Proceedings of the 50th Annual ASTRO Meeting S541

due to setup error and organ motion). Discrepancies $7% were verified by repeat measurement. Dose measurement promptedan 8.6% decrease in monitor units in one patient, 6.8%, 7.2%, and 7.6% MU increases were suggested in 3 other pts. MOSFETmeasurements suggested a need for MU correction in one pt, for whom correction was not indicated based on TLD measure-ments.

Conclusions: IMRT planning dose representations in the high dose region were not confirmed by TLD and MOSFET measure-ments in all pts. Tumor control and toxicities for anorectal IMRT may be affected by uncorrected discrepancies in delivered dose.Discordance in measured dose .5-7%, confirmed on repeat measurement, can be applied in correcting monitor units delivered.

Author Disclosure: R. Nordal, None; R. Popple, None.

2832 Dosimetric Impact of Minimizing Beam Segments in Breath Hold Liver Cancer IMRT

M. T. Lee1,2, T. Purdie1, C. Eccles1, T. Craig1, P. Lindsay1, M. B. Sharpe1,2, L. A. Dawson1,2

1Radiation Medicine Program, Toronto, ON, Canada, 2University of Toronto, Toronto, ON, Canada

Purpose/Objective(s): To investigate the relationship between number of segments used to deliver liver cancer IMRT and geo-metric uncertainties on delivered doses. The hypothesis is that, for plans that meet target coverage objectives and maintain normaltissue tolerances, reduced number of beam segments will increase the likelihood that delivered doses are similar to nominal planneddoses when geometric uncertainties are considered (i.e., more robust).

Materials/Methods: IMRT (30-50 segments) was previously found to facilitate dose escalation compared to 6 fraction breath-holdconformal RT, when prescription dose is individualized, based on effective liver volume irradiated. IMRT plans were re-optimizedafter reducing the segment number down to a minimum of 4 per plan. Re-optimized plans were acceptable if the minimum (0.5 cc)PTV dose was within 0.6 Gy of the baseline plan, max PTV dose \140% and normal tissue constraints (e.g., stomach maximumdose\30 Gy) were maintained. Plan complexity was scored according to the total monitor units (MU), number of segments, leafmovement and segment area. The Computational Environment for Radiotherapy Research (CERR), a Matlab based radiation planassessment tool (Washington University, St. Louis, MO), was used to evaluate and recompute the doses for the plans taking intoaccount residual geometric offsets in liver position for each fraction. Offsets were obtained from breath hold kV cone-beam CTsobtained in the treatment position following 2D image guidance (random, s, 1.6 mm mediolateral (ML), 1.1 mm craniocaudal (CC)and 2.3 mm anterior-posterior (AP); systematic, S, 1.6mm ML, 3.3 mm CC and 4.3 mm AP).

Results: 218 IMRT plans were assessed for 12 of 20 planned patients with 19 tumors. The mean GTV was 130 cc (range, 5 to465 cc). The median prescribed dose was 49.3 Gy in 6 fractions (range, 27.6-60 Gy). The median minimum number of segmentsfor acceptable, less complex plans was 10 (range, 4 to 22) with reduction in MUs by 20% (p = 0.001), as well as overall com-plexity (based on segment area, weights and leaf positioning) (p = \0.001). There were no significant differences for PTVEUDa=-20, (50.4 vs. 50.5 Gy, p = 0.86), maximum PTV dose (p = 0.75), liver Lyman NTCP (p = 0.085) or maximum dosesto stomach (p = 0.56) and bowel (p = 0.71). RTOG conformity index was higher with less complex plans (1.6 vs. 1.42, p =0.02). No significant differences in delivered doses were seen when geometric uncertainties were considered. A trend for higherGTV doses in less complex plans was seen (mean minimum dose to 99% GTV 49.8 Gy vs. 49.0 Gy, range of dose differences -0.8 to 3.1 Gy, p = 0.23).

Conclusions: Reducing IMRT complexity is feasible, with no adverse impact on delivered doses to GTV and normal tissues. Atrend for higher GTV doses with less complex plans was seen when residual geometric errors were considered.

Author Disclosure: M.T. Lee, None; T. Purdie, None; C. Eccles, None; T. Craig, None; P. Lindsay, None; M.B. Sharpe, ElektaOncology Systems, C. Other Research Support; Raysearch Laboratories AB, C. Other Research Support; Philips Medical Systems,C. Other Research Support; Philips Medical Systems, F. Consultant/Advisory Board; Elekta Oncology Systems, F. Consultant/Ad-visory Board; L.A. Dawson, Elekta Oncology Systems, B. Research Grant.

2833 Four-dimensional Heavy Charged Particle Beam Radiotherapy in Pancreatic Cancer

S. Mori1, G. Sharp2, M. Kumagai1, R. Hara1, H. Asakura1, S. Yamada1, R. Kishimoto1, H. Kato1, S. Kandatsu1

1NIRS, Chiba, Japan, 2Massachusetts General Hospital, Boston, MA

Purpose/Objective(s): Intrafractional motion affects the accuracy of the delivered dose in radiotherapy, particularly in carbonbeam radiotherapy. Although recent studies using 4D-CT have focused on the thoracic region, evaluation of dose variation dueto intrafractional respiratory motion is also necessary in abdominal regions, such as the liver and kidney, etc. However, thepoor image quality in abdominal 4D-CT resulting from respiratory phase-based resorting process error hampers calculation ofquantitative dose distribution. Here, we evaluated dose variation due to intrafractional motion using fast-rotate cone-beam CTand compared 4D dose assessments between respiratory-ungated and -gated charged particle pancreatic therapy.

Materials/Methods: Four-dimensional (4D) CT scan using a 256-multislice CT was performed in a patient with pancreatic cancerunder free breathing conditions. Two range compensators previously designed to ensure sufficient carbon beam coverage of PTVfor ungated and gated treatment strategies using 4D-CT data sets at respective phases were applied to 4D-CT, and then the carbonbeam dose distribution was calculated with a pencil beam algorithm. 4D-DVHs for the two strategies were calculated andcompared.

Results: Although abdominal region consists tissue density more than thoracic region, significant beam overshoot and undershootwere observed due to respiratory-induced intestinal gas bubble motion. Dose variation was minimized with respiratory-gated com-pared with ungated treatment. In contrast, DVHs incorporating respiratory function (4D-DVH) were similar.

Conclusions: We quantified 4D dose variation in the abdominal region using 4D-CT, which includes deformable registration.Dose assessments between the respiratory-ungated and -gated strategies did not greatly differ throughout treatment. 4D dose as-sessment more closely reflects actual clinical conditions than conventional planning, and is useful in defining treatment strategies.

Author Disclosure: S. Mori, None; G. Sharp, None; M. Kumagai, None; R. Hara, None; H. Asakura, None; S. Yamada, None;R. Kishimoto, None; H. Kato, None; S. Kandatsu, None.