A review of the treatment of brain metastases using SRS and HFSRT.
Written by Kumari Vatsala
A brain metastasis is a cancer that has spread to the brain from another location in the body. It occurs when cancer cells spread from their original site to the brain and is called a secondary brain tumor. The most common sites of
primary cancer which metastasize to the brain are the lung, breast, colon, kidney, and skin. Being one of the most deadly forms of cancer, it has a rather low possibility of cure or remission without surgical intervention, but
recent advancements in the SRS and HFSRT treatments provide a chance of survival & recovery for several patients worldwide.
What are SRS and HFSRT?
Stereotactic radiosurgery (SRS) is a non-surgical radiation therapy used to treat functional abnormalities and small tumors of brain. It leads to lasting tumor control with only minor side effects compared to whole brain radiotherapy, since there is only a little dose delivered to the brain.
Hypofractionated stereotactic radiotherapy (HFSRT) is a similar procedure reserved for large metastases or lesions close to organs at risk (eg. the brainstem).
It is not surgery in the traditional sense as there are no incisions made. Instead, SRS uses 3D imaging to target high doses of radiation to the affected area with minimal impact on the surrounding healthy tissue. It works by damaging the DNA of the targeted cells. The affected cells then lose the ability to reproduce, which causes the tumors to shrink. Stereotactic radiosurgery of the brain and spine is typically completed in a single session. It is also used to treat lung, liver, adrenal and other soft tissue tumors, and the treatment typically involves multiple (three to five) sessions.
Doctors use three types of technology to deliver radiation during SRS in the brain and other parts of the body:
● Linear accelerator (LINAC) machines use X-rays (photons) to treat
cancerous and noncancerous abnormalities in the brain and other parts of
the body.
● Gamma Knife machines use 192 or 201 small beams of gamma rays to
target and treat cancerous and noncancerous brain abnormalities.
● Proton beam therapy (charged particle radiosurgery) is the newest type
of stereotactic radiosurgery which is used to treat brain tumors in a single session, and it is also used to treat body tumors over several sessions.
How does SRS/HFSRT work?
All types of stereotactic radiosurgery and radiotherapy work in a similar
manner. The specialized equipment focuses many small beams of radiation on a tumor or other target. Each beam has very little effect on the tissue it passes through, but a targeted dose of radiation is delivered to the site where all the beams intersect. The high dose of radiation delivered to the affected area causes tumors to shrink and blood vessels to close off over time following treatment, robbing the tumor of its blood supply.
The precision of stereotactic radiosurgery means there’s minimal damage to the healthy surrounding tissues. In most cases, radiosurgery has a lower risk of side effects compared to the other types of traditional surgery or radiation therapy.
Why is radiosurgery preferred?
SRS and HFSRT are been increasingly used as the standard treatment for brain metastases for several reasons. For few metastases, the advantages compared to neurosurgical resection include its noninvasive approach, suitability for outpatient treatment, ability to treat surgically surgically inaccesible areas such as the brainstem, and ability to treat multiple lesions. For multiple metastases, the advantages compared to whole brain radiotherapy (WBRT) include improved local control, fewer neurocognitive side effects, and a shorter treatment course. In addition, retrospective series have shown that radioresistant histologies including renal cell carcinoma and melanoma have control rates after SRS that are similar to radiosensitive tumor types.
How does the treatment work in relation to WBRT?
In addition to avoiding or delaying WBRT to prevent associated side effects, the one day course of SRS may have a variety of implications on the life of the patients. WBRT generally takes 2–3 weeks of daily treatment. For patients with poor prognosis, this may represent a significant percentage of the time they have left. For all patients, this may delay starting systemic treatment since chemotherapy is usually withheld during WBRT.
Case Studies:
1. The European Organization for Research and Treatment of Cancer
(EORTC) 22952–260001 trial randomized 359 patients with one to three
metastases treated with SRS or surgery alone to either WBRT or
observation with brain MRI every 3 months. Patients treated with SRS
alone and surgery alone had a 2 year relapse rate at the initial site of 31%
and 59%, respectively. The rate of relapse at new sites was 42% and 48%
for SRS alone and surgery alone, respectively. WBRT improved the rate of relapse at the initial and new sites, but did not change overall survival
(median 10.9 and 10.7 months with WBRT and observation,respectively).
2. The Japanese Radiation Oncology Study Group (JROSG) 99–1 trial also
randomized 132 patients with one to four brain metastases to SRS alone
or SRS and WBRT. The 12 month brain tumor recurrence rate was 46.8%
in the WBRT and SRS group and 76.4% for the SRS alone group.
However, median survival and death from neurological causes was not
significantly different.
CONCLUSION:
SRS and HFSRT have become increasingly important treatment techniques in the management of brain metastases. An approach of SRS alone as initial treatment of brain metastases has allowed patients to delay or avoid WBRT and its associated side effects. Further studies are necessary to determine which patients may benefit from this approach. One of the most critical questions is how the benefit is defined and from whose perspective it is— patient, provider, payer, or society. Many centers with high volume practices feel comfortable treating multiple lesions at multiple time points in patients with an excellent performance status. However, whether the cost of this approach is justified has yet to be defined.
REFERENCES:
https://www.wikipedia.org/
https://emedicine.medscape.com
https://www.ncbi.nlm.nih.gov/pmc/articles
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