center of the target in a rotationally invariant manner. The rotation
of the film is determined separately based on the slope of the line between the points.
Field Size= D left + D right
Offset = 0.5 • (D left - D right)