Terms in this set (39)
What is the primary molecular interaction of ionizing radiation in the cell?
Water-->OH radical, which interacts with DNA, causing genetic damage.
What is Bragg's peak?
Protons deposit their energy over a specified depth over a more tightly defined area than do photons.
How does a linear accelerator work?
A very high energy current runs through a filament so that released into a microwave guide. Pulsing the microwave guide accelerates the electrons to nearly the speed of light and then the electrons impact a tungsten target, causing release of x-ray photons which are collimated, shaped, and modulated.
What are current technical abilities of LINACs?
<1mm accuracy and dose rates >10Gy/minute.
How does the Gamma Knife work?
201 cobalt sources are targeted at a focal spot. Each source can be controlled by 4, 8, and 16mm collimators.
What is the single dose sensitivity of the optic apparatus?
What are the four R's of radiobiology?
1) Repair of nonlethal injury
2) Reoxygenation of hypoxic tumor cells
3) Repopulation of tumor cells
4) Reassortment of tumor cells into more susceptible phases
What is the tolerance of whole brain to radiation?
45-50 Gy in 20-25 fractions, although this may yield substantial dementia and memory loss with time.
What is brachytherapy?
Radioactive source is placed within the patient and left for a predetermined period.
What characterizes radiation necrosis?
Coagulative necrosis affecting predominantly white matter due to small artery injury and thrombotic occlusion, occuring months to decades after injury.
What characteristic of radiosurgery is associated with development of radiation induced changes?
The 12-Gy volume is correlated with changes.
How is radiation necrosis treated?
Steroids, bevacizumab, resection.
What minimum distance between tumor and the optic apparatus is required for radiosurgery?
What are acute side effects of whole brain radiation?
Fatigue, hair loss, erythema, otitis.
What is the standard whole brain radiotherapy schedule for metastases?
30 Gy in 10 fractions.
Does whole brain radiotherapy provide a benefit after local control(surgery or radiosurgery) of visible metastases?
Debatable...improved local control but no difference in overall survival in RCTs to date.
What is the standard XRT regimen for malignant glioma?
60 Gy in 30 fractions.
What other CNS tumors are treated with XRT?
Primary CNS lymphoma and malignant meningiomas, hemangiopericytomas (30 x 2Gy).
What is the significance of a Lhermitte's sign after spine radiation?
A Lhermitte's sign is a transient finding of a self-limited, reversible myelopathy occuring 2-6 months after treatment and not associated with chroic progressive myelitis.
What is the typical time of onset of chronic radiation myelopathy?
Bimodal, with peaks at 13 and 29 months.
Are there any identifiable risk factors for chronic myelopathy in patients receiving less than 50 Gy?
No, it is extremely rare and idiosyncratic, although intensive chemotherapy may increase the risk.
What is the dose required to cause a 5% risk of myelopathy at 5 years?
In the range of 57 to 61 Gy.
What is a risk of high dose spinal XRT in children?
Kyphoscoliosis and short stature in pre-pubertal children.
What is the most objective informative tool for evaluating a radiosurgery plan?
The dose-volume histogram. Dose (%) on the X axis and Volume (%) on the Y axis. The ideal cumulative DVH shows a perfect horizontal line parallel to the x-axis denoting the entire PTV receiving 100% of the radiation dose and a vertical line parallel to the y-axis reflecting precise and steep falloff.
What are the limitations of SRS for the treatment of brain metastases?
Does not provide rapid relief of symptomatic mass effect.
Limited applicability for large metastatic lesions (>3cm diameter due to increasing normal brain radiation exposure).
What factors influence the survival of patients with brain metastases?
The RTOG RPA class is based on age, KPS, primary tumor status, and status of systemic disease.
What is the role of SRS in GBM?
There is no evidence for benefit of SRS in primary or recurrent GBM.
What is the role of SRS in other glial tumors (ependymoma, brainstem glioma, medulloblastoma)?
Debatable. There are case series but no RCTs.
What is the standard SRS dose for vestibular schwannomas?
12 to 13 Gy to the tumor margin.
What is the Koos classificaiton of vestibular schwannomas?
Class I--Intracanalicular tumor
Class II--Tumor in the CP angle but not reaching the pons
Class III--Tumor reaching the pons but not compressing
Class IV--Tumor compressing the pons and 4th V
What vestibular schwannomas are candidates for SRS?
Koos Classes I, II, and III, and some grade IV. Some centers, ie Marseille, use SRS as initial treatment of Koos I tumors (prior to tumor progression) in an attempt to preserve hearing.
What is an indication for fractionated stereotactic radiotherapy?
Very high control rates have been shown with optic nerve sheath meningioma.
What are two radiation therapy options for treatment of vestibular schwannomas?
Single fraction stereotactic radiosurgery or fractionated stereotactic radiotherapy.
Is the risk of hypopituitarism higher with SRS or fractionated radiotherapy?
Because the risk for hypopituitarism is lower with SRS than with conventional radiotherapy, SRS seems to be a valuable adjunctive treatment after unsuccessful surgery, as well as an alternative treatment in patients who tolerate medical therapy poorly or as a first-line treatment in patients with contraindications to surgical treatment of a small tumor.
What are the rates of AVM obliteration with radiosurgery doses?
Assuming that the nidus is completely covered, the chance of AVM cure is approximately 70%, 80%, and 90% for radiation doses of 16, 18, and 20 Gy, respectively
Is the risk of AVM hemorrhage increased, decreased or unchanged in the interval after radiosurgery?
The primary drawback of AVM radiosurgery is that patients remain at risk for hemorrhage until the AVM has eventually been completely obliterated. Despite early papers on AVM radiosurgery suggesting increased risk for bleeding before documented obliteration of AVMs,[30,64] later, more detailed analyses of this topic have concluded that the risk for AVM bleeding is either unchanged or reduced during this latency interval.[65-69] Karlsson and colleagues analyzed the large AVM experience at the Karolinska Institute and found that some measure of protection occurred as early as 6 months after radiosurgery for patients receiving an AVM margin dose of 25 Gy. Maruyama and associates performed a retrospective observational study of 500 AVM patients who underwent radiosurgery. In comparing the risk for bleeding before and after radiosurgery, they found a 54% reduction in bleeding risk during the latency interval.
Is repeated radiosurgery possible for AVMs?
Obliteration rates are similar after repeat radiosurgery--series have reported 60-70% obliteration. The complication rate may be higher.
What are radiosurgical options for large AVMs?
Hypofractionated stereotactic radiotherapy or staged-volume radiosurgery.
What factors predict success and complications of AVM radiosurgery?
The "Pittsburgh AVM score" is commonly used.
AVM score = (0.1) (AVM volume [cc]) + (0.02) (Patient age [yr]) + (0.5) (AVM location [hemispheric/cerebellar = 0, BG/thalamus/BS = 1])
TABLE 260-1 -- Modified Radiosurgery-Based Grading System for Arteriovenous Malformations
AVM SCORE* EXC DECLINE IN MRS
≤1.00 (n = 53) 89% 0%
1.01-1.50 (n = 83) 70% 13%
1.51-2.00 (n = 61) 64% 20%
>2.00 (n = 50) 46% 36%
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