Evaluation of the performance of the FFT-convolution in the thorax of an anthropomorphic phantom

Proceedings of the 36th Annual ASTRO Meeting

325

1148 EVALUATION OF THE PERFORMANCE OF THE FFT-CONVOLUTION IN THE THORAX OF AN ANTHROPOMORPHIC PHANTOM A. Boyer, Cl. Starkschall, and T. Willoughby Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, Texas 11030 m: A convolution algorithm has been developed to calculate x-ray dose distributions in three dimensions in an inhomogeneous medium. This study measured the performance of the algorithm in the thorax of an anthropomorphic phantom to determine how well it could compute dose distributions when the x-ray beam passed through lung, air, and bone, and when it missed a large portion of the patient altogether as in the case of tangential breast fields. wand Typical irregularly-shaped lung fields and typical tangential breast fields were specified by radiation oncologists for an anthropomorphic phantom. For each case, CT scans were made of the phantom with semi-radioopaque markers to delineate the treatment field positions and margins. The CT scans were transferred to a radiotherapy treatment planning system. The system employed a Fourier-convolution x-ray dose modeling algorithm to calculate the full three-dimensional dose distributions taking into consideration the heterogeneities measured by the CT scans. The phantom was loaded with lithium fluoride thermoluminescent dosimeters (TLD) and the treatment plans were executed on a linear accelerator. In each case the dose was measured with the TLD in the transverse plane containing isocenter and in transverse planes near the superior and inferior margins of the fields. The doses predicted by the computation were compared to the doses measured by the TLD and the differences were expressed as a percentage of the measured doses. m: In the lung study the measured doses within the Planning Target Volume (PTV) differed from the calculated doses ranging from -3.4% to +2.9% about a mean error of +O.l%. In the Irradiated Volume the measured doses differed from the calculated doses from -45% to +lOO% with 7 out of 11 of the differences falling within +5.1% to -0.9%. The large errors were near field edges where phantom positioning was critical. In the tangential field study, all of the measurements were in the Planning Target volume and they differed from the calculated doses between -4.4% (at the edge of the PTV) and +0.7% m: The convolution calculation based on CT scans predicted the dose measured in an anatomical phantom to within 5% in the Planning Target Volume. Most doses calculated in the Irradiated Volume are also within 5% of the measured dose although calculations near the edges of fields may have been subject to positioning errors. This investigation was supported in part by PHS Grant No. CA43840 awarded by the National Cancer Institute, DHHS.

1149 THE IMPORTANCE OF DOSE-VOLUME HISTOGRAM ANALYSIS IN DESIGN AND EVALUATION OF DOSE ESCALATION STUDIES. Pat&k E. Hanssens, M.D.. Andrzej Niemierko, Ph.D., Marcia M.Urie, Ph.D., Laurene G. Renard, M.D., Eugen B. Hug, M.D., John E. Munzenrider, M.D. Dept. of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, and Harvard Cyclotron Laboratory, Harvard University, Cambridge, Massachusetts. w : Evaluation of dose escalation studies assumes that the prescribed dose to the target is delivered homogeneously. However, due to the geometry of the target, the presence of abutting critical normal structures and the physical characteristics of radiation beams, the dos6 actually delivered may be non-homogeneous, which may result in significant tumor volumes receiving a lower dose than prescribed. This study describes the results of dose-volume histogram analysis of tumors abutting critical normal sttuctures treated with combined high dose proton and photon radiation after three dimensional treatment planning. Material Dose-volume histograms of 37 patients with chordoma of the skull base were analyzed. Doses are expressed as Cobalf Gray equivalent (CGE). The units of CGE are determined by the dose in Gray (Gy) multiplied by a relative biologic effectiveness (RBE) for modulated protons relative to 60 Cobalt radiation of 1.1. The prescribed dose (PD) was 69.4 CGE in 23 patients (Group I), 72 CGE in 7 patients (Group II) and 75.6 CGE in 7 patients (Group Ill). The normal tissue constraints (NTC) for the three groups were 53 CGE to the center of the spinal cord and brainstem, 64 CGE to the surface of the spinal cord and brainstem and 60 CGE to the optic nerves and chiasm. We calculated the minimum dose (Min. Dose), maximum dose (Max. Dose) and the &se delivered to 95%. 90%. 60% and 50% of the tumor volume (95%-dose, 90%~dose, 60%-dose, X&dose) and the percentage of the tumor volume that received the prescribed dose (%VPD). Results: The averages and (ranges) of each parameter for group I, II and Ill are shown below : FD -CGE-

%VPD -%-

Min. Dose

-CGE-

95%-dose -CGE-

QO%-dose -CGE-

60%-dose -CGE-

50% dose -CGE-

66.4 72.0

53.2 ( Q-75 ) 53.4 (34-75)

56.6 (50-63) 60.6 (57-64)

62.6 (56-66) 65.7 (60-66)

64.4 (60-66) 67.6 (61-70)

66.4 (60-69)

66.4 (64-70)

74.0

II

69.6

(64-72)

72.1

(70-73)

79.9

(74-66)

III

75.6

31.4

62.5

67.9

70.0

71.9

(70-74)

74.6

(74-75)

78.9

(76-85)

Group

I

(15-48)

(55-66)

(65-72)

(67-73)

Max. Dose -CGE(69-62)

The minimum dose in the tumor volume is determined by the normal tissue constraints (NTC). For the same NTC, the percentage of the tumor volume receivino the wescribed dose (%VPD) decreases as the wescribed dose increases. Therefore. the effective dose actuallv delivered (that is, the bse hhiih. when appiii homogeneously, c&s the same biological effect) is lowe; than the prescribed do&. This should be considered in.thedesign and evaluation of dose escalation studies. In addition to the recommendations of ICRU 50, NTc’s, %VPD and effective dose should be included when reporting dose-response data.