Fig. 1: Comparison of Tomo and SMLC in target coverage and normal tissue sparing

Author Disclosure: C. Yang, None; S. Narayan, None; F. Guo, None; C. Wu, None; J. Perks, None; S. Vijayakumar, None; J.Purdy, None.

2822 Dosimetric and Therapeutic Evaluation of Megavoltage Spatially Fractionated Radiotherapy (SFR) WithDifferent GRID Block Designs

K. Dou, A. S. Meigooni, N. J. Meigooni, M. Gnaster, S. B. Awan, S. Dini, E. L. Johnson, M. Kudrimoti

University of Kentucky, Department of Radiation Medicine, Lexington, KY

Purpose/Objective(s): Dosimetric characteristics of a newly designed GRID block for megavoltage radiotherapy wereinvestigated using TLD, film, and ionization chamber. Therapeutic advantage for this treatment modality was calculated usinga linear quadratic model. Efficacy was assessed for three different GRID designs of aperture size, pattern, and spacing and tissueradiosensitiviy.

Materials/Methods: A newly designed GRID block has recently become available from Radiation Product Design, Inc. for useon linear accelerators. Dosimetric characteristics of three GRID blocks with the aperture size of 14mm, 13mm, and 9.1mm(projected at isocenter) and center-to-center spacing of 24mm, 18mm, and 19mm, respectively, were evaluated for both 6MVand 18MV photons. The measurements were performed using films and TLDs in Solid WaterTM phantom and an ionizationchamber in water. The dose profiles were then used to evaluate the therapeutic advantage of the GRID therapy as a functionof dose, grid aperture size and spacing, and radiosensitivity with a linear-quadratic (LQ) model. Therapeutic ratio (TR) for thesame tumor survival fraction (SF) is defined here as TR � [SF of normal tissues with Grid field] / [SF of normal tissues withopen field]. Therapeutic ratios of these GRID blocks were determined for a single fraction with doses ranging from 0.5 to 30 Gy.

Results: A comparison of the therapeutic advantage of three different GRID blocks calculated using the measured beam profilesfrom each block is shown in Fig. 1 for 6 MV photons. The results in Fig. 1A are based on the 2-Gy survival fraction (SF2) of0.5 for both normal and tumor cells. However, Fig. 1B shows the TRs of GRID radiotherapy using different tumor SF2 valueswhile maintaining the SF2 of 0.5 for normal tissues. These results indicate that, for 6 MV x-ray beam, the TR of a specific cellline increases by, 1) increasing the dose, 2) increasing hole size, while keeping the same center-to-center spacing, 3) increasingthe spacing, and maintaining the same hole size, or 4) increasing the tumor SF2 (more likely representing radioresistant celllines).

Conclusions: GRID radiotherapy exhibited a significant therapeutic advantage over the open field radiotherapy when the tumorcells were more radioresistant than normal cells. The therapeutic advantage of the GRID therapy is also dependent on thetreatment dose, GRID aperture size and center-to-center spacing.

Author Disclosure: K. Dou, None; A.S. Meigooni, None; N.J. Meigooni, None; M. Gnaster, None; S.B. Awan, None; S. Dini,None; E.L. Johnson, None; M. Kudrimoti, None.

2823 Dosimetric Evaluation of Parallel Opposed Spatially Fractionated Radiation Therapy (GRID) for Deep-Seated Bulky Tumors

A. S. Meigooni, N. J. Meigooni, M. Gnaster, K. Dou, S. Dini, M. Kudrimoti

University of KY-Chandler Medical Ctr., Lexington, KY

Purpose/Objective(s): The efficacy of GRID radiation therapy has been proven for the management of bulky (�8 cm indiameter) malignant tumors with a single dose of 12–20 Gy1,2. Our early studies have proven that when small areas ofirradiated skin and subcutaneous tissues shielded by the grid block served as centers of re-growth for normal tissues overlyingthe tumor volume. The clinical data obtained thus far for GRID therapy consisted of single radiation field with the prescribeddose ranging from 10 to 20 Gy at dmax. However, for deep seated bulky tumor, such as in the abdomen or pelvis, a single fieldmay deliver larger dose to the overlying normal tissue than the tumor. In this investigation the parallel opposed beam is beingevaluated for treatment of deep seated bulky tumors, using two different grid patterns.

Materials/Methods: Dosimetric characteristics of single field and parallel opposed radiation field were experimentallydetermined with film in Solid Water phantom and using an ion chamber in water. The measurements were performed with SADtechnique using both 6 and 18 MV x-ray beams from a Varian Clinac 2100EX linear accelerator. Two different Grid blockpatterns, fabricated by Radiation Products Design, Inc. were studied. The cerrobend GRID blocks of 7.5 cm thickness weremanufactured with hole diameters of 9.1mm and 14mm (projected at isocenter) and 18 mm center-to-center spacing.

Results: A comparison of dose profiles at tumor (i.e. mid-plane) and depths of 1.5 cm and 6.5 cm, in a 23 cm thick phantom,using a single and parallel opposed beam techniques with the 14mm grid block is shown in Figure 1. Similar results wereobserved for the 9.1 mm grid block. The results of these investigations indicated that with both 9.1mm and 14mm GRID blocks,

S672 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006