DOS 523 - Week 6 Discussion
Initial Post: Precision Therapy Acronym Soup: SRS, SRT, SBRT, SAbR
During my internship so far, I observed a stereotactic radiotherapy (SRT) procedure to treat Grave's disease and a stereotactic body radiotherapy (SBRT) procedure to treat a small lung lesion. I have not yet observed a stereotactic radiosurgery (SRS) procedure. I spoke with physicist Sébastien Gros (Oral communication, April 1, 2015) who helped me understand the practical differences and the similarities between the techniques. The three techniques are closely related with a few key distinctions. They are all capable of being delivered on a linear accelerator (linac) and all of them require extreme precision in patient positioning because these techniques are typically used when sparing of immediately adjacent normal tissue is required. The distinction between SRT and SBRT is simply the location of where it is delivered. The "B" in SBRT means that it is delivered in the body, in order to distinguish the technique from SRT, which is only used for intracranial and facial nerve applications. Both SRT and SBRT use a fractionated schedule of delivery, although that schedule may be accelerated in a hypofractionated regime of as few as 5 treatment sessions. SRS is often delivered as a single high dose fraction, but it is sometimes stretched out to as many as 5 fractions.
Much like standard forms a radiotherapy, the goal of SRT is to disrupt cell division processes without completely destroying the local tissue. SRS, on the other hand, is an ablative technique where the goal is to deliver sufficient dose to the target area to kill all the cells in the target region. SRS is sometimes called Stereotactic Ablative Radiotherapy (SAbR), a name which can also apply to SBRT if the SBRT regime is set up to have ablative doses and fractionation.1
According to Sebastien, SRS and SRT can treat a variety of conditions, including malignant tumors such as glioblastoma multiforme (GBM), and metastases of tumors from the breast, lung, kidney, or melanoma. Benign conditions like acoustic neuromas and trigeminal neuralgia are also often treated with SRS and SRT, as are arteriovenous malformations.
The pros of SRS and SRT are primarily in the form of reduced radiation exposure to other adjacent tissues. SRS and SRT typically have sub-millimeter setup tolerances because no PTV margin for setup error is used. The extreme level of accuracy means that tumors immediately adjacent to critical structures can be treated. The high level of accuracy can also be a con, because one of the most sure ways of guaranteeing setup accuracy is to use a stereotactic frame that actually screws into the patient's skull through their skin. This is an invasive procedure, but it does allow the patient's position in space to be known with submillimeter accuracy. Frameless options for patient setup do exist for SRS and SRT systems, and these typically involve special reinforced thermoplastic masks and advanced tracking systems that allow the patient to be be moved relative to the isocenter in submillimeter increments.
Many linacs are capable of delivering SRS and SRT treatments, but some machines are specialized for the purpose. Loyola has had a Novalis system since 2003, which is a specially designed treatment head with smaller than usual multileaf collimator (MLC) leaves, called a micro-MLC, mounted on a Varian linac. This machine has reached the end of its service life and is about to be replaced with a new Varian Edge system, which also has a micro-MLC treatment head. The Edge has the latest image guidance tools available, and it can also deliver more traditional treatments instead of being an SRS/SRT "one trick pony". This means we will have the latest SRS/SRT capabilities while also expanding capacity for traditional treatments, instead of having a specialty machine that sits idle for most of the day. GammaKnife is another treatment system that can be used to deliver SRS, but it can not deliver SRT. GammaKnife uses a constellation of cobalt-60 sources that are shielded behind banks of doors that can be selective opened and closed to provide a shot of radiation to a precisely targeted area. CyberKnife is another system that can be used for SRS, SRT, or SBRT. It uses a small linear accelerator mounted on a robotic arm that can be positioned to deliver precise treatment from any angle, not just from selected angles in a single plane. CyberKnife has real-time tracking capabilities and can follow moving targets precisely.
Tomotherapy and particle therapy (in particular proton therapy) are other forms of precisely targeted treatment, but these do not have the same level of setup accuracy that would be needed for true SRS/SRT. During my time at Seattle Cancer Care Alliance Proton Therapy - A ProCure Center, I helped prepare many patients for intracranial proton planning. Our neurooncologist, Dr. Jason Rockhill, splits his patient load between the proton center and a GammaKnife center based on the size of the lesions and the precision needed for treatment. GammaKnife is used for small, regularly shaped lesions that require extreme precision, and the Bragg peak of proton therapy is used to high advantage with moderate to large sized lesions, where the lack of exit dose prevents the whole brain from receiving a low dose bath. The Bragg peak of a proton beam can also allow retreatment in some areas if the most logical photon path to the tumor has already received a high dose and a new path that might point directly at a critical organ at risk must be used. The Bragg peak will ensure that the beam stops before it reaches the organ at risk.
- Solberg TD, Siddon RL, Kavanagh B. Historical development of stereotactic ablative radiotherapy. Stereotactic Body Radiation Therapy. Berlin: Springer; 2012:9-35.