Mark Gehring

Defining the role of radiosurgery in the management of brain metastases

The role of stereotactic radiosurgery in the management of recurrent and newly diagnosed brain metastases was evaluated prospectively. From December 1988 to March 1991, 58 lesions in 40 patients were treated with accelerator-based stereotactic radiosurgery. All patients were followed for a minimum of 6 months or to death. The primary purpose was to determine the impact of radiosurgery on local control and its subsequent effects on quality of life. An overall tumor control rate of 82% with a complete response rate of 43% were achieved. As anticipated, the response rate for smaller tumors was substantially better than that for larger tumors (78% for lesions < 2 cm3; 50% for lesions > or = 10 cm3). Although the overall in-field progression rate was 18.5%, only 1/23 (4%) complete responders subsequently recurred. The in-field failure rate is highly comparable with recently published surgical data. Progression outside the brain was noted in two-thirds of patients. One quarter of the deaths were neurologic. The median survival for this minimally selected patient population was 6.5 months. Stereotactic radiosurgery was also associated with improved quality of life as measured by Karnofsky score, neurologic function, and steroid dependence. Long-term steroid dependence was encountered in only four patients. We conclude that stereotactic radiosurgery can be used effectively in patients with brain metastases. In this series, a high tumor response rate was achieved which was associated with improved quality of life.

Investigation of the convolution method for polyenergetic spectra

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two convolution approaches that properly account for beam hardening effects. Dose deposition kernels calculated previously using the EGS4 Monte Carlo code are convolved with the primary terma to give the dose for monoenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fluence spectrum to yield the polyenergetic dose distribution. Alternatively, a single convolution for the polyenergetic beam is considered, where a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels for a diverging beam geometry were also examined. The depth dose data produced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.

Treatment of arteriovenous malformations with stereotactic radiosurgery employing both magnetic resonance angiography and standard angiography as a database

Twenty-one arteriovenous malformations were prospectively evaluated using magnetic resonance angiography and compared with stereotactic angiography. The goals were to establish the feasibility of magnetic resonance angiography, compare it to stereotactic angiography, employ magnetic resonance angiography in follow-up, and semiquantify flow. A correlative evaluation between flow and response to stereotactic radiosurgery was carried out. Phase contrast angiograms were obtained at flow velocities of 400, 200, 100, 60, and 20 cm/sec. The fractionated velocities provided images that selectively demonstrated the arterial and venous components of the arteriovenous malformations. Qualitative assessment of the velocity within the arteriovenous malformations and the presence of fistulae were also determined by multiple velocity images. In addition, 3-dimensional time-of-flight magnetic resonance angiograms were obtained to define the exact size and shape of the nidus. This technique also permitted evaluation of the nidus and feeding arteries for the presence of low flow aneurysms. Correlation between the two imaging modalities was carried out by subjective and semiquantitative estimation of flow velocity and estimation of nidus size. The following velocity parameters were employed: fast, intermediate, slow, and none (arteriovenous malformation obliterated). In 19 of 21 (90.5%) arteriovenous malformations, magnetic resonance angiography was equal or superior to stereotactic angiography for flow quantification and visualization of the nidus. Only 2 of 21 arteriovenous malformations were better demonstrated by stereotactic angiography than by magnetic resonance angiography (failure rate of 9.5%). The nidus size in one case was clearly underestimated by stereotactic angiography and would have resulted in a geographic miss without magnetic resonance angiography. Seven post-radiosurgery arteriovenous malformations were evaluated for follow-up with both magnetic resonance angiography and stereotactic angiography. In 6 of 7 arteriovenous malformations, magnetic resonance angiography response matched stereotactic angiography response. Correlation of flow with outcome was carried out for 14 arteriovenous malformations using magnetic resonance angiography only. Interestingly, all nine arteriovenous malformations with intermediate or slow flow demonstrated partial or complete obliteration; whereas only 3 of 5 fast flow arteriovenous malformations achieved a response with a median follow-up of 10 months. This early analysis suggests that slower flowing arteriovenous malformations may obliterate faster after stereotactic radiosurgery and flow parameters could be employed to predict response. In conclusion, magnetic resonance angiography permits semiquantitative flow velocity assessment and may therefore be superior to stereotactic angiography. An additional advantage of magnetic resonance angiography is the generation of serial transverse images which can replace the conventional CT scan employed for stereotactic radiosurgery treatment planning.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A three-dimensional volume visualization package applied to stereotactic radiosurgery treatment planning☆

A comprehensive software package has been developed for visualization and analysis of 3-dimensional data sets. The system offers a variety of 2- and 3-dimensional display facilities including highly realistic volume rendered images generated directly from the data set. The package has been specifically modified and successfully used for stereotactic radiosurgery treatment planning. The stereotactic coordinate transformation is determined by finding the localization frame automatically in the CT volume. Treatment arcs are specified interactively and displayed as paths on 3-dimensional anatomical surfaces. The resulting dose distribution is displayed using traditional 2-dimensional displays or as an isodose surface composited with underlying anatomy and the target volume. Dose volume histogram analysis is an integral part of the system. This paper gives an overview of volume rendering methods and describes the application of these tools to stereotactic radiosurgery treatment planning.