Department of Radiation Oncology, University of Rochester, 601 Elmwood Avenue, Box 670, Rochester, NY 14642, USA
Reproducibility of Internal Target Positions for
Breath-held Conformal External-beam Radiotherapy
W. G. O'Dell, C. R. Maurer, Jr., M. C. Schell, A. Sandhu, P. Okunieff
Department of Neurosurgery, Stanford University, 300 Pasteur Drive, Room S-012, Stanford, CA 94305-5327, USA
Recent advances in Stereotactic Radiosurgery/Conformal Radiotherapy have made
it possible to deliver precise radiation therapy to small lesions while preserving
function to surrounding structures. Unfortunately, the application of 3D
conformal radiotherapy to mobile tumors in the lung and liver is geared toward
slowing the progression of disease rather than obtaining a cure. Here, the
traditional therapeutic approach is to measure the range over which the tumor
moves during the respiratory cycle and to then irradiate a volume that encloses
the entire tumor over its entire motion range. The oncologist’s dilemma is that
prescribing a lethal radiation dose to the area would not only kill the tumor but
also damage a sufficiently large volume of healthy tissue to cause significant
clinical repercussions, including death. Our ultimate goal is to hit, with a very
focused and high-dose radiation beam, moving targets within the body with such
high precision that we will cure these cancer patients of their disease while
sparing the surrounding healthy tissue.
The fundamental physiologic questions relevant to this approach are:
- What are the respiratory-derived motions typical of lesions in the lung
- What is the reproducibility in lesion positioning using end-expiration
- Can we use passive breathing and/or breath-holding without additional
complications for the treatment of lung and liver tumors?
IV. Radiotherapy Planning
1. Margin Specification: 7x7x10mm [APxRLxSI] |
The specified margin is > 3 s in each direction.
Thus, there is > 99.87% certainty of enclosing the
entire tumor on any given treatment fraction.
2. Radiation Dose|
For each tumor, two arcs were prescribed, offset by 20 degrees
(the yellow and blue arcs shown in the figure). Each fraction was
administered during a separate ~30s breath-hold. The 2-arc
protocol was repeated 10 times over a 2-week period to give a
total tumor exposure of 50 Gy using 6 MV X-rays.
3. Patient Repositioning using ExacTrac|
Automatic registration of 8 surface markers affixed to the patient’s chest
and abdomen was performed using the ExacTrac system (BrainLAB, AG) incorporated
into our Novalis treatment facility. The positions of all 8 skin markers
were monitored in real-time in all three dimensions to determine the extent
of variation in their position. A deviation of any surface marker position
exceeding 2mm in any direction, using the initial planning scan locations
for reference, signaled cessation of treatment until the patient repositioned
himself through another relaxed exhalation.
4. Lesion Localization Verification|
Repeat CT scans were obtained thrice during the 2-week treatment. These scans
were utilized to perform “virtual” treatments whereby the isodose
distributions from each plan were superimposed on each target for all time
points. Figure: repeat-day tumor contours overlaid onto the planning CT for
Lesion 3. The central yellow, purple and pink curves mark the day 1,
day 3 and day 6 locations.
V. Treatment Results
At the 7-week follow-up 4 of the 5 tumors had completely disappeared and the volume
of the radio-dense material at the site of the remaining lesion had decreased
significantly [Graph: CT1 is at treatment day 1; CT2 is at day 3; CT3 at day 6;
CT4 at day 9; and FU at 7-week Follow-Up]. There were no indications of clinical
side-effects and pulmonary function was normal. At the 6-month follow-up, a
radiologist determined that all tumors had been completely eradicated, however
there was evidence of pneumonitis at the sites of lesions 1&3.
Lesion 3: Before||
The variability in target 1D position over repeated end-expiration breath-holds
was found, on average, to be 1.1—2.3mm with the largest motion in the SI direction.
Using these values as a guide, radiotherapy treatments were planned and administered
to 5 lung lesions in a single patient. 50Gy doses were given to each lesion while
maintaining a favorable dose-volume histogram for the surrounding lung tissue. We
have shown the feasibility of using breath-holding and surface marker registration
to perform fractionated external-beam radiotherapy of internal, mobile targets.
Although a curative response of these tumors was likely achieved, our future work
will focus on obtaining a larger number of trials and better statistical analysis
of tumor position variability to enable us to further limit the margin size,
decreasing the incidence of pneumonitis. Real-time motion modeling and beam
gating are also being developed.
Balter, J., et al.
Improvement of CT-based treatment-planning models of
abdominal targets using static exhale imaging.
International Journal of Radiation
Oncology, Biology, Physics, 1998. 41(4): p. 939-43
Holland, A., J. Goldfarb, and R. Edelman
Diaphragmatic and cardiac motion during suspended breathing: preliminary
experience and implications for breath-hold MR imaging.
Radiology, 1998. 209(2): p. 483-9
The Authors wish to acknowledge the support of BrainLAB GmbH, and the University
of Rochester‘s James. P. Wilmot Cancer Center