Solutions to Problems



Solutions to Problems

P. 149:

An I-192 source was delivered on Monday with an apparent activity of 10mCi. The following Monday the activity will be?

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P. 150:

A Pd-103 seed is found, assayed, and determined to be 0.35 mCi. It is concluded to have come from a batch of seeds 5 days earlier. The original activity of the seed was?

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P. 152: (sorry, a mistake was made in this question. The number of days should be 17)

An unknown source is assayed 17 days after its discovery and found to be 85% of its original value. What is the source?

Quick solution: Going by the rule of thumb, Ir-192 decays approximately 1% every day. Therefore 17 times 1% would equal 83%.

Long solution:

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Assays are never perfect therefore if given the choices of Pd-107, I-125, Cs-137, Co-60, or Ir-192. Ir-192 is the closest isotope to 72.5 days

P. 153

What is the decay constant of an isotope that has a half life of 60 days?

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P. 154

The exposure rate coming from a Radium source in a patient is 20 mR/hr. If you need to reduce the exposure to 2 mR/hr or less behind the lead shield how thick does the portable shield need to be if the HVL of Radium is 8.0 mm of lead?

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P. 155

Calculate the equivalent square for a 7cm x 29cm field size/

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P. 158

A patient is simulated with a 10.5 cm x 8 cm field with a 100 cm SSD technique. The prescription calls for 200 cGy at 11 cm depth on a Varian 2100 EX linear Accelerator. The physician changes his mind and would like a SAD technique (100 cm). What is the equivalent square that will be used to look up the TMR for the new SAD technique?

Equivalent Square = 9.1 @ 100cm SSD. You would then need to find the equivalent square at the treatment depth. This would be the area you are trying to treat. This area does not change therefore the equivalent square at a depth of 11cm for the SSD technique would be the same for the SAD technique.

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P. 160

A 9 cm x 14 cm right lateral pancreatic field is treated to a depth of 6 cm on a Varian 21 EX unit (100 cm SSD) to 180 cGy/fx. What is the opposed kidney dose if the kidney is located at a depth of 10cm.

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P. 163

A physician prescribes 200 cGy/fraction to a midline point in the pelvis. The patient separation is 20 cm. What is the dose at a depth of 2.0 cm for the posterior field if the equivalent field size is 15 cm2 using a 6 MV beam?

Assuming Ap/Pa technique. (should have been included in the problem)

D2 = Entrance + Exit

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P. 166

The PDD at depth 7 cm for Varian 21 EX unit 100cm SSD is 78.7%. What is the PDD at 90 cm SSD for the same equivalent square field size? (I apologies again, 6x should be listed in the question)

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P 171

The spine field for the cranio-spinal treatment has a length of 22 cm at 100 cm SSD, what is the collimator rotation?

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Shortcut: (Hientz 57 rule)

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P. 176

You have left TLD badge in the accelerator room by accident. The badge was located about 1 meter outside of the beam from isocenter. Five patients had been treated, each with a prescription dose of 180 cGy. Estimate the exposure to you badge?

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The answer could be in Roentgens or milli Roentgens. Know how to convert.

P. 185

What is the Ctp factor if the temperature is 74 degrees Fahrenheit and 752 mmHg in the treatment room?

First convert Fahrenheit to Celsius

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Second Solve for Ctp:

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