Monte Carlo and convolution dosimetry for stereotactic radiosurgery.

The dosimetry of small photon beams used for stereotactic radiosurgery was investigated using Monte Carlo simulation, convolution calculations, and measurements. A Monte Carlo code was used to simulate radiation transport through a linear accelerator to produce and score energy spectrum and angular distribution of 6 MV bremsstrahlung photons exiting from the accelerator treatment head. These photons were then transported through a stereotactic collimator system and into a water phantom placed at isocenter. The energy spectrum was also used as input for the convolution method of photon dose calculation. Monte Carlo and convolution results were compared with the measured data obtained using an ionization chamber, a diode, and film.

Irradiation of bilateral orbital lymphoma: a non-coplanar technique with case reports.

Bilateral synchronous involvement of the orbits by lymphoma is an uncommon event. Therefore, the irradiation techniques for this disease are not well refined. Depending on the technique used, one has to decide between underdosing the anterior segment of the orbit and the frequently involved lacrimal glands or to tolerate shield related in homogeneities in the range of 40 to 50% as well as higher doses to the midline structures, e.g. the optic chiasma. To avoid the above shortcomings we developed a technique by which half the prescribed dose is delivered by pairs of beams angulated in the transverse plane and the other half by pairs of beams angulated in the sagittal plane. Film dosimetry in a phantom confirms that the entire orbit, including its anterior portion, is well covered, the lens is protected, and the dose to the mid-sagittal structures is not increased. The dose inhomogeneity is in the range of 20% which represents a substantial improvement over alternative methods. Two irradiated patients with bilateral orbital lymphoma are presented with local control and no ophthalmic or other side effects as of last follow-up.