196

154

Radiation Oncology, Biology, Physics October 1989, Volume 17, Supplement 1

MONTE CARLO AND CONVOLUTION DOSIMETRY FOR STEREOTACTIC RADIOSURGERY

SS Kubsad, M.S., TR Mackie, Ph.D., MA Gehring, B.S., BR Paliwal, Ph.D., MP Mehta, M.D. and TJ Kinsella, M.D.

Department of Human Oncology, University of Wisconsin Medical School, Madison, Wisconsin 53792

Stereotactic radiation therapy procedures using the Brown-Roberts-Wells (BRW) frame and small x-ray beams are being used for brain neoplasms and arteriovenous malformations. The radiation field sizes range from 0.5 cm to 4.0 cm in diameter. The presence of lateral disequilibrium and large dose gradients along the central axis are extremely difficult to measure using standard ionization chambers. However, Monte Carlo methods can provide dosimetry information such as the energy and angular spectrum, and absorbed dose distributions of the beam in heterogeneous media.

We have developed geometry packages to simulate the Clinac-2500 accelerator treatment head (ATH), stereotactic collimating system (SCS) and a semi infinite water phantom placed at the isocenter. ATH modelling includes target, primary collimator, flattening filter for 6 MV x-rays, transmission chamber, secondary collimator, upper and lower jaw collimators. SCS modelling consists of lead-filled collimators with diverging circular holes and stereotactic transmission monitor chamber to detect the radiation as it exits the SCS. Water phantom variable in dimensions placed at the isocenter with regions of interests of 0.2 cm in height and 0.5 cm to 4.0 cm in diameter.

The Electron Gamma Shower Version 4 (EGS4) Monte Carlo code simulates radiation transport of 6 MV x-ray spectrum into a phantom modelled to be at the isocenter of the accelerator to obtain the depth dose distributions. The transport parameters used in this simulation are; ESTEPE=0.04, SMAX=0.2, ECUT=0.521 MeV and PCUT=O.Ol MeV with divergent beam. The Monte Carlo results were compared with the measured data obtained using an ionization chamber and with an independent calculations using the convolution method. Representative depth dose data for 2.0 cm cone size of the above three methods and for 2.0 and 4.0 cm in diameter are presented in Figures 1 and 2. The results for cone sizes from 0.5 cm to 4.0 cm in steps of 0.25 cm will be presented.

0 1 2 S 4 5 6 7 6 !3 10 Depth (cm)

155 DQSF DISTRIBUTIONS ;iiJD TREATMENT PLANNING OF LEKSELL STEREOTACTIC GAMMA KNIFE FOR RADIOSURGERY Tindrew \Ju , PhD., A.M. Kalend, PhD., 7i.H. Maitz, MS., L.D. Lunsford, MD., J.C. Flickinqer, MD., and W.D. Bloomer, MD Departments of Radiation Oncology and Neurosuigeiy, University of Pittsburgh, School of Medicine, Joint Radiation Oncoloqy Center and Pittsburqh Cancer Center, Pittsburgh, PA 15213

Ste;eotactic ;adiosurgery has been used in treatment of ce;-tain brain diseases such as arterio-venous malfunctions, pituitary adenonas, and acoustic neui-inonas foi several de- cades. The main goal of such a procedure is to delive; a single la;_qe radiation dose to a small brain lesion with a high degree of spatial accuracy. To achieve this goal, the radiation-pyoduced unit has to be precisely aligned mechan icclly and the radiation beams slrould be highly focused and have a rapid fall-off outside the target volume. In addition to llcavy charged particle beams from cyclotrons or ::-;ays from isocentric mounted linear accelerators, the gamma knife unit provides highly focused gamma rays produced from 231 Cobalt-60 sources. The 201 channels are drilled in a heni-spherical helmet which is r,xrde of G cm t!iiclr cast iron. They intersect at the focus with a mechanical precision of +/- 0.3 i.!::. '.,..-i-e are four helmets with removable collimators of 4 different sizes, i.e. 4, 8, 14, i;..c. G rm diameter fields at the focus.

The calibration of dose-Late output of l&mm collimator helmet was pcifoimed using an ion chanbe;_. The ;esults were verified with other methods of dose measurements such as TLDs and diodes. The ;elative dose factors foi the other three collinatoz helmets we;e also measured using TLDs and diodes. The proceduzcs and ;-esults al;c presented and din- cussed in this paper.

Due to its multiple-beam nature, the dosinetry calculation algo:ithn in the tieatraent planning of the gamma knife ai-e built on the single beam dose characteristics. Measurc- nents were made fOi the sinqle beam profile WI)_ -'-h 200 of the 201 collinato;s occluded,

Proceedings of the 31st Annual ASTRO Meeting 197

leaving one ape;_ture open, With bac::g;ound radiation, Lihich is transmitted lih;_ough the collimator plugs, subtracted. The results of both single and multiple beans p;ofiles are also p;_esenttTc!.

Zn te;ms of the spatial accuracy of the unit, a lead "shot" \Jas placed at the centei- of a spherical phantom and also at the focus of the helmet. A film was taped at the opposite side of the phantom and caught the images of the "shot" in each of every beam. This illustrates that all beams do pass through the focus. Furthermore, the deviation of the mechanical and i-adiation centers Were quantitatively measured with a specially de- signed gauge and is also reported.

156

TREATMENT VOLUMB SHAPING WITH SELECTIVE BEAM BLOCKING USING THE LEKSELL GAMMA UNIT

John C. Elickingfr,, M.D.1; Ann Maitz,, M.S. 1 1 2

Andrew Wu,, Ph.D. ; Andre Kalend,, Ph.D. ; L. Dade Lunsford,, M.D. ; and

Deparments of 1Radiation Oncology,, 2 Neurological Pittsburgh School of Medicine,, Joint

Surgery and Radiologyl, University of

Pittsburgh,, PA Radiation Oncology Center and Pittsburgh Cancer Institute,,

The Leksell Gm Unit at the University of Pittsburgh utilizes 201 highly focused Cobalt-60 beams arranged in a hemispherical array. Selective beam blocking can be used to modify the treatment isodose volume into various ellipsoid shapes oriented in different directions to better match the shape of the target volume being treated. Techniques are presented for blocking groups of individual sources in different specific patterns so that either the x,, y or z diameter of the isodose volume can be enlarged. Dose distributions for different blocking patterns were calculated using specially developed 3-D treatment planning software and were confirmed with film densitometry. The appropriate use of selective beam blocking to more closely match treatment isodose volumes to the target volumes being treated should be helpful in reducing the incidence of complications expected from stereotactic external beam irradiation with the Leksell Gamma Unit.

157 TUMOR ATPase, CREATINE KINASE, AND ADENYLATE KINASE KINETICS DETECTED BY 2-D 31P NMR

Paul Okunieffl, Kcbede Beshah*, Peter Vaupell, and Leo Neuringcr*

1 Massachusetts General Hospital, Dept. Radiation Medicine, Harvard Medical School, Boston, MA 02114, and *Massachusetts Institiute of Technology, Francis Bitter National Magnet Laboratory, Cambridge MA 02139.

3 lP nuclear magnetic resonance (NMR) spectroscopy has been used to measure the overall metabolic state of a tissue during the period required to perform the measurement. The major applications of high resolution NMR to in vivo tumor biology has been the non-invasive and serial measurement of changes in metabolic state that occur before, during, or after a change in physiological parameters or treatment status. Such measurements are useful for calculating high energy phosphate ratios, pH, and indirectly for estimating tumor oxygenation and efficiency of blood flow.

3 IP NMR techniques are now available that have the potential to expand these measurements to include the m of high energy phosphate utilization during steady-state metabolism. Hence, the energetic state and the rate at which metabolites are synthesized and consumed can be evaluated in the same tumor simultaneously. We have recently applied the technique of steady state saturation transfer to the measurement of pseudo-first order reaction rate constants of ATP synthesis via the reaction ADP + Pi ---> ATP and by the creatine kinase

catalyzed reaction PCr + ADP ---> ATP + Cr. 1-D and 2-D 31P NMR studies of tumor were performed using an NMR probe designed for the study of tumors in small animals and operating at 145.6 MHz (8.5 T) for phosphorus. Pulse sequences optimized for measuring 2-D NMR coherence transfer and nuclear Overhauser spectroscopy, were performed to simultaneously detect, and in some cases quantify, several chemical reaction rates in tumors. These measurements are non-invasive and can be performed serially and non-invasively in murine tumors growing S.C. in the hind foot dorsum.

The fraction of total cellular ATP utilized each second (and at steady state synthesized each second) can be as high as 20-25 % in fast growing tumors. Slower growing tumors have lower rates ( < l-5% per second) which can be below the detectability limits of some or all of the techniques employed. The ATP turnover rates decrease with tumor size consistent with the expected progressive depletion of metabolites and associated slowing of growth rate. Measurements of ATP utilization rates also correlate with tumor growth rate.

Reaction rate kinetics such as those catalyzed by ATPase, creatine kinase, and adenylate kinase can be measured in vivo using 1-D steady state saturation studies and 2-D 3 lP NMR spectroscopy. From these measurements metabolic rate can be evaluated directly, and indirect estimates of growth kinetics can be made. This rate information, when supplemented with the energy state information, should substantially increases the potential of NMR technology to predict the response of tumor to treatment.

Supported in part by NIH grants CA48096, CA13311, and RR00995, and by the ACS Career Development Award.