S548 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008

HDR brachytherapy treatments. Cone Beam Simulator CT may provide the ability to move from 2D to 3D Image Guided Brachy-therapy.

Author Disclosure: S. Devic, None; T. Vuong, None; M. Evans, None; E. Podgorsak, None.

2847 Dosimetric Comparison of Radiation Techniques to the Prone Pelvis for Rectal Cancer: 3-field Based on

Bony Landmarks (2D) vs. 3-dimensonal Conformal (3D) vs. Intensity Modulated Radiation Therapy(IMRT)

N. E. Martin1, A. O. Nawaz2, B. Schuller2, K. R. Kozak1, T. S. Hong3, L. A. Kachnic2

1Harvard Radiation Oncology Program, Boston, MA, 2Boston Medical Center, Boston, MA, 3Massachusetts General Hospital,Boston, MA

Purpose/Objective(s): As intensified preoperative therapies are explored for locally advanced rectal cancer, radiation deliverytechniques to reduce toxicities will become increasingly important.

Materials/Methods: Ten consecutive patients (pts) with locally advanced rectal cancer treated with pelvic radiation comprised thestudy cohort. Seven pts treated in the prone position using a bowel displacement device are analyzed here. The primary clinicaltarget volume (CTV) was defined as the gross tumor volume expanded by 1.5 cm radially and 2 cm craniocaudally plus the entiremesorectum (perirectal and presacral regions). The internal and external iliac vessels to the level of the acetabulum with a custom 1cm radial margin were defined as separate CTVs. Planning target volumes (PTVs) were defined by 5 mm expansions of the CTV.Three plans for coverage of (1) the primary and internal iliac CTVs; and (2) the primary, external and internal iliac CTVs werecompared for each patient as follows: (1) a 3-field approach using ‘‘standard’’ bony landmarks to define the apertures (2D); (2)a 3D plan where 8 mm expansions on CTVs defined the conformal aperture edge; and (3) an IMRT plan designed to conformto the PTV. For the 2D and 3D plans, dose was prescribed to 97% to the primary and at-risk nodal CTV to 45 Gy followed bya 5.4 Gy boost in 28 fractions. For IMRT, dose painting was used to cover the at-risk nodal PTV to 45 Gy and the primaryPTV to 50 Gy in 25 fractions. IMRT plans were optimized with target dose conformation as the primary objective. The femoralheads, bladder, external genitalia, and small and large bowel were delineated. Tabulated dose-volume histograms were comparedfor the 42 generated plans using the Kruskal-Wallis test.

Results: All 3 techniques produced comparable coverage of the targets. Generally, greater organ sparing was noted with IMRT forplans covering both the internal and external iliacs. For the bladder, in plans covering both nodal volumes, the V40 was 95.4, 73.3,and 38.1% for the 2D, 3D, and IMRT respectively (p\0.001). For the femoral heads, the V40 was 28.7, 10.4 and 2.6% for the 2D,3D, and IMRT plans covering the internal iliacs respectively (p = 0.001). Prone positioning eliminated much of the small and largebowel from the pelvis, and while the dose to small volumes decreased by as much as 10 Gy with IMRT, this did not reach statisticalsignificance.

Conclusions: When compared to standard 2D or conformal 3D techniques, IMRT can potentially reduce dose to normal struc-tures, while still maintaining full target coverage, for pts undergoing preoperative radiation for rectal cancer. Greater dosesparing to normal structures was noted for IMRT plans in which external iliac nodes were included in the at-risk CTV to45 Gy.

Author Disclosure: N.E. Martin, None; A.O. Nawaz, None; B. Schuller, None; K.R. Kozak, None; T.S. Hong, None; L.A. Kachnic,None.

2848 MR Guided Focused Ultrasound for Cancer Therapy: Enhancement of Drug Delivery in Prostate Tumor In

Vivo

L. Chen, Z. Mu, P. Hachem, A. Konski, G. Freedman, C. Ma, A. Pollack

Fox Chase Cancer Center, Philadelphia, PA

Purpose/Objective(s): The purpose of this work was to commission and to evaluate the role of MR guided high intensity focusedultrasound (MRgHIFU) in cancer therapy. We specifically focused on investigating the feasibility of the enhancement of drug de-livery in prostate cancer for gene therapy with AS-MDM2 and for chemotherapy with both doxorubicin and docetaxel using a nudemice model.

Method/Materials: We conducted experiments with an InSightec ExAblate 2000 HIFU system with a 1.5T GE MR scanner thatis approved by FDA for the treatment of uterine fibroids. This system is clinically being investigated in our department for treatingbone metastases, prostate and breast cancers under local IRB approval. The phased array transducer is housed in a sealed bath andconnected to a motion system. The focal region is cigar shaped, �2mm in diameter and 10mm in length. The sequence of ourstudies is as follows. First, we performed experiments on an acoustic phantom to determine the optimal ultrasonic parameters in-cluding frequency, acoustic power, and exposure duration. Second, we developed techniques for the treatment of prostate tumor-bearing mice. Third, we evaluated the ‘‘optimal’’ ultrasonic parameters for drug delivery in prostate tumors in mice using doxo-rubicin. Fourth, based on the ‘‘optimal’’ ultrasonic parameters derived from the doxorubicin experiments we studied theMRgHIFU enhancement of AS-MDM2 in implanted prostate tumor in mice in vivo by measuring the protein expression levelof MDM2, p53 and p21 after treatment using immunohistochemical staining. We also studied the efficacy of MRgHIFU forthe enhancement of AS-MDM2 delivery in inhibiting prostate tumor growth in vivo. Finally, we used a radioactive tritiateddocetaxel (3H-docetaxel) to determine the uptake in prostate tumor by quantitatively measuring the radioactivity using a liquidscintillator.

Results: Phantom studies demonstrated that MRgHIFU could provide adequate temperature elevation for tissue ablation; acous-tic power .10W leading to temperature elevation (DT) .7�C. At a lower acoustic power (5 W) we could keep DT \4�C,which is adequate for drug enhancement. The results were then applied to animal studies. Tissue damage was achieved atthe predetermined region through MR real-time guidance. Our data shows increased doxorubicin concentrations in prostatetumor in MRgHIFU treated animals (0.2 ± 0.02 mg) vs. those without MRgHIFU treatment (0.11 ± 0.03 mg); Docetaxel