92 Radiation Oncology ??Biology ??Physics October 1984, Volume 10, Sup. 2

31 ASSESSMENT OF LIVER RADIATION TOLERANCE USING DOSE VOLUME HISTOGRAMS

M.M. Austin-Seymour, G.T.Y. Chen, J.R. Castro, W.M. Saunders, S. Pitluck, K.H. Woodruff, L. Chere, M. Kessler

University of California, Lawrence Berkeley Laboratory, Biology and Medicine Division, Berkeley, CA 94720

Ten patients with carcinoma of the pancreas or biliary system re- ceived whole liver radiation in conjunction with primary radiation therapy because the liver is a frequent site of failure in these diseases. Doses to the whole liver ranged from the biological equivalent of lo-24 Gray (Gy) using fractions of 2-3 Gy. Doses to the primary lesion ranged from 53.5-70 Gy. Because part of the liver received dose from treatment of the primary in addition to dose from whole liver radiation, we have analyzed liver dose using histograms relating dose and volume. All patients had pretreat- ment computed tomography (CT) of the upper abdomen. Dose distri- butions were calculated on each axial slice utilizing all ports. A 3-dimensional dose matrix was therefore constructed. The liver was contoured on the appropriate axial slices and a histogram of dose within the liver was determined. These dose volume histo- grams are of the integral type and display on the ordinate the percent volume of liver which is irradiated in excess of the dose specified on the abscissa. For example, the sample histogram shows that 100% of theyiver received a dose of 15 Gy or greater. This represents the whole liver portion of the treatment. The sample histogram also shows that 18% of the liver received a dose in excess of 30 Gy. Dose volume histograms permit a 3-dimensional anal- ysis of liver radiation dose. While the histograms do not take into account certain factors such as dose per fraction, they are a useful adjunct to isodose distributions in the evaluation of a treatment plan.

One patient developed radiation hepatitis. She received 69 Gy to the primary and 21 Gy to the whole liver using fractions of 3 Gy. Four weeks after completing treatment she developed ascites, peripheral edema and jaundice with elevated liver function tests. A liver spleen scan showed very little functioning liver and a liver biopsy revealed diffuse fibrosis, focal hepatocyte necrosis and veno-occlusive changes. No other patient developed symptoms of liver dysfunction. Four of the patients who expired had autopsies and microscopic evaluation of the liver did not reveal any radiation damage. The dose volume histograms of these 10 patients permit a preliminary analysis of the radiation tolerance of the liver. The patient who developed radiation hepatitis received the highest liver dose in this series. Half of the liver was radiated to a dose of 30 Gy or qreater. Another pa- tient received a dose of 30 Gy or greater to 32% of the liver. This patient had a primary lesion of the biliary system and she is alive without evidence of liver dysfunction 15 months following radiation. Our preliminary results suggest that liver doses in excess of 30-35 Gy should be limited to 30% of the liver volume when whole liver radiation is used in conjunction with primary radiation therapy of the pancreas or biliary system.

Supported by NIH Grant lPOlCA19138 and DOE Contract DE-AC03-76SF00098

32 THE ACCURACY OF THE LINE AND POINT SOURCE APPROXIMATIONS IN IR-192 IMPLANT DOSIMETRY

J. F. Williamson, Ph.D. and W. Swindell, D.Sc., Ph.D.

Division of Radiation Oncology, University of Arizona Health Sciences Center, Tucson

The Ir-192 ribbons commonly used in Interstitial implant therapy consist of 3 mm long by 0.5 mm diam- eter seeds encapsulated in steel or platinum and spaced at intervals of 1 cm. Such details of source con- struction are usually ignored by the-dose calculation algorithms currently in use for We have analyzed the dosimetric errors introduced by two approximations widely usd in (1) treating each seed as'an isotropic point source; and (2) treating each ribbon as line of continuously distributed radioactivity.

METHODS: Three dimensional dose rate distributions (1 mm3 grid) were calculated idealized implant geometries assuming:

(a) each Ir-192 seed is a point source, (b) each ribbon is an unfiltered line source, and

treatment planning. clinical practice: an unfiltered

for various

(cl a single ribbon dose rate table derived from Monte Carlo calculations (Phys. Med. Biol. 28:1021, 1983). The distribution of radioactivity within the 3 mm seed and self-absorption and oblique filtration of Ir rays were modeled.

Comparison of the 3-D distributions was facilitated by graphing the function F against dose rate, defined as the fraction of the implanted volume that receives at least the given dose rate (i.e., F = 1.0 for Dmin and F = 0.0 for D,,j

RESULTS: The discrepancies between the three models depend mainly on the criterion adopted for dose prescription and, for the line source model, on the rule used to assign active length, L, to the line source corresponding to a ribbon with N seeds.

Proceedings of the 1st Annual ASTRO Meeting 93

If the radioactivity within ehe ribbon is considered to be distributed along a line extending from the tip of the first seed to the end of the last, i.e., L = (N-.7) cm, large errors result. The isodose contours covering 10%. 50%, and 90% of the implanted volume disagree with those predicted by the more accurate model (c) by 10-14X, 8-122, and 20% respectively when the entire volume is con- sidered and by lo-152 when only the central plane is considered. If each seed In the ribbon is replaced by a 1 cm line source (L = N cm), the differences in model (b) relative to (c) are reduced (Z-7% central plane, 2-20% entire volume). 2 to 5%.

Errors introduced by the point source model range from

Our analysis indicates that both approximations are sufficiently accurate for use in clinical dosimetry, although the linear source density in the line source approximation must be defined cor- rectly. Dosimetric differences between stainless steel and platinum clad Ir seeds will also be discussed.

33 DOSIMETRY CORSIDERATIONS OF CT-GUIDED VOLUMETRIC INTERSTITIAL BRACHYTHERAPY OF MALIGNANT BRAIN TUMORS

F.G. Abrath' , S.D. Henderson', J.R. Simpson', C.J. Moran' , J.A. Marchosky* and D.V. Rae'

1 Mallinokrodt Institute of Radiology, St. Louis, MO 63110, and 2Missouri Baptist hoosltal, St. Louis, MO 63131

Malignant gliomas, in general, are oonsidered a local disease, in that they are normally confined to one area of the brain and rarely metastasize. The standard treatments of surgery and external radiation therapy has this far been unsuccessful in controlling these tumors. With respect to the latter modality, the delivery of sufficiently high doses to completely irradiate the tumor cell population is limited by the radiation toxicity of the surrounding normal brain. The delivery of high total doses of radiation to a local area can be achieved by interstitial implantation of radioactive sources, i.e., brachytherapy. Over the past two years, an afterloading technique has been developed and refined to stereotactically implant radioactive sources (Ir-192) in malignant brain tumors of patients. Radiation dosimetry is integral in this procedure, and encompasses the immediate prescription of total dose, dose distributions and shielding considerations, and the long-term evaluation of tumor response, normal tissue response, and perhaps ultimately contribute to an understanding of the disease process itself. This report will detail the immediate dosimetry considerations and experience of interstitial brachytherapy of malignant brain tumors with Ir-192.

The implantation procedure integrates a stereotaxic system with computerized tomography (CT), which provides tumor position, volume, and guides the placement of catheters. A radiolucent ring-frame immobilizes the head as burr holes are made at 1 cm intervals with the aid of a template. Catheters containing dummy sources 1 cm apart are then inserted to the desired depth and their position verified in 3 dimensions to insure complete coverage of visible tumor volume (1 cm outside visible tumor periphery). Once catheters are secured by buttons sutured to the skin, the anesthetized patient is moved to the ICU. With adequate shielding in place, the dummy sources are replaced by ribbons of Ir-192. Specific activity of the Ir-192 has ranged from 0.7 to 1.0 mg Ra eq. The radioactive sources are removed after the desired dose is delivered.

Implants (a total of 31) have covered tumor volumes as large as 30 cm , and have contained as many as 40 catheters with 150 seeds. With these volumes the use of orthogonal films to locate sources and generate isodose curves becomes formidable. Fortunately, the present technique of volumetric implantation allows the application of brachytherapy dosimetry models. The rigid catheters containing the ribbons of Ir-192 have closely resembled 3-dimensional planar implants. CT scans with the pre-loaded dummy sources were used to designate spatial coordinates of sources. A computer program converted position data and source strength into dose contours in any plane. The implantation time for the desired dose to the periphery (80-120 Gy) was calculated. Dose rate contours were scaled to match the magnification and superimposed on preimplant CT scans. Maximum and minimum doses were determined from the various planes for the center, periphery, and at 1, 2, and 3 cm from the periphery. In general, the doses were uniform in the various planes at each distance.

Verification dosimetry has been carried out with TLDs placed in a catheter located in a plane along the tumor periphery. In vivo isodose values compared to idealized plans agree within &. Futher dosimetry is being carried out for various large volume implants using a water phantom scanning system.

34 I 125 INTERSTITIAL BRACHYTHERAPY FOR PRIMARY MALIGNANT BRAIN lUMORS: TECHNICAL ASPECTS OF TREATMENT PLANNING AND IMPLANTATION METHODS

Peggie A. Findlay, M.D.*, Donald C. Wright, M.D.t, Frank S. Harrington*, Robert W. Miller, M.S.*, and Eli Glatstein, M.D.*

*Radiation Oncology Branch, Division of Cancer Treatment, National Cancer Institute, and the tSurgica1 Neurology Branch, National Institute of Neurological Communicative Disorders and Stroke, of the National Institutes of Health, Bethesda, Maryland 20205

Patients harboring high-grade primary malignant brain tumors have proven resistant to even the most aggressive current combination therapy. External radiation is limited by dose dependent damage to normal tissue. Since time to tumor regrowth is progressively delayed with increasing radiation dose, and the