Emergence of stereotactic body radiation therapy and its impact on current and future clinical practice

Abstract

Stereotactic body radiation therapy (SBRT) is generally a tumor-ablative radiation modality using essential technologies capable of accurately and precisely damaging the target with a high dose while geometrically sparing innocent normal tissues. The intent, conduct, and tissue biology are all dramatically distinct from conventionally fractionated radiotherapy such that new understanding is required for its optimization. It is most practical, tolerable, and tumoricidal in its most potent form treating tumors in the lung and liver. However, it is increasingly being used for tumors adjacent to bowels and nervous tissue, albeit with somewhat less ablative potency. Its strengths include high rates of tumor eradication via a noninvasive, convenient outpatient treatment. Its weakness relates to the possibility of causing difficult-to-manage toxicity (eg, ulceration, stenosis, fibrosis, and even necrosis) that may occur considerably later after treatment, particularly in the vicinity of the body's many tubular structures (eg, organ hila, bowel). However, clinical trials in a variety of organs and sites have shown SBRT to result in good outcomes in properly selected patients. Given its short course, lack of need for recovery, and favorable overall toxicity profile, there is great hope that SBRT will find a prominent place in the treatment of metastatic cancer as a consolidative partner with systemic therapy. With considerable published experience, available required technologies and training, and many patients in need of local therapy, SBRT has found a place in the routine cancer-fighting arsenal.

Fig 1.

Comparison of stereotactic body radiation therapy (SBRT) plan in the left panel versus historical postage stamp anterior/posterior-directed field arrangements shown in the right panel. The SBRT plan uses advanced imaging and guidance to reduce the necessary margin around the tumor. In addition, it spares the high-dose (60 Gy, yellow) and intermediate-dose (30 Gy, green) volumes in exchange for a considerably larger low-dose (10 Gy, orange) volume.

Fig 2.

Dose-volume histogram of a prostate stereotactic body radiation therapy (SBRT) plan illustrating 50 Gy delivered on protocol over five treatments. The patient was immobilized in a custom-designed setup including a rectal balloon to limit the collateral dose. Note the intensity-modulated radiation therapy–facilitated sparing of specifically the rectal wall (at the expense of the rectal lumen) to avoid circumferential damage to rectal mucosa clonagens.

Fig 3.

Pre– and post–stereotactic body radiation therapy (SBRT) imaging of an unresectable pancreatic head cancer showing a favorable treatment response.

Fig 4.

Three-dimensional, axial, sagittal, and coronal views of dosimetry for stereotactic body radiation therapy (SBRT) for adjuvant therapy of breast cancer after lumpectomy in a patient treated on protocol. The tumor bed delineated by the heavy blue line constitutes normal breast tissue (possibly admixed with residual cancer cells). In this setting, radioablation is not advisable for this target because the higher therapeutic dose includes considerable late reacting normal tissue. However, the mechanics of SBRT still allow reduction of low and intermediate doses to adjacent breast, lung, and heart, facilitating hypofractionation.