Calculating Polarity Correction Factor

MDPhysics.com — Tue, 29 Dec 2009 08:40:23 +0000

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 (P_pol) 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 P_pol by the following equation:

P_pol = Abs [(M⁺_raw - M^-_raw)/2M_raw]
M⁺_raw is the reading when positive charge is collected and M^-_raw is the reading when negative charge is collected

There may be some ambiguity when first encountering this equation as to which reading one should use for the M_raw value in the denominator. There are quite a few possibilities, but only one correct answer. So, which reading do you use for M_raw in the denominator?

1- The average of M⁺_raw and M^-_raw
2- The larger value of M⁺_raw and M^-_raw
3- The smaller value of M⁺_raw and M^-_raw
4- Either M⁺_raw or M^-_raw can be selected
5- None of the above

Click here for the answer

Wireless Dose Verification System for Cancer Patients

MDPhysics.com — Thu, 09 Jul 2009 03:10:48 +0000

The DVS-HFT (Dose Verification System) is the first wireless and implantable dosimeter system. The device was developed by Sicel Technologies, Inc. In March 2009, Sicel Technologies, Inc announced that it had received clearance from the U.S. Food and Drug Administration to begin marketing the DVS-HFT for measuring actual radiation dose at the tumor site when treating breast and prostate cancer patients undergoing external beam radiation therapy. The company also makes and markets OneDose, patient dosimetry verification systems specifically designed for radiation oncology therapy (we use them in our cancer center). DVS-HFT is specifically used for higher radiation doses over fewer treatments. This technique is called Hypo-Fractionated Therapy (HFT) and is highly effective in treating certain tumors.

“The DVS-HFT is an important advance in hypo-fractionated therapy, because physicians can now confidently determine the prescribed dose has been accurately delivered throughout the course of treatment, which is a key determinant in the long-term successful treatment of a tumor,” said Arnold Malcolm, MD, Associate Professor Vanderbilt Center for Radiation Oncology. “This device may actually accelerate the use of hypo-fractionated protocols, like those being adopted for treatment of prostate cancer, which may offer benefits across the healthcare spectrum.”

For further clinical use, technical understanding, reimbursement information, etc. go to:
http://www.siceltech.com/DVSSmartMarker/index.html

Photos from HealthyLiving (Summer 2009) published by the Boca Raton Community Hospital

MDPhysics: A Medical Physics Blog » Dosimetry

Calculating Polarity Correction Factor

Wireless Dose Verification System for Cancer Patients