S696 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009

therapy, we will obtain values to be used in hypo-fractionated treatment regimens intended for Brain/Head & Neck, Thorax, Ab-domen and Pelvic sites for use by clinicians as a reference.

Materials/Methods: Recent technical advances have created a need for better understanding the correlation of a/b ratio and con-ventional normal tissue tolerance dose limits for various fractionation schemes as dose escalation and hypo-fractionation schemeshave been attempted. The Linear-Quadratic (L-Q) Model was used to determine equivalent prescription dose for hypo-fractionatedschemes assuming a/b ratio of 3 and 10 for cancerous and normal tissues respectively. Recent publications have raised valid ques-tions on the assumption of a/b ratio of 3 for cancerous tissue. This provokes additional questions on the validity of hypo-fraction-ation and dose escalation regimens currently used. Keshwar, Milano and Timmerman have compiled extensive data from publishedliterature as well as RTOG protocols on a/b ratio, percentage of volume irradiated, total dose delivered and clinical end points fora variety of organs. The L-Q model has been used to calculate and create equivalent hypo-fractionated dose for a range of a/b from0.1 to 10.0 and total dose of 20 Gy to 90 Gy delivered in one (1) to five (5) fractions. These equivalent doses have been combinedwith clinically accepted and published dose limits used in various RTOG protocols.

Results: The resulting data has been consolidated for various clinically relevant organs in four body regions: Brain/Head & Neck,Thorax, Abdomen and Pelvis. Convenient, user-friendly, reference tables for normal tissue tolerance doses have been created forclinical use.

Conclusions: This development of concise tables of data that include clinically relevant, inter-connected parameters in one placewill enable clinicians to customize hypo-fractionated treatment schemes. By converting well-known conventional normal tissuetolerances to a convenient, user-friendly, digital reference, this data can facilitate further testing of the a/b hypothesis and safe uti-lization of hypo-fractionated treatments in clinical radiotherapy. These reference data tables may also influence developing con-sensus for hypo-fractionation schemes intended for various clinical sites.

Author Disclosure: R. Wynn, CK Society, B. Research Grant; NCI R-20, B. Research Grant; R. Bhatnagar, CK Society,B. Research Grant.

3145 Stereotactic Radiosurgery Options for Small Metastasis Brain Tumors with High Resolution Geometric

Setup

K. Li, J. C. Grecula, J. F. Montebello, J. Z. Wang, N. Gupta, L. Lu, H. Zhang, S. S. Lo, N. A. Mayr

The Ohio State University, Columbus, OH

Purpose/Objective(s): With the development of molecular imaging technique, it is possible to define early stage tumors whiletheir volume is very small. Therefore, there is an increasing need to understand the effectiveness of current Stereotactic Radiosur-gery equipments in treating very small targets. In this study, we compared and evaluated treatment options for the highest physicalresolution Stereotactic Radiosurgery equipments from 2.5mm Moduleaf Linac based IMRT (Moduleaf), Nomos MIMiC ARC to-motherapy with 4mm beak (Nomos) and Leksell Gamma Knife C Model with 4mm helmet size (GK).

Materials/Methods: An isolated target with the size less than 50mm3 was identified and treatment plans were developed for Mod-uleaf, Nomos and GK respectively. First, the target was identified in the GK Treatment Planning System (TPS) and transferred toNomos Covus 8.1 TPS and X-Knife RT TPS. Prescription was 50% isodose line covering the target in GK TPS, and the pluggingpattern web engine was employed to generate a more conformal plan. The 2.5mm Moduleaf plan was computed using Xknife RTTPS with a twelve-field setup. For Nomos TPS, the arc length was set to 340 degrees. Comparisons of all 3 treatment plans wascarried out using dose-volume histogram (DVH), target-volume ratio (TVR), which is defined as the ratio of volume that receivesa dose higher than or equal to the minimum tumor dose and the tumor volume, maximum dose to the prescription dose ratio(MDPD), ratio PITV (PIV/TV), radiation conformity index (TVPIV/TV), and a combined conformity index (TVPIV

2/(TV�PIV)),where PIV is the prescription isodose surface volume, TV is target volume, and TVPIV is the intersection of TV and the PIV.

Results: With normal treatment planning efforts, plan evaluation parameters from GK TPS, Nomos TPS and XKnife RT TPS are1.0, 1.5, 2.0 for TVR, 2.0, 1.05, 1.05 for MPPD, 2.3, 1.5, 2.0 for PITV, 1.0, 1.0, 1.0 for TVPIV/TV, and 0.43, 0.67 and 0.5 for(TVPIV

2/(TV�PIV)), which is the inverse of the TVR for both plans from Nomos Covus 8.1 TPS and X-Knife RT TPS becausethe prescription dose surface covered the target volume.

Conclusions: For targets smaller than 50mm3, the Gamma Knife treatment plan gave the best target volume ratio. Both Nomos arcTomotherapy and X-Knife LINAC based IMRT gave better dose homogeneity. In addition to improvements in localization accu-racy, smaller sizes of Nomos MLC and better IMRT planning algorithms for 2.5mm Moduleaf are needed to improve the planconformality for very small targets.

Author Disclosure: K. Li, None; J.C. Grecula, None; J.F. Montebello, None; J.Z. Wang, None; N. Gupta, None; L. Lu, None;H. Zhang, None; S.S. Lo, None; N.A. Mayr, None.

3146 Pattern of Failure Study in Ocular Patients with Macular Melanoma using Fusion of Fundus Images in

Eyeplan Software

I. K. Daftari1, K. K. Mishra1, J. M. O’Brien2, T. Tsai3, S. S. Park4, T. L. Phillips1

1Department of Radiation Oncology, University of California- San Francisco, San Francisco, CA, 2Department ofOphthalmology, University of California- San Francisco, San Francisco, CA, 3Retinal Consultants Medical Group, Sacramento,CA, 4Department of Ophthalmology & Vision Science, University of California-Davis, Sacramento, CA

Purpose/Objective(s): To study the pattern of local failure and determine if tumor localization can be improved in patients treatedwith proton beam radiotherapy (PBRT) for ocular melanoma using the new version of EYEPLAN software, which allows for reg-istration of high-resolution fundus photos.

Materials/Methods: The new version of EYEPLAN (V3.05a) software allows for the registration of a fundus photo so that it canbe displayed as a background image for currently displayed graphics. The image is registered by clicking on the center of the foveaand optic disc in the planning fundus view. This is used in conjunction with tantalum marker rings, surgeon’s mapping, ultrasound,

Proceedings of the 51st Annual ASTRO Meeting S697

and 3DMRI to draw the tumor contour accurately. In order to determine if the fundus image helps in tumor delineation and treat-ment planning, we identified 303 patients with choroidal melanoma who were treated with PBRT between 1995 and 2008. Allpatients were treated to a dose of 56 GyE/4 fx. The seventy-nine pts with macular lesions, which can be more difficult to outlinewith rings due to their posterior location, were then chosen and the treatment plans were reviewed and re-planned using EYEPLAN.For patients with local failure (LF), the pre-treatment fundus images were fused to the original treatment plan to check if the originalfundus image tumor volume was covered. The process was repeated with post-treatment fundus images to record the exact locationof failure with respect to the target volume and dose received.

Results: The mean follow-up of macular melanoma patients was 28 ± 21 (range 6 to 91) months. The 3-yr local control rate andoverall survival was 84.5 ± 6.1% and 79.9 ± 6% respectively. Tumor growth was seen in 6 eyes. Among the 6 recurrences, 3 weremanaged by enucleation and 3 had distant metastasis of which 2 patients died. Re-planning these 6 patients with their original fun-dus photo superimposed showed that in 4 cases the treatment field adequately covered the tumor volume. The remaining 2 patients,had 3 clips placed around the tumor due to difficulty defining these posterior tumors. Overlaid fundus photos clearly indicated thearea of marginal miss. Re-planning with the fundus photo showed improved tumor coverage in these macular lesions.

Conclusions: Local control was excellent in patients receiving 56 GyE of PBRT for macular melanoma. The superposition of thefundus photo would have clearly allowed improved localization of tumor in one-third of those patients who failed. Posterior lesionsare better defined with the additional use of fundus image since they can be difficult to mark at the time of clip surgery. Our currentpractice standard is to use the direct superimposition of the high-resolution fundus photo in addition to the surgeon’s clinical andclip mapping of the tumor and ultrasound to draw the tumor volume.

Author Disclosure: I.K. Daftari, None; K.K. Mishra, None; J.M. O’Brien, None; T. Tsai, None; S.S. Park, None; T.L. Phillips,None.

3147 Isocenter Shift in Image-guided Proton Therapy (IGPT) of Prostate Irradiation

C. Cheng1, I. J. Das2, P. A. S. Johnstone2

1Morristown Memorial Hospital, Morristown, NJ, 2Indiana University, Indianapolis, IN

Purpose/Objective(s): In IGRT of prostate cancer with photons, organ motion may result in changes in rectal dose from 15 - 25%in shifts .10mm in the AP-PA direction, while the coverage of CTV may remain within 5%. The sharp distal fall-off in a protonbeam (90% to 10% in 2 mm) makes it even more critical than photons due to changes in depths, inhomogeneities and knowledge ofthe exact location of the prostate. In this study, we investigate the effect of organ motion in IGPT of prostate cancer.

Materials/Methods: Seven consecutive CT scans of a prostate patient obtained daily prior to treatment with a Siemens CT-on-Rails system for setup verification is examined. A single un-modulated 186 MeV proton beam suffices for the purpose of this study,instead of a full SOBP. The range of the beam is 24.5 cm in water, corresponding to the water-equivalent path length (WEPL) at thedistal edge of the CTV through isocenter with the beam at 270o. The lateral spread of the beam is also larger than that of a fullSOBP, providing a more conservative estimate on the lateral coverage of the CTV. Image fusion between the pre-treatment CTand the planning CT provides the shift adjustment to account for organ motion and setup error. The change in the thickness ofbone along the beam path, the corresponding change in the WEPL as a result of the shift correction, and the change in the rectalwall volume inside the proton field are determined for each CT scan.

Results: For the seven pre-treatment CT scans, the changes in the bone thickness along the beam path are 1, 6, 4, 5, 4, 6 and 1 mmrespectively, after the shift correction. The corresponding changes in the WEPL are -2, 0, -1, -1, 5, 5, and 4 mm respectively. A 5mm change in WEPL will put the distal edge of the CTV on the edge of the Bragg peak. The distance of the posterior edge of theCTV to the anterior rectal wall changes from that in the planning CT by: -7, -3, -11, -2,- 2, -7, and -7 mm respectively (minus signindicates decrease in the distance). Since the lateral spread in water is about 14 mm (FWHM) for a 186 MeV proton beam due tomultiple scattering at the end of its range, the negative change in rectal wall distance implies that a larger rectal volume is movedinto the field. The largest change occurs in the day-to-day rectum cross-section, as much as 400% change compared to that in theplanning CT is observed.

Conclusions: The sensitivity of a proton beam relative to inhomogeneities and depth presents the dosimetric problem not correctedby patient repositioning in the x-, y- and z-directions. Our study shows that shift correction may still lead to either increase in therectal dose, or a decrease in tumor coverage. A treatment couch that offers 6-degrees of freedom would improve the accuracy ofIGPT, but still may not be sufficient to account for tumor shape change due to large changes in rectal and bladder filling.

Author Disclosure: C. Cheng, None; I.J. Das, None; P.A.S. Johnstone, None.

3148 Dosimetric Comparison of Protons and Photons for Stereotactic Body Radiation Therapy for Lesions Near

the Chest Wall

J. G. Brabham1, K. Shahnazi2, C. Allgower2, M. M. Fitzek2,1, H. R. Cardenes1, A. J. Fakiris1, M. E. Ewing1, I. J. Das1,2

1Indiana University School of Medicine, Indianapolis, IN, 2Midwest Proton Radiotherapy Institute, Bloomington, IN

Purpose/Objective(s): To compare the dosimetric parameters of photon-based stereotactic body radiation therapy (SBRT) withthose of stereotactic body proton therapy (SBPT).

Materials/Methods: The treatment plans of 10 SBRT patients with lesions within 1 cm of the chest wall treated between 2007 and2008 were selected for analysis. Five patients had lung lesions and 5 had liver lesions. Median lesion volume was 15 cm3 (range 10-336 cm3). All plans had been created in accordance with institutional SBRT planning guidelines using at least 9 non-coplanar fieldswith 6 MV and 16 MV photons prescribed to the 80% isodose line in the Varian Eclipse treatment planning system. The planningCT scans and all contoured structures were then transferred into the CMS XiO treatment planning system at our proton center andprospectively planned using passively scattered 206 MeV protons with a maximum range of 27 cm prescribed to the 100% isodoseline. The individual generating the SBPT plans was blinded to all information from the SBRT plans other than treatment volumesand prescription dose. The dosimetric data from each pair of SBRT and SBPT plans were analyzed by dose-volume histogram andcompared for both target coverage and organs-at-risk (OAR) sparing.