S372 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

2376 Interfraction Seminal Vesicle Motion and Target Margin Assessment for Fiducial-Guided Intensity

Modulated Radiotherapy for Prostate Cancer

M. H. Stenmark, K. A. Vineberg, D. W. Litzenberg, D. A. Hamstra, M. Feng

University of Michigan, Ann Arbor, MI

Purpose/Objective(s): To determine if differential interfraction motion necessitates specific planning target volume (PTV) mar-gins for the seminal vesicles (SV) during fiducial-guided radiotherapy of the prostate.

Materials/Methods: 20 patients were enrolled in an IRB-approved prospective study, which included fiducial implantation and 3sets of CTs during treatment. SV motion relative to the prostate was analyzed by CT at 60 interfraction time points for these pa-tients, assuming perfect set-up to the prostate with daily guidance. Following mutual information image registration of the prostatefor each instance, the simulation SVs were expanded in 1 mm increments until the expansion fully covered the union of SV volumesfor each patient to evaluate the geometric extent of motion. To evaluate the dosimetric consequences of variable SV positions, twointensity-modulated radiotherapy (IMRT) plans were generated for each patient including, 1) the prostate and proximal (1cm) SV(PSV), and 2) the prostate and full SV (FSV), both with a standard 5 mm PTV margin for optimization. Treatment planning wasperformed by a single operator using a multi-criteria hierarchical optimization method. Dosimetric coverage of both the proximaland full SVs was assessed at each instance and compared with the initial treatment plan.

Results: Positional variability was greater for the FSV as compared to the PSV. A 5 mm PSV-PTV margin yielded 95% geometriccoverage (C95%) in 95% of cases; median coverage of 99% (range 93-100%, SD 2%). However, a 5 mm FSV-PTV margin onlyresulted in C95% SV in 55% of cases; median coverage of 96% (83%-100%, SD 5%). To achieve C95% for the FSV in .95% ofcases an 8 mm margin was required (median 99%, 95-100%, SD 2%). To obtain C99% in .95% of cases, the PSV required a 7mmexpansion, while the FSV required a 10 mm expansion. Despite the lack of full geometric coverage of all PSV instances with a 5mmPTV expansion, dosimetric coverage was adequate. For plans optimized to the prostate and PSV, motion did not impact the volumeof PSV covered by the 100% isodose line (V100%), mean 95% (SD 3%), or the 95% isodose line (V95%), mean 99% (SD 1%). Forplans optimized to the FSV, dosimetric coverage was compromised in some patients, as V95% was .95% in only 70% of cases.However, overall, mean V100% was 89% (range 65%-98%, SD 8%) and mean V95% was 96%, (range 89%-100%, SD 4%).

Conclusions: The SVs move differentially from the prostate and exhibit greater variation with increasing distance from the pros-tate. For plans targeting just the prostate and PSVs, 5mm PTV expansions are adequate. However, despite daily localization of theprostate, larger PTV margins are required for cases where the intent is to cover the FSVs.Supported by NIH/NCI (1 R21 CA110485-01A1).

Author Disclosure: M.H. Stenmark, None; K.A. Vineberg, None; D.W. Litzenberg, None; D.A. Hamstra, None; M. Feng, None.

2377 The Use of MRI Imaging in IMRT Treatment Planning for Prostate Cancer

S. Vora, A. C. Silva, W. W. Wong, J. M. Collins, S. E. Schild

Mayo Clinic Arizona, Phoenix, AZ

Purpose/Objective(s): Dose escalation improves biochemical control rates based on phase III trials. However, higher doses havebeen associated with increased risk of bladder and rectal toxicity. Improvements in MRI imaging allow better definition of suspi-cious areas within the prostate. The purpose of this study is to assess the clinical utility of incorporating MRI imaging and its’ im-pact in treatment planning.

Materials/Methods: 25 patients with clinically localized prostate adenocarcinoma underwent 1.5 T 8-channel surface coil MRI ofthe prostate using high resolution axial, coronal, and sagittal T2 weighted series, axial T1 weighted series, contrast enhanced dy-namic and delayed T1 weighted fast gradient-echo series and a diffusion weighted sequence. No endorectal coil was used. The MRIwas performed following placement of internal fiducial markers and CT simulation. The MRI images were reviewed for the fol-lowing factors: early arterial enhancement, restricted diffusion, and T2 hypointensity. If patients had $2 factors, the area was iden-tified as suspicious. MRI images were fused with the planning CT scan. The entire prostate+3mm margin (PTV) was planned toreceive 77.4 Gy/43 fractions (1.8 Gy daily dose) and MRI suspicious areas (GTV) received concomitant boost to 83 Gy/43 fractions(1.93 Gy daily dose). Rectal/Bladder tolerance doses were maintained to keep D30\70 Gy, D10 \75 Gy, D1.8cc\81 Gy. If theconstraints could not be met or if imaging was inconclusive, the GTV dose was either reduced to 81 Gy or no boost was delivered.

Results: Of 25 patients, 22 (88%) patients had MRI imaging that contained suspicious areas consistent with malignancy. IMRTtreatment plans generated .99% coverage of the PTV and .97% coverage of the GTV. 15 pts (68%) received boost dose of 83 Gyand the remaining 7 (32%) received 81 Gy. Review of all MRI imaging demonstrated that placing the internal fiducial markers priorto the MRI caused artifacts which can alter the GTV. Nine patients (36%) had evidence of post-fiducial placement artifact withhemorrhage most commonly observed.

Conclusions: We found MRI imaging helpful in IMRT treatment planning, allowing us to define a tumor volume for dose esca-lation. Longer follow-up will be needed to assess biochemical control and toxicity. We now perform MRI imaging prior to internalfiducial marker placement to reduce artifacts and improve the accuracy when defining the GTV.

Author Disclosure: S. Vora, None; A.C. Silva, None; W.W. Wong, None; J.M. Collins, None; S.E. Schild, None.

2378 MRI-based Treatment Planning for Prostate Brachytherapy

J. M. Albert, M. Zhang, T. L. Bruno, R. J. Kudchadker, D. A. Swanson, S. J. Frank

M. D. Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Transrectal ultrasound (TRUS) is the standard imaging modality for brachytherapy treatment planning.However, MRI provides better anatomical delineation of the prostate. With more precise anatomic detail, MRI may confer advan-tages during treatment planning. Endorectal coil MRI (ecMRI), often used for staging, provides even greater anatomical resolution