Medical Physicists Role

By John McCormick  |  Posted 2004-03-08 Print this article Print

Perhaps nothing shows the ravages of faulty calculations as clearly as cancer.

The patients who were suffering in Panama had cancers of the pelvis. Pelvic organs such as the intestines and kidneys are acutely sensitive to radiation. Before a cancer patient such as Garcia is exposed to radiation, a doctor devises a treatment plan that determines what dose of radiation can safely be directed at the tumor. The physician considers the tumors position and depth in the body, the likelihood that the cancer has spread to surrounding tissue, the location and sensitivity of nearby organs and the best angles of attack.

As part of the plan, the doctor figures out how to place metal shields, known as "blocks," above the area where the tumor is located. These blocks, usually made of lead or a metal alloy called cerrobend, protect normal or sensitive tissue from the gamma rays to come.

The doctor hands his plan to a medical physicist, who feeds information on the size, shape and location of the blocks into a software package. These packages generally create a 3-D picture of how the dose will be distributed, showing how the radiation will "sum" as beams coming in from different angles intersect at depth in the patients tissue. Once the doctor prescribes a dosage, the software calculates the duration of treatment.

The physicists in Panama were carrying out a doctors instruction to be more protective, adding a fifth block to the four the hospital often used on patients in cancer treatments. The extra block could help protect patients whose tissues were especially sensitive due to previous surgeries or radiation treatments.

Multidatas planning software was designed to calculate treatments when there were four or fewer blocks, according to the companys general business manager, Mick Conley. Saldaña, however, read Multidatas manual and concluded she could make the software account for a fifth block.

According to an August 2001 report from the IAEA, Saldaña found the software didnt only work if she entered the dimensions of each block individually, up to four. She found it also allowed her to enter the dimensions of all five blocks as a single, composite shape-for instance, a rectangle with one triangular block sitting in each corner and a fifth square block protruding, tooth-like, down into the rectangle from the top.


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So, using the mouse attached to her computer, she entered on the screen the coordinates of the specially shaped block— first the inner perimeter of the shape and then the outer perimeter. This is when she felt she was "home free."

After all, when Saldaña entered the data for this unusual-looking block, the system produced a diagram that appeared to confirm its dimensions. She seemed to be getting confirmation from the system itself that her approach was acceptable.

Next Page: Ravages of miscalculation.


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