Radiation protection tutorial warmup quiz

Question 1

What is the annual dose limit for radiation workers and the public?

Answer 1

• Radiation workers: 20 mSv.
• Public: 1 mSv.

Question 2

What are the guideline TADR2000 values for area classification?

Answer 2

• Controlled: > 3 microSv/hr.
• Supervised: > 0.5 microSv/hr.
• Unsupervised: > 0.15 microSv/hr.

Question 3

What guideline IDR would be acceptable in terms of not having to worry about the area at all? What is the caveat to this for linacs? Where does this come from?

Answer 3

• 7.5 microSv/hr.
• This would lead to excessive shielding for normal clinical use of most linacs.
• It comes from the 3/10 of the old annual dose limit of 50 mSv which is then corrected for workload to a per hour value.

Question 4

Is patient scatter or leakage larger for a radiotherapy linac? What does this mean for shielding calculation?

Answer 4

• Leakage.
• This means leakage dose should be used for the calculation of secondary barriers.

Question 5

By what factor does a primary barrier typically have to attenuate the primary beam by for a linac? What about the secondary barrier?

Answer 5

• 1 000 000.
• 1000.

Question 6

What is the typical IDR for a modern flattened linac beam? How might this change for an FFF beam?

Answer 6

• 600 MU/min = 6 Gy/min.
• 2400 MU/min = 24 Gy/min.

Question 7

What is the typical distance in a linac bunker from the isocentre to the primary barrier? What about the secondary barrier?

Answer 7

• 5 m.
• 7 m.

Question 8

What should head leakage be limited to in relation to the dose rate of the primary beam at isocentre?

Answer 8

0.1% of the primary beam at isocentre.

Question 9

What differences are apparent for ISL corrections when considering primary and secondary barriers?

Answer 9

• For primary barriers, the distance from isocentre plus 1 m should be used due to the distance of the source from the isocentre.
• For secondary barriers, the distance from the isocentre can be used as it is assumed the isocentre is the location of the source of scatter.

Question 10

What is the general method for determining primary dose rate on the other side of a primary barrier for a linac bunker? How is this different for secondary barriers?

Answer 10

• Determine the IDR. This is typically 600 MU/min = 6 Gy/min for modern flattened linac primary beam. For the secondary barrier, leakage should be considered (which outweighs patient scatter in typical radiotherapy). This is limited to 0.1% of the primary beam.
• Distance from isocentre to location of interest determined for ISL correction. For primary barriers, an extra 1 m is added to account for the distance from the tube to the isocentre.
• Determine the shielding thickness and corresponding TVL given the linac beam energy and shielding material.
• ISL correct the primary/leakage IDR at isocentre and correct for the attenuation of the shielding (10^-TVL) to determine the IDR on the other side of the barrier.