Treatments and Technology

Stereotactic Body Radiation Therapy


Overview


Stereotactic radiation therapy employs special equipment to deliver cancer destroying doses of radiation while decreasing the normal tissue included in the treatment field.  Using special equipment to position the patient and localize the tumor allows precise delivery of radiation therapy to the tumor.  This has been used for many years for tumors in the brain for both benign and malignant tumors.  Treatment in the brain is typically performed in one session, using rigid immobilization, and it is called stereotactic radiosurgery (SRS) since it is typically performed in one session.
SBRT-NM1

Rigid immobilization is not possible for tumors in the body and there is the added problem of daily variation in patient setup and positioning, organ changes between treatments (for example, variable filling of bladder or bowels), and organ motion during treatment (due to the natural motion of the heart and lungs).  Recent advances in patient immobilization, in imaging, and daily tumor localization and tracking now allow treatment with nearly the same precision as those treatments for the brain.  These advances have enabled the development of stereotactic body radiation therapy (SBRT).  This technique allows the treatment from many different points of entry which allows for smaller total doses to normal tissue, and allows the delivery of much higher doses to the tumor during each treatment and increases the likelihood of tumor control.

SBRT is most useful in situations in which we can clearly identify the tumor, use imaging to localize the tumor before and during treatment, and in tumors that are not too close to the skin surface or involve critical normal structures.  Tumors in the lung frequently meet these criteria, and that is where we have the most experience and most mature results with SBRT.  Other treatment sites have included liver metastases, spinal metastases or paraspinal tumors, solitary bone metastases, and primary tumors of the prostate and pancreas.

SBRT-NM2The standard of care for a patient with a Stage I non-small cell carcinoma of the lung involves a lobectomy with nodal dissection.  However, there are many poor risk patients who cannot tolerate a lobectomy because of compromised pulmonary or cardiac function.  Standard radiation therapy for these poor risk patients involves doses of about 70 Gy given over a period of 7 weeks (30 to 35 daily treatments).  With this treatment, the local control rate is about 50-60%.  SBRT is a better option for these poor risk patients.  There are multiple reports of SBRT for early stage lung cancer with local control rates consistently greater than 90%.  Side effects are uncommon but are dependent on the size and location of the tumor.

SBRT can be given on an adapted standard linear accelerator, such as a Varian Trilogy and Clinac iX, on an helical linear accelerator, called TomoTherapy, or on a robotic arm linear accelerator (CyberKnife).  It is done as an outpatient, typically in 3 to 5 treatments, depending on the tumor type, size and location.  The CyberKnife typically requires an additional procedure for the placement of markers into the tumor because the imaging on that machine is done with 2-dimensional x-rays; these markers are typically placed percutaneously with some risk of a pneumothorax or bleeding.  The advantages of the Trilogy or Tomotherapy systems are that no fiducial markers are required as the tumor imaging is done with CT scans (so there is volumetric 3-dimensional imaging for target localization), and the time in the treatment room is typically much shorter for these than for a CyberKnife session.   In addition, the Trilogy or Tomotherapy units have more shielding in the head of the machines, as compared to the Cyberknife machines which have been shown to have higher “leakage” rates which thus delivers more unwanted radiation to normal tissues even further away from the target.

SBRT-NM3The advantages of SBRT for lung cancer are that a treatment course is completed rapidly, a smaller volume of normal tissue is treated with lower risks of complications, and tumor control appears to be significantly better.  There are current studies to evaluate the optimal dose and number of treatments for early stage lung cancers.  There is also a Dutch study comparing lobectomy to SBRT for regular risk Stage IA lung cancer patients.  Additional studies will also be necessary to determine the role of adjuvant chemotherapy following SBRT or the roll of SBRT as a portion of a standard course of radiation.  Based on the results from treatment of primary lung cancers, liver metastases, bone metastases and other sites, it is likely that SBRT will be used with increasing frequency in the coming months and years.

 

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