Overdose problem associated with treatment planning software for high energy photons in response of Panama's accident.

BACKGROUND AND PURPOSE: The purpose of this study was to quantify dose distribution errors by comparing actual dose measurements with the calculated values done by the software. To evaluate the outcome of radiation overexposure related to Panama's accident and in response to ensure that the treatment planning systems (T.P.S.) are being operated in accordance with the appropriate quality assurance programme, we studied the central axis and pripheral depth dose data using complex field shaped with blocks to quantify dose distribution errors. MATERIAL AND METHODS: Multidata T.P.S. software versions 2.35 and 2.40 and Helax T.P.S. software version 5.1 B were assesed. The calculated data of the software treatment planning systems were verified by comparing these data with the actual dose measurements for open and blocked high energy photon fields (Co-60, 6MV & 18MV photons). RESULTS: Close calculated and measured results were obtained for the 2-D (Multidata) and 3-D treatment planning (TMS Helax). These results were correct within 1 to 2% for open fields and 0.5 to 2.5% for peripheral blocked fields. Discrepancies between calculated and measured data ranged between 13. to 36% along the central axis of complex blocked fields when normalisation point was selected at the Dmax, when the normalisation point was selected near or under the blocks, the variation between the calculated and the measured data was up to 500% difference. CONCLUSIONS: The present results emphasize the importance of the proper selection of the normalization point in the radiation field, as this facilitates detection of aberrant dose distribution (over exposure or under exposure).

Shielding for scattered radiation to the testis during pelvic radiotherapy: is it worth?

OBJECTIVES: To assess the value of external shielding of the testis during pelvic radiotherapy. MATERIAL AND METHODS: Nineteen patients, receiving radiotherapy to the pelvis with the lower border of the field at the obturator foramen, were randomly selected. A 5 half value layer cerrobent shield was positioned at the inferior border of the field. The dose to the testis was measured with and without the shield. Observations were made regarding the reflex cremaster contraction and phantom measurements were done at different distances from the perineum. RESULTS: The mean radiation dose to the testis for patients receiving treatment with no shield was 7.4cGy (1.3) and it was 5.7cGy (-/+2.5) for patients with external shield, this difference was statistically significant by the paired t test p<0.0001. This accounted for a 22 % decrease in the dose received by the testis. The position of the testis with the contraction of the cremaster muscle and the dartos fascia after manipulation of the testis during diodes placement changed up to 3.5 cm (mean 1.5). Phantom measurements showed 37% increase in the dose with 2cm change in the position of the testis to the pelvic direction. CONCLUSION: External shield at the inferior border of the pelvic field is a simple, easy reproducible, convenient shielding method. Clam-shell scrotal shield is not free of drawbacks, but still its benefits overweigh its harms and should be used with caution.