Calculating Polarity Correction Factor

December 29, 2009 · Posted in Board Certification, Dosimetry · Comment 

AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams contains many details to which we should pay close attention when performing necessary measurements for the calibration of high-energy beams. One such measurement is calculating the polarity correction factor (Ppol) for ionization chambers used in electron beam dosimetry. Since polarity effects vary with beam quality and other conditions (such as cable position), one must correct for these effects by making measurements each time clinical reference dosimetry is performed. To correct an ion chamber’s raw reading for polarity effects it is necessary to take readings with both polarities applied and tabulate Ppol by the following equation: Read more

ABR Practice Exam Question 6 – D90/V100 Prostate Seed Implant

August 30, 2009 · Posted in Board Certification · Comment 

From a post-plan CT following a prostate seed implant, a physicist reported that D90 is equal to 80% of the prescribed dose and V100 is 95. What does he means by that statement?

1- The minimum dose to 90% of the target volume is equal to 80% of prescribed dose; and, 100% of the target volume receives 95% of prescribed dose.

2- The 90% isodose line covers 80% of the target volume; and, 95% of the target volume receives 100% of the prescribed dose.

3- 100% of the target volume receives 95% of the prescribed dose; and, the 90% isodose line covers 80% of the target volume.

4- The minimum dose to 90% of the target volume is equal to 80% of prescribed dose; and, 95% of the target volume receives 100% of the dose.

Click here for the answer to ABR Practice Exam Question 6

ABR Practice Exam Question 5 – Cesium-131 Brachytherapy

August 26, 2009 · Posted in Board Certification · 5 Comments 

Which of these statements is/are correct regarding a Cesium-131 brachytherapy source?

1- It emits gamma rays with a peak energy of 662 KeV.

2- Its half-value layer is around 5.5 mm lead.

3- Its half-life is 9.7 days.

4- Statements 1 and 2 are correct only.

5- None of the statements are correct.

Click here for the answer to ABR Practice Exam Question 5.

ABR Practice Exam Question 4 – Accelerated Partial Breast Irradiation (APBI)

August 22, 2009 · Posted in Board Certification · 1 Comment 

Which of these statements is/are not correct regarding “Accelerated Partial Breast Irradiation” (APBI)?

1- APBI can be treated with external beam and mammosite only.

2- External beam based approaches seem to minimize the risk of seroma formation and infection compared to brachytherapy.

3- Radiation dose to uninvolved ipsilateral breast, heart and lung using brachytherapy technique is lower than other techniques.

4- IMRT technique improves upon 3D-CRT approach by enhancing conformity dose to tumor with a possible increase radiation dose to the uninvolved ipsilateral breast, heart and lung.

5- The PTV used for brachytherapy planning is typically smaller than the PTV for external beam technique, however, brachytherapy results in a significant dose inhomogenity.

Don’t take the exam if you don’t know the answer of this question.

Some of our readers have recommended that we do not post the answer at the same time as we post the question. We think it is a good suggestion, as it gives the readers a chance to think about the answer. Therefore, the answer will be posted within a couple of days.

Click here for the answer to ABR Practice Exam Question 4.

ABR Practice Exam Question 3 – Time Error

August 15, 2009 · Posted in Board Certification · Comment 

To measure Time Error for a therapy machine (Co-60), a physicist took two different output measurements at two different time intervals. The result is listed below:

minute Exposure
T1 = 1 R1 = 100
T2 = 2 R2 = 201

What is the time error for this machine?

1- 0.59 seconds
2- 1 seconds
3- 0.29 seconds
4- 1.18 seconds
5- Cannot be determined from the data

Note: Co-60 machines may be out of the picture, but there may be a question on C0-60 machines on the exam. Although Time Error can be applicable to all therapy machines, it’s more applicable to Co machines.

Click here for the answer to ABR Practice Exam Question 3.

ABR Practice Exam Question 2 – Shielding

August 11, 2009 · Posted in Board Certification · Comment 

In a shielding evaluation (Survey) of a new vault that housed a Varian linear accelerator, In-Any-One-Hour Time Averaged Dose Equivalent Rate (Rh) was found to be 2.5 mr/hr in the unrestricted area. Which of the following are true?  (There may be multiple answers)

1- Rh is only applicable for a restricted area.

2- It is over the limit for restricted area and some action should be taken.

3- It is over the limit for unrestricted area and some action should be taken.

4- Rh is only applicable for radioactive materials shielding survey.

5- Value of Rh depends on average number of patient treatments in any one hour and the weekly time averaged dose equivalent rate.

Click here for the answer to ABR Practice Exam Question 2.

ABR Practice Exam Question 1 – Dose Decay

August 9, 2009 · Posted in Board Certification · Comment 

For those of you preparing to take the ABR exam this September, here is a problem for you (because it’s August–one month away from your test date–and you can never do too many practice problems!). Stay tuned for more practice problems in the coming month.

A prostate I-125 seed implant patient would like to know what fraction of the dose is delivered to the tumor in 10, 60, and 80 days after the implant.

Click here for the answer to ABR Practice Exam Question 1.

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