First Year Exam: MU Calcs

Question 1

What is an “equivalent field”

Answer 1

Any field that has the same depth-dose characteristics as an irregular field

(it can be square or circular)

You can even do rectangular, but that in itself would further need another equivalent square field so there’s no reason to really do that…

Question 2

What field size should you use for S_c for a SSD setup? What about SAD?

Answer 2

You always use field size at 100 cm for S_c regardless of whether you’re SSD or SAD

This should make sense, S_c is dependent only on collimator scatter. The patient setup makes no difference

Question 3

What field size do you use for S_p for a SSD setup?

Answer 3

Field size at the surface of the phantom/patient

Question 4

What field size do you use for Sp for a SAD setup?

Answer 4

Field size at the target

Question 5

When are you allowed to use S_cp instead of S_c or S_p?

Answer 5

When the field size you would use for S_c and S_p are both the same

This happens in a 100 SSD setup

or

If you set your target to 100 SAD

Question 6

What field size do you use for PDD?

Answer 6

Always use field size at the surface

Question 7

What field size do you use for TMR?

Answer 7

Always use field size at point of measurement

It’s not always 100 SAD. Sometimes they’ll ask for some point off of 100 SAD

Question 8

Where does TG-71 recommend normalization point for photon outputs be set to?

Answer 8

10 cm

Note: most clinics, including ours, use d_max (which is also acceptable according to the TG report). But TG-71 recommends 10 cm because it’s easier and more clinically relevant

Question 9

In what scenario would MU calcs still be used?

Answer 9

Emergency setups that don’t require simulation/would be more efficient without simulation

Question 10

Why does TG-71 actually recommend using 10 cm as normalization depth? (4 reasons)

Answer 10

1. Different machines at same energy will be matched to a more clinically relevant depth
2. May decrease differences in programmed MU when moving patients from one machine to another
3. Some TPS systems specifically require measured output factors at d = 10 cm, thus you’d have to take this measurement anyway. May as well kill two birds with one stone
4. Some field size dosimetric quantities vary less at depths > d_max

Question 11

What normalization depth does TG-71 recommend for electrons?

Answer 11

d_max

Question 12

If your secondary MU check is failing, what are some steps you can take to explore this issue? (5 suggested steps)

Answer 12

1. Verify a calculation error hasn’t been made by reviewing the parameters used in the MUV (this includes beam modifiers and beam parameters that are used for the TPS)
2. Confirm an appropriate comparison point was used (in field and away from heterogeneity)
3. Confirm geometric information between MUV and TPS are consistent (should be since the body contour and radiological densities are typically assigned in the MUV)
4. Investigate if the algorithm for calcing itself has some magnitude and direction of error that is expected to be constant and predictable that wasn’t taken into account
5. If you can’t figure out whats causing the discrepency, alert the Physician and see how they would like too proceed given some known uncertainty

Question 13

What values are typically taken into account for photon MU calcs (7)?

Answer 13

1. PDD or TMR
2. OAR
3. Collimator scatter
4. Phantom scatter
5. Beam modifier transmission factors
6. Mayneord factor (only for PDD)
7. Inverse square corrections

Question 14

What values are typically taken into account for electron MU calcs (5)?

Answer 14

1. Applicator factor
2. Cutout factor
3. The treat to isodose line
4. Inverse square correction (SSD_eff factor)
5. Electron depth dose (but note, if you normalize to an isodose like, then the EDD just becomes the treat to isodose line, as is the case most of the time). So EDD will often = 0.9 or 0.8

Question 15

What is the equation for the applicator factor of rectangular field (AxB)?

What is the analogous equation for the EDD?

Answer 15

S_e(AxB) = [AF_AxA*AF_BxB]^1/2

EDD(AxB) = [EDD_AxA*EDD_BxB]^1/2

Question 16

For the square root rule for the EDD and output factor of a rectangular field/applicator, what setup table is the data references at?

Answer 16

If SSD = 100, 105, or 110, or any other SSD that you already have a EDD or AF table for, then use the respective SSD

If it’s for an SSD that you don’t have a table for, then assume SSD = 100, and then you need to apply an additional SSD_eff factor

Question 17

When do you use the SSD_eff factor?

Answer 17

Only when you use a SSD that you don’t have a table for. Typically this is anything other than SSD = 100, 105 and 110

Question 18

What is the equation for the SSD_eff factor?

Answer 18

[(SSD_virtual + d_max) / (SSD_virtual + d_max + gap)]²

Where the ‘gap’ is the difference between the two SSD setups

So if you have SSD = 103 cm setup, and you only have a SSD = 100 cm table, then gap = 3 cm

Question 19

Conceptually speaking, what is the “Virtual Source Distance” (VSD)?

Answer 19

It’s an arbitary point in space in which electrons follow a psuedo inverse square relationship relative to

Question 20

What is an alternative to the extended SSD technique that can be used for electron MU calculations?

Answer 20

The Air-Gap technique

Question 21

What are the rules for selecting which physical wedge factor to use in a wedge factor (100 SSD) table?

Answer 21

If you’re using the wedged PDD table - use field size at surface and wedge factor at d_max, regardless of depth of target. This is because the table already accounts for the WF at the target, but you need to still normalize to d_max so you have to use the WF for that instead

If you’re using open PDD table - Use field size at surface and wedge factor at depth of target

If you’re using openTMR table - use field size at target and depth if target

DO NOT USE WEDGED TMR TABLE!!!! You haven’t seen any problem solutions that used it, so don’t try it. Just use the open TMR table if need be, then follow the above rule.

Question 22

What does SPD stand for?

Answer 22

Source to point distance

Question 23

True or False

For extended SSD or extended SPD setups, you can use TMR for any of them?

Answer 23

True

TMR is independent of distance from source. All you would have to do is adjust for output at the point of interest (inverse square), and field size at point of interest

If you’re comfortable with using TMR instead of SSD+Mayneord factor, then it’s MUCH easier to just use TMR for any extended SSD stuff

Question 24

True or False

For extended SSD problems, if your isocenter is not in the patient, you cannot use TMR table

Answer 24

False

Nothing is stopping you. Your calculation point does not need to be at isocenter at all. You would just need to do a inverse square correction to account for change in output at isocenter

Question 25

Why do you not have to do the SSD_eff factor, when you are at 105 or 110 SSD?

Answer 25

Because the applicator factor tables are available for 105 and 110 SSD. These are normalized to 100 SSD. So the factors themself already include the SSD_eff factor inherent in them.

If you were at an SSD where you didn’t have a table, then you would have to do the SSD_eff factor and use the AF at 100 SSD

Question 26

True or False

Regardless of the air gap in electron MU calcs, you always use the 100 SSD table

Answer 26

True

Unless you have anything more accurate, use 100 SSD table. The actual depth dose curve does not change that much, only the output changes drastically (which is accounted for in your output factor)

Question 27

If you’re given a 6 x 10 cone, and 107 SSD, what is your SSD_eff? You look at the VSD table, and realize it only has 6x6 and 10x10 cone sizes. How do you find, VSD?

Answer 27

You use the VSD for 6x6, because that will allow you to overestimate MU’s (so avoid under-covering target).

There is no special rules to derive a 6 x 10 cone VSD. VSD is something that you have to measure, can’t really calculate it. So if the table doesn’t include a VSD for a certain cone size, just use the cone size that will give you the higher of the two MUs

Question 28

Select all of the following that would cause a %DD curve to change

Energy

Physical Wedges

Field Size

Dynamic Wedges

SSD

Answer 28

Select all of the following that would cause a %DD curve to change

Energy

Physical Wedges

Field Size

Dynamic Wedges

SSD

NOTE: Dynamic wedges don’t attenuate the field, and such they don’t change %DD at central axis