Stereotactic Radiosurgery, SBRT and Proton Therapy

Stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) deliver very high radiation doses in a few sessions to a precisely defined target, with a steep dose fall-off outside it. SRS is used for the brain; SBRT applies the same technology elsewhere in the body, most often the lung and prostate. Proton therapy pushes precision further by depositing most of its energy at the tumour and almost none beyond it.

Dr. Garima Singh, Principal Consultant - Radiation Oncology, BLK-Max Super Speciality Hospital, New Delhi, India, covered both techniques in her masterclass, and doctors returned to them in the questions: the complications of stereotactic treatment, SBRT doses in early lung cancer, and her experience with children. The foundations these techniques build on, IMRT and image guidance, are explained in IMRT, IGRT and tomotherapy; the full overview is in the complete guide to advances in radiation oncology.

What stereotactic radiosurgery and SBRT are

Dr. Singh defined stereotactic radiosurgery by dose and number of sessions: more than 5 Gy per session, delivered in one to five sessions. Concentrating a high dose into few sessions increases the efficacy of the treatment against the tumour. Stereotactic radiotherapy (SRT) refers to the delivery of the same high-precision dose in multiple fractions to a defined target volume while sparing normal structures.

The vocabulary maps onto anatomy:

• SRS and fractionated SRT for lesions in the brain.
• SBRT, also called stereotactic ablative radiotherapy, for lesions in the rest of the body.

The technology can be delivered on several platforms. Dr. Singh listed linear accelerator-based radiosurgery, tomotherapy, the Gamma Knife, the CyberKnife and protons. What they share is precise targeting and a dose that falls away sharply at the edge of the target.

Indications for stereotactic radiosurgery in the brain

The range of SRS indications is wide. Dr. Singh grouped them as follows.

Her illustration was an MRI series from a patient with a metastatic brain lesion treated with SRS: six months after treatment, the metastatic disease had completely resolved.

Side effects of stereotactic radiosurgery: radiation necrosis and oedema

Asked by Dr. Ivan Ipavu from Uganda about the complications and prognosis of stereotactic treatment, Dr. Singh began with the principle that the risk depends on the site treated and on what lies next to it.

The two main complications in the brain are:

1. Radiation necrosis, death of brain tissue in the high-dose region. In Dr. Singh's figures, its incidence is below 10% when brain lesions are treated with SRS or fractionated radiosurgery.
2. Brain oedema, swelling around the treated lesion.

The risk rises when the tumour lies near a critical structure, such as the motor cortex or the brainstem. Dr. Singh noted that robust data on the dose the motor cortex tolerates are lacking; on the limited data available, she cited about 15 Gy to the motor cortex in a single SRS session. With higher-dose fractionated SRT or SRS, Dr. Singh stressed, the clinician must be especially vigilant about how close the organs at risk lie to the target while planning.

With that vigilance, her centre treats metastatic brain lesions safely with minimal complications. Her caveat was that stereotactic treatment is a sophisticated technology that needs expertise and careful dose painting to deliver.

SBRT for early lung cancer and lung metastases: doses by tumour location

For early-stage lung cancer, surgery remains the treatment of choice. Dr. Singh's answer to Dr. Ipavu's question on SBRT in early lung cancer set out where radiation comes in.

• Inoperable early lung cancer: when a patient cannot have surgery because of other illness, SBRT is the alternative.
• Lung metastases: when cancer from the breast or another site has spread to the lung, SBRT can be given upfront. SBRT for lung metastases is generally reserved for patients with a limited number of metastatic deposits (oligometastatic disease).

The dose depends on where the tumour sits. Lung tumours are classed as peripheral, central or ultracentral, where ultracentral means closely abutting the heart or other mediastinal structures. The closer the tumour lies to those structures, the more the dose has to be spread over additional sessions.

• Ultracentral tumours: Dr. Singh gives around 6 to 7 Gy per fraction over up to 10 fractions, 60 to 70 Gy in total.
• Peripheral tumours: 50 Gy in five sessions, or in some cases 55 Gy in five sessions.

Tumour volume and location together decide the schedule, and with that tailoring, she said, SBRT can be given to the lung safely. The dose constraints for organs at risk in SBRT are set out in published guidelines, which her centre follows.

Two practical points complete the picture. Because a small lung tumour moves with every breath, SBRT relies on gating or tumour tracking, described in the IMRT, IGRT and tomotherapy article. And in prostate cancer, Dr. Singh listed SBRT as an option for early-stage disease, with hypofractionated radiation used for locally advanced disease.

Proton therapy: the physics of minimal exit dose

Proton therapy is, in Dr. Singh's words, a significant advance in radiation oncology. The difference from conventional X-ray radiation is physical. An X-ray beam deposits dose all along its path, including after it has passed through the tumour. A proton beam deposits the majority of its energy at a specific depth, the Bragg peak, which can be set at the tumour, and has minimal exit dose beyond it.

Two consequences follow:

• Less damage to nearby healthy tissue, which makes protons especially useful for tumours next to critical organs.
• A very low integral dose, the total radiation absorbed by the body outside the target.

Dr. Singh described proton therapy as an emerging major treatment modality for:

• Craniospinal irradiation, for example in medulloblastoma.
• Craniopharyngioma.
• Skull base tumours, where better organ sparing is needed.
• Prostate cancer, where better local control is needed.
• Paediatric malignancies generally.

Protons, along with carbon ions, also allow dose escalation in prostate cancer beyond the 80 Gy achievable with IMRT and image guidance.

Proton therapy and radiotherapy for children: why integral dose matters

The paediatric case is where integral dose becomes decisive. Dr. Abraha Gebreegziabher from Ethiopia asked about Dr. Singh's experience with childhood cancers. She has treated paediatric patients, including around ten from Ethiopia.

Her first principle is that paediatric malignancy has to be treated very carefully. In medulloblastoma, where craniospinal irradiation is given, dose planning must be very precise to prevent radiation-related toxicity.

Her second principle concerns life after cure. Survival in tumours such as ependymoma and medulloblastoma is good, so the most important long-term problem is secondary malignancy, a new cancer caused by the radiation years later. That risk is driven by low-dose spillage across the body, the integral dose. Paying close attention to that spillage is, in her words, very important in children.

For paediatric patients needing long treatment fields, her centre generally uses tomotherapy, with very good outcomes. Proton therapy, with its lower integral dose, is the other route to the same goal. Dr. Singh's answers to the paediatric and stereotactic questions are recorded in full in the Live Discussion.

How stereotactic treatment and protons fit the wider picture

Stereotactic treatment and proton therapy sit at the far end of the same progression that runs from 3D conformal radiotherapy through IMRT and image guidance. Each step tightens the dose around the target. SRS and SBRT exploit that precision to give doses high enough to ablate a tumour in a handful of sessions, which also shortens treatment for patients who have travelled; the cost of radiotherapy page gives indicative pricing. Protons exploit physics to minimise the exit dose, which matters most when the patient is a child with decades of life ahead.

For referring doctors, the practical message from Dr. Singh's answers is to consider stereotactic treatment for small, well-defined targets: brain metastases, benign brain tumours such as meningioma and acoustic neuroma, early lung cancer in a patient who cannot have surgery, and lung metastases. Whether a given patient is suitable depends on the lesion's size and location relative to critical structures, which is a planning judgement for the radiation oncologist and the tumour board.