2840 Comparing Two Strategies of Dynamic Intensity Modulated Radiation Therapy (dIMRT) Using HelicalTomotherapy (HT) With 3-Dimensional Conformal Radiation Therapy (3DCRT) in the Setting ofHypofractionated Treatment of the Pelvic Nodal Basin in High-Risk Prostate Cancer

J. Yuen1, G. Rodrigues1, K. Trenka2, T. Coad2, S. Yartsev2, M. Lock1, G. Bauman1

1Department of Radiation Oncology, London Regional Cancer Program, University of Western Ontario, London, ON,Canada, 2Department of Clinical Physics, London Regional Cancer Program, London Health Sciences Centre, London,ON, Canada

Purpose/Objective(s): A large RCT showed a benefit in men with an estimated risk of nodal involvement �15% based ontumor stage, PSA and grade. Unfortunately, the use of non-conformal pelvic radiotherapy may be associated with such toxicityas proctitis and ileitis. To date, contouring the pelvic vessels and adding a 1–2cm margin has been used as a surrogate for pelvicnodal regions, but there is uncertainty as to the optimum margin needed and difficulty tracking the smaller vessels. An alternatestrategy is proposed to target the pelvic nodal basin while conformally avoiding critical structures.

Materials/Methods: 10 computed tomography datasets were selected. 3DCRT and dIMRT/HT planning was undertaken todeliver 68Gy in 25 fractions to the prostate while simultaneously delivering 45Gy in 25 fractions to the lymph node bearingregions of the pelvis in a single phase. To incorporate the regional nodal irradiation in dIMRT/HT one paradigm (nodal/conformal targeting) is to use the iliac vessels as a surrogate structure in targeting the pelvic nodes. The other approach (pelvictargeting/conformal avoidance) is to address the soft tissues of the pelvis using forward planned optimization to limit dose toidentified critical structures (small bowel, bladder, rectum, and femoral heads). The dIMRT/HT and 3DCRT solutions werecompared to each other using paired t-tests.

Results: Compared to 3DCRT the two dIMRT/HT approaches were statistically superior in their delivery to the PTV (8/8 dosepoints; p�0.001) while also sparing dose to the critical structures of rectum, bladder, femoral heads and small bowel (23/24dose points; p�0.045). Considering the two dIMRT/HT techniques, pelvic targeting delivers similar nodal coverage (8/8 dosepoints; p�ns), more homogenous PTV delivery (2/2 dose points; p�0.002), higher and more homogenous pelvic tissue dose(6/6 dose points; p�0.03), at the cost of slightly higher critical structure dose (Ddose, 1–3Gy over 5/10 dose points; p�0.03).

Conclusions: dIMRT/HT nodal and pelvic targeting is superior to 3DCRT in dose delivery and critical structure sparing in thesetting of hypofractionation for high-risk prostate cancer. The pelvic targeting paradigm, with its additional small normal tissuecosts, is a potential solution to deliver highly conformal pelvic radiation in the setting of true nodal location uncertainty. Effectsof movement in normal tissue volumes were not considered here. Future studies incorporating this uncertainty would be usefulin delineating the optimal strategy for nodal basin irradiation.

Author Disclosure: J. Yuen, None; G. Rodrigues, None; K. Trenka, None; T. Coad, None; S. Yartsev, None; M. Lock, None;G. Bauman, None.

2841 Cobalt-Based Tomotherapy Using a MIMiC Multi-Leaf Collimator

G. J. Salomons, M. V. Rogers, N. Chng, A. T. Kerr, L. J. Schreiner

Kingston General Hospital, Kingston, ON, Canada

Purpose/Objective(s): Tomotherapy is a rotational implementation of IMRT based on the modulation of a fan beam ofradiation from a source resulting in a highly conformal distribution. Although linac based tomotherapy has entered into clinicaluse we believe that the simple nature of Co-60 teletherapy can extend the adoption of tomotherapy.

Materials/Methods: We have shown that conformal dose delivery is readily achievable with Co-60 in the tomotherapy settingand the differences between treatment plans generated with Co-60 and 6MV beam energies (Fig. 1) are not clinically significant.Further, megavoltage CT scanning (MVCT) using Co-60 (Fig. 2) provides accurate imaging for patient set-up verification.

Results: For these reasons we have moved ahead with a clinically viable Co-60 tomotherapy unit consisting of a NOMOSMIMiC MLC mounted on the CCSEO MDS Nordion T-780C Co-60 unit (Fig. 3). In this work we will show the beamcharacteristics of the Co-60 tomotherapy unit and discuss the challenges of mounting a MIMiC on our Co-60 unit given the size

S683Proceedings of the 48th Annual ASTRO Meeting

of the Co-60 source and its 80cm SAD. CT images from a MIMiC collimated Co-60 beam will also be presented and theaccuracy of the CT numbers and their relationship to electron density will be discussed, particularly in regards to metallicimplants.

Conclusions: Our results will show that Co-60 holds the promise of a simple, robust tomotherapy source.

Author Disclosure: G.J. Salomons, MDS Nordion, C. Other Research Support; North American Scientific (NOMOS), C. OtherResearch Support; Varian Medical Systems, C. Other Research Support; MDS Nordion (not yet confirmed), D. SpeakersBureau/Honoraria; M.V. Rogers, None; N. Chng, None; A.T. Kerr, None; L.J. Schreiner, None.

2842 Field Junctioning Over Spinal Cord Using Dose-Volume-Histogram Analysis

T. W. Zusag, S. Ladsaria, R. A. Abrams

Rush University Medical Center, Chicago, IL

Purpose/Objective(s): Junctioning treatment volumes over spinal cord is a common yet potentially risky clinical situation.Algorithms based on visual or radiographic confirmation of distances between edges of diverging entry fields require simplebeam geometries, and cannot be used with oblique or noncoplanar beams, dose gradients in three dimensions, or when thepatient’s position is changed for the second treatment course. Because of these restrictions we developed a methodology forabutting high-dose regions over spinal cord in retreatment of patients planned with 3D techniques.

Materials/Methods: From December 2004 to April 2006, 6 patients presented for 8 courses of retreatment requiring 9 separatejunctions of target volumes over spinal cord. The shortest interval between treatments was 2 months, the longest 2 1/2 years.Four patients with lung cancer irradiated with multiple concurrent conformal oblique beams were subsequently retreated forspine or spinal cord metastases. In one patient 3 junctions were managed, 2 of them concurrently. Two patients had sarcomas

S684 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006