Prior to enrollment all patients will be evaluated with a physical exam, review of pathology and laboratory values to confirm diagnosis, and baseline imaging studies.
Accelerator
Physicians will treat with a stereotactic radiosurgery system using 6MV photons to deliver stereotactic body radiotherapy.
Doses
Patients will receive a total dose ranging from 50-75 Gy in 5 fractions (10-15 Gy/fx). Dose escalation will be via the traditional "up and down" scheme.
In determining the radiation dose and fractionation scheme for this protocol, we used the linear-quadratic formalism for radiation cell killing to "equate" schemes that vary the dose/fraction and number of fractions. This concept of biologically equivalent dose (BED) states that the total effect is given by:
nd x (1 + d/(alpha-beta ratio))
where n is the # of fractions and d is the dose/fraction. The "alpha-beta ratio" characterizes the radiation response of a particular tissue; a higher value is indicative of a tissue that responds acutely to the effects of radiation. Due to their highly proliferative nature, most tumors fall into this category.
This final dose scheme (total dose 75 Gy) is biologically equivalent to the previously studied doses in the literature (60 Gy in 3 fractions), meaning the first two sets of patients will be treated to a radiobiologically smaller (and likely safer) dose. We would favor treating in five fractions, as opposed to three, to allow more repair of normal tissue, reoxygenation of tumor cells, and redistribution of tumor cells to more radiosensitive parts of the cell cycle. Using a smaller fraction size, 10-15 Gy compared to 20 Gy, will also help reduce late effects of radiation therapy. SBRT treatment will be given on an every other day schedule, excluding weekends. The prescription dose will be prescribed to the isodose line best encompassing the planning target volume (PTV) depending on the volume of tumor (HCC).
Localization, immobilization, and simulation
Within 5 - 10 days after fiducial placement, pPatients will undergo 4D FDG-PET/CT simulation with the goal of evaluating tumor motion to allow for gated treatment when indicated. This goal will be accomplished by using the Real-time Position Management (RPM) system (Varian Medical Systems, Palo Alto, CA) to create a retrospective 4D CT scan. Following the institutional protocol, a helical CT scan and a 4D positron emission tomography (PET) scan with a patient with body immobilization device will be acquired. A patient will not eat or drink anything for four hours before the PET scan. Before the PET scan, blood sample will be taken from either a finger stick or a vein in the arm to check the sugar level. An injection of a small amount of a radioactive drug called FDG ( \[F18\] fluorodeoxyglucose) which is a chemical similar to sugar will be administered into a vein in the arm or hand. Approximately 45 to 60 minutes after the injection of FDG, the patient will be asked to urinate (to empty the bladder).
The patient will be set up in the PET/CT scanner using a vacuum cushion for immobilization in the supine position with feet tied and hands across the chest or above the head. There will also be a respiration-monitoring device called a marker block placed 5cm below the patient's xyphoid process. An infrared camera at the foot of the CT table will capture the images of the marker block and relay them to the RPM computer, which in turn will translate the images into a respiratory pattern. The audio coach (which instructs the patient in regulating breathing) will be calibrated to both patient comfort and time of expiration, inspiration, and full breathing cycle. The placing of the patient in a body immobilization device will take about 10-15 minutes. The patient will need to lie still for about 30 minutes before the completion of the 4D PET scan. The PET/CT scanner will then be programmed to acquire a retrospective 4D CT scan with a set of images for each phase of the breathing cycle. This scan will take place immediately after the PET scan. It will take around 5-10 minutes. The physician or physicist will then select the number of breathing phases to use while the software program selects the best image for each selected breathing phase.
The entire FDG-PET/CT scan procedure is expected to take about 2 hours.
Treatment Planning
Treatment planning will be carried out using the planning station for the radiosurgery equipment being used for treatment. The gross tumor volume (GTV) will be contoured on the fused image set. Two GTV volumes will be contoured; the gross tumor as seen on CT alone and the gross tumor corresponding to FDG avidity. No margins will be added for clinical target volume (CTV), but custom margins will be added for the planning target volume (PTV) based on the findings of the 4D FDG-PET/CT motion study assessment. The treatment will be prescribed to the isodose line that best covers the planning target volume, which will typically be the 80% isodose line.
Treatment Delivery
SBRT will take place within 14 days of the treatment planning scan. The planning data containing the coordinates of tumor isocenter, the external infrared markers, and the implanted markers are transferred to the appropriate platform depending on the treating machine. If the patient meets the criteria of gating technique then treatment delivery will be accomplished using the appropriate gating technology. Depending on the technology used external infrared markers attached to the patient's skin or a marker block placed on the patient's chest is used to determine the breathing pattern. The size of beam-on window will be determined based on the target motion as detected by the 4D FDG-PET/CT scan. The threshold for gated treatment delivery is determined based upon the target motion due to respiration.
The daily initial positioning during treatment delivery will be performed using lasers and skin marks and infrared optical markers as appropriate. The target isocenter will be verified using daily imaging. Depending on the platform used, the moving target will be positioned within the beam under infrared and/or image guidance
