A comparison of commercial treatment planning systems when calculating dose under shielding blocks

Several commercial computerized treatment planning systems were compared when calculating dose under shielding blocks in soft tissue and lung tissue. A test case was studied in which a water-equivalent phantom containing a removable cork insert was exposed to 60Co radiation with and without a lead block in the beam. For simplicity, all cross sections were square and only central axis doses were considered. Manufacturers of treatment planning systems were asked to calculate doses for the test case for comparison with corresponding experimental values determined from ionization chamber measurements. When the initial replies seemed to indicate the existence of errors or misinterpretations, the manufacturers were informed and asked to submit revised values. The results for eight systems exhibit wide variations in the doses under the block, with differences of up to 88% between measurements and calculations. Most of the systems did not take into account heterogeneity and shielding simultaneously. While in comparison with the maximum dose the dose differences under the block are not large and their clinical significance may be debated, better accuracy and more of a consensus would be expected when elaborate computer systems dedicated to radiation therapy deal with a relatively simple problem. The results emphasize the need for caution when users implement the options offered by their treatment planning computers.