2nd Year: SRS/SRT Site Policy, Procedures and Planning

Question 1

What IMRT QA do we do for SRS/SRT? What are the gamma criteria?

Answer 1

**SRT: **Only MapCheck
SRS Single Lesion: Only Lucy+EBT XD Film
SRS Multi-Lesion: Lucy+EBT XD Film AND MapCheck

• Lucy+Film is 3%, 1 mm, 10% threshold, 95% pass rate (level 2: drop to 90%) (level 3: investigate)
• MapCheck is 3%, 2 mm, 10% threshold, 95% pass rate (level 2: drop to 90%) (level 3: ArcCheck (only if SRT) and Lucy/Film)

Note: ArcCheck is never a fallback for SRS per our protocols. Only in rare cases for SRTs

Question 2

What is the small field correction factor for our A16 ion chamber?

Answer 2

1.027

Question 3

In cone planning, everything in the body contour is assumed to be water. What is the attenuation differences induced error that results from the skull also being assumed to be water in a 6 MV beam?

Answer 3

Approximate thickness of skull is 0.75 cm
Skull has a density of 1.4x that of water
Equivalent thickness of skull is 1.05 cm
1.05 - 0.75 = 0.3 cm

For a 6 MV beam, attenuation is approximately 4%/cm. So error is…

0.3 cm * 4%/cm = 1.2%

The TPS over-estimates dose at target by 1.2%

Question 4

With cone plans, is the plan approved by the physician in cone-planning or eclipse? Why?

Answer 4

The plan is approved in eclipse

This is because in the past there was isuses with plan normalizations being difference between cone planning and eclipse. When something is approved in cone-planning, then sent to eclipse, the normalization of the approved plan could potentially change in transit.

Question 5

Which mask do we use for SRS? What about SRT?

Answer 5

For both, we use the encompass open mask

We use this with OSMS

Question 6

For whole brain or hippocampal sparing, which mask do we use?

Answer 6

Either the Aquaplast short maskw ith open eyes and mouth
OR
Encompass open mask, no OSMS

Question 7

Prior to treatment delivery, there are a lot of last second checks that the morning physicist has to perform. What are the ones of importance relating to daily QA results? (5 listed)

Answer 7

1. WL Max Deviation < 1 mm and Mean Deviation < 0.5 mm
2. If Cone SRS, Cone WL Deviation < 1 mm
3. OSMS daily QA < 0.5 mm
4. MPC for beam geometry and 6FFF and 10FFF beam check all pass
5. Isocal verification within tolerance

Question 8

An SRS/SRT patient is currently on the treatment couch, setup, ready for treatment. What are the pre-treatment delivery steps that you take with the MD and therapists? (10 listed)

Answer 8

1. Usual timeout is performed
2. Take the first CBCT
3. Therapists fuse the first CBCT and get approval from MD
4. Apply first CBCT shifts
5. Take OSMS reference image
6. Perform second CBCT with the MD present
7. MD will perform a fusion. If the fusion is within < 0.5 mm and 0.5 deg, approve the fusion but do not apply the shifts
8. If shifts exceed above tolerance, apply shifts, then take another verification CBCT
9. Lock couch linear axes before beam-on
10. Beam on

**Note: **the second CBCT is purely as a double check to make sure that the first CBCT shifts were good

Question 9

Which MRI sequence is used for planning of SRS/SRT?

Answer 9

T1 is the go-to. In the past we only used T1

However, since there is a shortage of the good contrast, we have shifted to a newer contrast that is difficult to localize some lesions, so the rad oncs have also requested a T2-Flair be taken since this helps localize swelling around lesions

Question 10

How long is a MR image valid which is used for treatment planning?

Answer 10

14 days

The patient must be treated within 14 days of the MR used for treatment planning being scanned

Question 11

What 4 documents does the sim therapist need in aria before the start of a CT Sim for SRS/SRT?

Answer 11

1. Patient consent
2. Sim order
3. Pathology report
4. MRI report

Question 12

Do you include the encompass mask in body contour for a cone plan? Why or why not?

Answer 12

NO! Never contour mask for cone plan

Cone plan assumes everything in body contour is water. So it’ll assume the mask is water, and any air gap between mask and skin is also water. It’ll also mess up the SSD parameter

Question 13

Do you insert the encompass couch insert for cone planning?

Answer 13

You can if you want, but it’s pointless. Only what is in the body will be considered for cone planning. Anything outside of the body will be assumed to be air

Question 14

What setup material are needed during CT sim?

Answer 14

Encompass board isnert
Encompass hand grips
Encompass grey E0 head rest
Encompass masks
Blue table pad
Red knee sponge
Hair tie and/or shower cap for patients with long hair

Question 15

Why is patient hair important to account for in SRS/SRT setup?

Answer 15

You want reporoducibility

So you want want to make sure that the patient will not change their facial hair (ideally no facial hair) or head hair between sim and treatment, and that both are tame enough for a reproducible setup

Question 16

Is the patient simmed with chin up or chin down? Why?

Answer 16

Chin down to a comfortable position

Better for OSMS

Question 17

For SRS/SRT, where do you place the user origin BB’s on patient?

Answer 17

Trick question, you don’t

The encompass tabletop insert has 3 BB’s embedded in it. You use those. The purpose of them is to ensure that the 35 cm FOV covers the entire head + some inferior

Question 18

What are the scan protocols for SRS/SRT?

Answer 18

SFOV=DFOV= 35 cm
Slice thickness = 1.00 mm
kVp = 120
Smart mA at max = 700 mA
Rotation period = 0.5 s

Question 19

For MR to CT image registration, what is the recommended technique for registration?

Answer 19

Rigid registration
Align to intensity range, bony
Set ROI to encompass as closely as possible the entire skull

Question 20

What are some identifying features for a good MR to CT registration for SRS/SRT?

Answer 20

Bony anatomy alignment, especially around eye
Soft tissue alignment, especially around ventricals

Question 21

For Multi-Lesion LINAC SRS, where do you place isocenter?

Answer 21

centroid of total target volume

Question 22

For a single lesion SRS, what are the size dependent rules for use of dynamic jaw tracking? Why does this rule only apply to single lesion SRS?

Answer 22

If lesion is large enough that field size required for good coverage is > 2 cm, use dynamic jaw tracking with MLC modulation

If lesion is small enough that field size required for good coverage is < 2 cm, fix the jaw size at 2x2 cm2 and use MLC modulation

For multi lesion your jaws have to be dynamic since the projected distance between lesions is angle dependent. You can’t just have one jaw setting otherwise you can miss at certain angles. You can have an all encompassing jaw angle, but then it’d be large

Question 23

When creating a GTV/CTV/PTV structure for the physician, what do you do?

Answer 23

Make sure they are all high resolution
Name them according to plan and lesion number

Question 24

What margin is used for GTV –> PTV?

Answer 24

2 mm

Question 25

For IMRT SRS, what structurs must be included in the calc?

Answer 25

Encompass couch insert
Mask contoured with the body
The entire body

Question 26

What dose grid size is used for SRS? What algorithm?

Answer 26

1 mm dose grid size
AAA

Question 27

What is the most shallow lesion depth that a cone plan may be used to treat?

Answer 27

1 cm

Question 28

In general, what are the constraints that stop one from doing cone planning?

Answer 28

One of these must be true…

Depth < 1.5 cm
Multi-lesion
Lesion near bone or air
Irregularly shaped lesion
High density materials near treatment area

Question 29

What tolerances do we use for isocal verification?

Answer 29

Max Imager Shift kV and MV < 0.035 cm
Max deviation from central beam < 0.05 cm
In-plane imager rotations < 0.2 deg

Question 30

What is the tolerance for OSMS daily QA that we use?

Answer 30

0.5 mm

Question 31

What collimator jaw size is used for cones? What is the maximum allowed collimator size per Varian?

Answer 31

5 x 5 cm2

Max allowed is 5.6 x 5.6 cm2

Question 32

For cone plans, what is the dose grid size?

Answer 32

0.5 mm

Question 33

What is required slice thickness of CBCT for SRS/SRT?

Answer 33

1 mm

Question 34

What pieces of information are to be listed on a written directive for SRS/SRT? (8 listed)

Answer 34

1. Patient name
2. DOB
3. Patient MRN
4. Beam energy
5. Modality
6. Target dose
7. Dose/Fx
8. Treatment site

Question 35

What is the tolerance on stereotactic CBCT couch shifts from shifts off original plan that we use in our clinic?

That is, when patient is aligned to BB’s, we then apply the shifts given from the plan and take a CBCT. What is the maximum shifts that the CBCT can yield before a double check of plan printout/registration is needed?

Answer 35

5 mm in any translational motion
2 deg in pitch, roll or rotation

Question 36

What is the intracranial SRS OSMS tolerances? Why is the translational seemingly high?

Answer 36

2 mm translational magnitude
1 deg tolerance on pitch, roll, rotation

OSMS is a combination of inherent OSMS uncertainty, isocentricity, and patient motion. To make the OSMS tolerance 1 mm is asking for alot. 2 mm is more reasonable, and remember that the GTV –> PTV margin is also 2 mm

Question 37

What information is discussed during SRS/SRT time out?

Answer 37

Patient armband and photos in room

Beam parameters (collimator angle, gantry start and end angles, couch kicks, energy, MUs)

Patient name, MRN, DOB

Fraction number

Dose per fraction

Treatment site

Question 38

What selection criteria is used for SRS/SRT over other modalities for intracranial?

Answer 38

Patients should be very high functioning (high karnofsky performance status) and must have a good likelihood of longevity based on characteristics of primary disease

Question 39

At our site, what is the larger diameter met that can be treated with SRS?

Answer 39

3 cm

Question 40

True or False

For treatment planning optimization, both entrance and exit through eye are not allowed.

Answer 40

False

Only entrance through eye is not allowed. Exit is allowed

Question 41

Why do we not allow our jaws to go down below 2x2 for treatment plans?

Answer 41

2x2 cm2 is the lowest jaw setting that we trust small field measurements for

We do have small field measurements below 1x1, we just don’t trust them as much

Question 42

What are the coverage goals and hotspot constraint for SRS PTV and GTV for our site?

Answer 42

PTV: V100% > 95%
GTV: Dmin = 20 Gy
Dmax < 125% (130% at the absolute maximum)

(This is for 18 Gy in 1 Fx)

Question 43

What is the IMRT SRS/SRT fall-off that we expect?

Answer 43

10%/mm

Question 44

Describe the optimization use of ring and outer brain structures at our site

Answer 44

.5 cm ring: max dose = 90% of prescription
1 cm ring: max dose = 50% of prescription (loosen if difficult with multi met SRT brain patients)
2 cm ring: max dose = 10 - 30% of prescription (loosen for multi-met if not achieveable)
Outer brain (2 cm crop from all PTVs): max dose = 10-20% of prescription

Question 45

What is ideal conformity index for SRS/SRT? What is the upper limit for our site?

Answer 45

Ideal = 1.0
Upper limit = 1.3

Question 46

Which is easier to achieve a 1.0 conformity index for, multi-met or single-met?

Answer 46

Single-met is easier

No bridging

Question 47

What isodose line do we evaluate for dose bridging?

Answer 47

It depends on the proximity of the nearby lesions

If they are really close to one another, you may have to settle with the bridging of the 80% line, and in some cases even the 100%

If they are further, you look at the 30-50% lines

In general, the further they are, the lower the isodose you look at is

Question 48

What is the MU/arc limit for VMAT?
What about for SRS Arc mode?

Answer 48

VMAT: 2000 MU/arc
SRS Arc: 10,800 MU

Question 49

What is the frequency in which we do our E2E tests for SRS/SRT/SBRT? What two types are there?

Answer 49

Annual

“Hiden target test” and Dosimetric test

Question 50

MPPG 9a recommends a collimator size indicator daily test for “clinically relevant apertures.” Do we actually do this daily?

Answer 50

Yes and No?

We do a check at 20 x 20. But how relevant is it for small fields, which is the scope of MPPG 9a? You can argue it’s good enough, but you can also argue the other way. This has come up before in our clinic

Question 51

Per MPPG 9a, what is the one specified patient-specific QA test that they recommend?

Answer 51

Dry-run collisional checks

Question 52

What are the QA tests we perform daily that align with TG-142 and MPPG 9a for required for a stereotactic program? (9 listed)

Answer 52

1. Laser localization
2. Collimator size indicator
3. Radiation isocentricity
4. IGRT positioning
5. Imaging Subsystem interlocks
6. Stereotactic interlocks????
7. Output constancy
8. ODI @Iso
9. OSMS Daily QA

Question 53

What are the QA tests we perform monthly that align with TG-142 and MPPG 9a for required for a stereotactic program? (7 listed)

Answer 53

1. Radiation isocentricity
2. Couch position indicators
3. Output constancy
4. Laser Localization
5. MLC travel speed
6. Leaf position accuracy
7. Image quality tests

Question 54

How do we perform E2E for stereotactic at our site?

Answer 54

Two types, Dosimetric and Hidden Target

**Dosimetric: **Scan Lucy with Film and A16 inserts using CT sim and the scan protocols used for treatment. Create sample treatment plans with non-coplanar arcs and lesion at center of measurement media. Obtain either film dose plane or A16 mean dose. Deliver treatment on machine. Compare measured doses vs TPS. For film, do the usual procedure with Film QA Pro. For A16, measure the per field collected charge and add together, then correct reading to get mean dose in volume.

**Hidden Target: **Scan Calypso calibration cube on CT scanner and send to Eclipse. Contour beacons and create reference points at center of contours. Create a test plan with iso at the geometric center of the beacons. Export RP Dicom to calypso console. Align cube to isocenter according to the calypso array only. Use CBCT to then match to planning CT. Verify that post CBCT positions agree with the CBCT shifts.

Question 55

In the past we used to MRI after CT Sim. How was this done, what was the benefit?

Answer 55

We used to give MRI the patient’s mask, and they had an encompass couch top over there. So they can reproduce our setup

Benefits are same setup, less movement

Question 56

How often do we dry run for SRS/SBRT?

Answer 56

The ABR answer is to say always

The real answer is only when there’s a potential issue. For example…
- SBRT has no couch kicks, so your dry run can just be with the patient on the couch, rotate gantry 360 deg
- Single iso SRS at the center of the brain, or any SRS with iso near center will clearly not have a collision issue

Question 57

Which cone algorithm do we have, CDC or ECDC?

Answer 57

ECDC

Question 58

At our site, we have recently started requesting the T2 Flair MRI with the T1. Do we have to do a separate registration for the T2 Flair to CT during TP? Why or why not?

Answer 58

In theory no, in practice maybe

The T2 flair is taken immediately after the T1 is taken. So in theory the patient should be in the exact same setup as the T1. So if the T1 is registered to the CT, that registration should also be good for the T2. But you always want to double check just incase the patient moved around in betwene the two sets

Question 59

What are some general general practices you should employ when doing MRI to Ct registration for SRS brains?

Answer 59

• Always get a manual match close before auto matching
• Always align to bone first (turn on bone contour intensity range to bone)
• Make sure your VOI box for the automatch is tight on the skull (inferior border is BOS)
• Make sure skull and bones around eyes are properly aligned
• Double check registration with the soft tissue
• After double checking the T1 registration, ALWAYS make sure the registration is good for the T2 as well