SRS

Question 1

the formula to cover the CTV for 90% of the patients with the 95% line…
PTV margin = 2.5 (upper case sigma - quadratic sum of SD of all systematic errors) + 0.7 (lower case sigma- quadratic sum of all SD of all random errors)

Is this valid for SRS?

Answer 1

No- smaller number of fractions impacts model. Random errors become systematic errors in the limit of 1-5 fractions

Question 2

advantages of linac based srs

Answer 2

better conformity for irregular targets, improved dose homogeneity inside the target, less time consuming planning, shorter treatments, linac not limited to cranial

Question 3

advantages of linac and frameless

Answer 3

patient comfort, ease of tx and workflow, comparable positioning accuracy, hypofractionated treatments

Question 4

in srs, dose and dose fractionization depends on…

Answer 4

Volume…fractionate when more than 3 cm diameter,
Location…next to critical struction
Disease…different for different types

Question 5

Radionecrosis

Answer 5

V 24Gy… 16.8 cc

Question 6

imaging

Answer 6

metastatic...mri with gadolinium
T1 post contrast...thin slice
small non enhancing lesions may be seen on T2...CT head w/contrast if mr not avail.,but may miss small post fossa lesions
AVM...CtA, DSA, MRA
Trigeminal Neuralgia...T1 post, FIESTA

Question 7

MRI issue

Answer 7

distortions…TG-54,

TG-117 QC

Question 8

Value in arc

Answer 8

as number of mets increases, MU ration decreases…can treat with fewer Mus when using arc, which saves integral dose and time

Question 9

MLC width?

for

Answer 9

11% diff in Conformity index… 5 mm vs

Question 10

target structure…

Answer 10

make sure it is not jagged, as this causes issues with dose hot spots…

Calc grid size

Question 11

where do you get critical structure constraints

Answer 11

TG-101

Cranial nerve 12.5 to 15 Gy