DRAFT in progress
Clinical Applications of a Kilovoltage EPID
Arthur Boyer1, Christopher King1, Albert Koong1, Quynh Le1, Stine Korreman2,
Gary Luxton1, Todd Pawlicki1, Calvin Huntzinger3, Peter Munro3
1. Radiation Oncology Department, Stanford University. Stanford, CA
2. Department of Radiation Physics, Rigshospitalet, Copenhagen, Denmark
3. Varian Medical Systems. Palo Alto, CA
Image-guided radiotherapy (IGRT) can be carried out using both planar imaging and
cone-beam imaging. The purpose of this presentation is to describe specific clinical applications
of digital kilovoltage flat panel array integrated into a medical linear accelerator.
We have been evaluating a linear accelerator that has been designed for IGRT by the
addition of a kilovoltage x-ray source and a high performance kilovoltage imager to the gantry of
the accelerator. These additions are called the On-Board ImagerTM (OBI) by the manufacturer.
The x-ray source can operate at 40-150 kVp; the image receptor has a field size of 40cm x 30 cm;
can operate at up to 30 fps; and is sufficiently sensitive to image at exposure rates of 20 nGy/s.
The x-ray source and the kilovoltage imager are attached to robotic arms that allow great
flexibility in source and imager positioning. For instance, the imager can be placed between 0-80
cm from isocenter. In addition, the arms have active feedback to maintain high geometric
precision as the gantry moves. Given the technical capabilities of this system we are trying to
determine what specific clinical advantages can be realized by using planar digital kilovoltage
images to guide radiation therapy.
Hypofractionated radiotherapy for localized prostate cancer is justified on radiobiological
grounds by assuming a low value of the / ratio for prostate tumors of around 1.5 Gy
(significantly less than that of the 10Gy value observed for most tumors). This causes prostate
tumors to possess a high sensitivity to dose-per-fraction leading to a high therapeutic ratio for
hypofractionation. Using gold fiducials implanted in the prostate gland, 3-5mm margins can be
achieved on the GTV with the aid of OBI to verify the daily setup. A study is on-going that
investigates regimens from 5.5 Gy per fraction for 8 fractions over two weeks (total dose 44 Gy)
to 7.25 Gy per fraction for 5 fractions over 1 week (total dose 36.25 Gy).
Single fraction stereotactic radiotherapy of locally advanced pancreatic cancer is being
investigated in order to achieve several therapeutic goals. First, local control of pancreatic tumors
minimizes the risk of developing gastric or duodenal obstruction. Furthermore, local control
contributes to pain control in pancreatic cancer patients. And finally, radiosurgical ablation of the
primary tumor can theoretically prevent distant seeding from the primary tumor. All of these
factors are of clinical importance in pancreatic cancer patients. Treatment on this protocol
requires placement of 3-5 gold fiducials for targeting purposes. The fiducials will be placed
directly into the tumor under CT guidance or under direct visualization during surgery or
laparoscopy when possible. In conjunction with the imaging system, fiducials will serve to
identify the precise location of the pancreas tumor relative to these markers during radiosurgery
and confirm that the tumor does not move significantly with respect to the bony skeleton over the
course of treatment.
Following a more conventional regimen of large-field chemo-irradiation, patients will
receive a single fraction 25Gy stereotactic radiosurgical boost to the GTV. The hypodense
lesion representing the gross tumor volume (GTV) will be outlined on sequential axial
computed tomography images. The dose will be prescribed to the maximum isodose volume,
which completely covers the GTV. Dose to the adjacent normal tissue will be minimized.
During the treatment, live images of the patient are to be obtained with the OBI system.
DRAFT in progress
Fiducial locations in these images are extracted which are compared to the fiducial locations in
the CT scan of the patient to estimate the tumor movements.
Accurate delivery of radiation treatment to tumors in the abdomen and thorax is limited
by respiratory motion. In order to ensure that the tumor is irradiated in all phases of the
breathing cycle, a large margin has traditionally been added to the radiation fields. However,
this means that surrounding healthy tissues are also irradiated to high doses. By synchronizing
the radiation treatment to the breathing cycle, tumor motion related to breathing can potentially
be eliminated, the size of the radiation fields can be reduced, and normal surrounding tissues
can be spared. The OBI system will be used in studies to investigate the potential improvements
of respiratory management to synchronizing radiation treatment to the breathing cycles in
patients receiving radiation treatment to the thoracic or abdominal tumors.
We have recently treated 17 patients in feasibility study and noted no grade 3-5
toxicity in these patients with a minimal follow up of 3 months for all patients. With these data
establishing the relative safety of large single-fraction conformal irradiation of the lung and
surrounding structures, we have decided to proceed with a dose escalation study to determine the
maximal tolerated dose (MTD) for single fraction STR in patients with lung tumors.
The patient will undergo CT guided percutaneous placement of two to four gold fiducials (1
mm in length). The fiducials will be placed using a 18-19 gauge needle under computed
tomography (CT) guidance and local anesthesia. Ideally within 7 days of fiducial placement, a
radiation therapy immobilization device (such as the Alpha Cradle) will be custom made for each
patient who will then undergo a contrast CT scan through the entire thoracic cavity using 3mm
thick slices. An electronic device produces audio tones of different pitches to coach patients to
attain a desired regular breathing pattern. After the patient is able to achieve and comply with the
desired breathing pattern, a triggering signal can be sent directly to either the CT-scanner or the
linear accelerator to trigger radiation beam-on or -off, in correlation with the patients breathing
cycle. Instead of a standard treatment planning CT scan using free breathing, a respiratorycorrelated CT-scan will be performed, and used for treatment planning.
Within two weeks of the initial treatment planning imaging study, linac-based radiosurgery
will be administered. The Alpha Cradle will be used to minimize movement of the chest, spine,
and abdomen during treatment. During treatment, real time x-ray images of the patients chest are
obtained. Fiducial locations in the images are extracted and compared to the fiducial locations in
the CT scan of the patient to estimate tumor movements. Throughout the duration of the
treatment course, the pattern of breathing, the performance of the gating procedure and the patient
tolerance will be monitored and evaluated. After completion of the treatment course, the final
dose distribution will be evaluated, and compared to what would have been achieved without use
of respiratory correlation. Approximately 25 patients with thoracic tumors and 25 with
intraabdominal tumors will be recruited to the treatment study.
In addition, all patients will undergo pulmonary function tests before and at 3 months after
completion of radiation to evaluate for lung function changes using respiratory-correlated CT and
treatment. These changes will be compared to the patients in the feasibility portion to determine if
respiratory-correlated treatment can minimize radiation-related loss of lung function.
These are examples of the types of high precision image guided radiotherapy procedures that
are being investigated using the latest available technology. As more experienced is gained with
the technology, and as the technology itself is refined and improved, these procedures will become
more common place in the repertoire of cancer management practices available to radiation
oncology.