Fetal radiation doses for model C gamma knife radiosurgery.

OBJECTIVE

To assess fetal radiation doses in a pregnant patient undergoing gamma knife stereotactic radiosurgery (GKRS) and to quantify the components of extracranial radiation doses by means of phantom measurements.

METHODS

A patient in her 25th week of pregnancy received GKRS with the Model C gamma knife for a solitary metastatic melanoma of the brain. A single isocenter with the 14-mm collimator and trunnions was used to deliver 20 Gy to the lesion defined to the 80% isodose line. Fetal radiation doses were assessed with phantom measurements before GKRS and then measured during GKRS. In addition, doses to the lateral canthi, thyroid, sternum, and pelvis were monitored with thermoluminescent dosimeters in 20 consecutive patients during their GKRS. Phantom measurements were also performed to identify extracranial radiation doses to these structures.

RESULTS

Fetal radiation doses were 0.31, 0.20, and 0.15 cGy to the top, middle, and bottom of the uterus, respectively. These approximately corresponded to 0.01% of the maximum tumor dose of 25 Gy. The mean extracranial doses in 20 patients were 36.9, 5.8, 3.3, and 0.6 cGy to the lateral canthi, thyroid, sternum, and pelvis, respectively. Phantom measurements supported the results obtained from the patient measurements and further refined estimates of doses to extracranial sites. These measurements were also compared with those of a previously reported study with the use of the Model U.

CONCLUSION

Measured radiation doses in a pregnant patient to extracranial sites, including those to the fetus, were very low. We think that GKRS is a safe treatment, particularly with the Model C, and could be recommended to carefully selected patients with brain metastases who are in the second and third trimester of pregnancy.