Results: Based on the MCNP5 calculations performed in this study, the existence of air pockets in the MammoSite® causedhigher doses to be deposited at the tissue-air pocket interface. The maximum dose difference from dose point A and B, 7.19%,resulted from the 125 cc balloon and a 1.50 cm radius air pocket. For dose point A and C, because of their different distancesfrom the source, the maximum dose discrepancy, 61.98 %, occurred as a result of a 1.5 cm radius air pocket with a 70cc (5–6cm) balloon. In order to compare the actual dose delivered to the tissue-air pocket interface and the planned dose to the sametissue without being pushed away by the air pocket, the inverse square factor was not included in the dose discrepanciescalculation. Therefore, point A and C had large dose differences. It was found that, in general, larger air pockets will result inlarger dose discrepancies despite their balloon sizes.

Conclusions: Air pockets located outside of a MammoSite® balloon may cause dose discrepancies between the delivered doseand the planned dose (based on conventional treatment planning systems) at the tissue-air pocket interface up to 61.06 % undersituations presented in this study. A more detailed study on the dose effect of air pockets is currently under investigation.

Author Disclosure: Y.J. Huang, None; M. Blough, None; N. Papanikolaou, None.

2860 A New Approach for Dose Calculation Around Elongated Brachytherapy Sources

S. B. Awan1, A. S. Meigooni1, R. Mokhberiosgouei1, M. Hussain2

1University of Kentucky, Lexington, KY, 2University of the Punjab, Lahore, Pakistan

Purpose/Objective(s): TG-43 protocols are based on polar co-ordinate systems (PCS) and are designed for dose calculationaround seed type sources (�1cm) and commercially available treatment planning systems are designed accordingly. RecentlyRadioCoilTM sources with active lengths ranging from 1 cm to 6 cm have been introduced for interstitial implants. Thecommercially available treatment planning systems are not compatible for dose calculation with the elongated sources. Anintermediate solution was introduced to resolve this problem by subdividing the source into small segments and using TG-43U1source dosimetry data in PCS. However, high angular dose gradient near the end of the active length of the source resulted indiscrepancies between the Monte Carlo simulated and treatment planning data in this region. In this project, the dosimetriccharacteristics of a 5cm long RadioCoil™ source has been determined using TG-43U1 based formalisms in cylindricalco-ordinate system (CCS). A comparison of Monte Carlo simulated dose profiles with the CCS is utilized to justify its advantageto the PCS based dosimetry.

Materials/Methods: TG-43U1 parameters have been modified from spherical to CCS in order to calculate the dose distributionaround an elongated brachytherapy source. Figure 1 shows the comparison between the CCS and PCS based TG-43 formalism,where R and Z are the distances in transverse, longitudinal axis of the source. In this formalism Ro (1cm) and Zo (0 cm)represents the reference point. Definitions of the other parameters are similar to those described in TG-43U1 report.

Dosimetric characteristics of a 5cm long RadioCoil™ brachytherapy source have been determined in liquid water usingMonte Carlo simulation technique. These parameters were obtained both in spherical and CCS. Dose profiles at distancesranging from 0.2 to 3.0cm along the transverse direction of the source were calculated using these parameters and the resultswere compared with Monte Carlo simulated data.

Results: A comparison between the Monte Carlo simulated dose profile and calculated values using CCS and PCS are shownin Figure 1. These results indicate that at short distances, the CCS replicates the Monte Carlo simulated data better than PCS(2% vs. 15%, respectively). However, at larger distances, both systems are equally comparable to the Monte Carlo simulateddata.

Conclusions: TG-43U1 based formalisms in CCS can better represents the dose distribution along a elongated source than thePCS. Therefore, the authors recommend the update or modification of the TG-43U1 protocol for elongated sources based oncylindrical coordinate system.

Balloon FillingVolume (cc)

BalloonSize (cm)

Air Pocket Radius(rair)

Point A & B DoseDifference (%)

Point A & C DoseDifference (%)

34 4-5 0.25 1.34 26.2834 4-5 0.5 2.68 40.1934 4-5 1.0 3.96 57.7070 4-5 0.5 2.62 33.6070 4-5 1.0 4.66 50.3470 4-5 1.5 6.80 61.0670 5-6 0.5 2.43 34.2970 5-6 1.0 4.81 51.0970 5-6 1.5 6.76 61.98

125 5-6 0.5 2.64 30.24125 5-6 1.0 5.05 45.65125 5-6 1.5 7.19 56.38

S695Proceedings of the 48th Annual ASTRO Meeting

Author Disclosure: S.B. Awan, None; A.S. Meigooni, None; R. Mokhberiosgouei, None; M. Hussain, None.

2861 Ru-106 Eye Plaques for Treatment of Ocular Melanoma - Practical Issues

F. Mourtada, J. Horton, D. Gombos, A. Garden

M.D. Anderson Cancer Center, Houston, TX

Purpose/Objective(s): At our institution, Ru-106 plaques have been used in to complement traditional I-125 COMS plaquesfor radiotherapy management of uveal melanoma. Over 45 patients have been treated with Ru-106 since the launch of thisprogram in late 2003. We report on our experience with treatment planning considerations and quality assurance of Ru-106beta-emitting plaques.

Materials/Methods: Six sources (2 of each model: CCB, COB, and CCA, manufactured by BEBIG GmbH, Berlin, Germany)were commissioned since Dec. 2003. The CCB and CCA plaques are fully circular with a 20-mm and 15-mm diameter,respectively. The COB is 20 mm in diameter with a notch. Measurements of the absolute dose rate and relative dose uniformitywere obtained using radiochromic films and a hemispherical eye phantom. A high resolution CCD densitometer (PeC 100,Photoelectron Co.) was used to digitize the films (0.13 mm pixel resolution). Sr-90/Y-90 source traceable to the NationalInstitute of Standards and Technology (Gaithersburg, MD) was used for film calibration. A scaling function, for converting themeasured dose rate in plastic to that in water, was estimated with Monte Carlo simulations (MCNPX code, Los Alamos NationalLab., Los Alamos, NM)). Our overall measurement uncertainty is �11% (2 ). The measured dose distributions are used forplaque commissioning and treatment planning. Treatment planning was done using an Excel spreadsheet and a SURFERsoftware program (Golden Software, Inc., Golden, CO) for isodose contour plotting.

Results: The absolute dose rate, along the central axis, for each source model is found to be in good agreement (within �10%)with the manufacturer’s reported values. All 6 tested plaques were found to have good dose uniformity at measured a nominaldepth of 2 mm from the inner surface of the plaque (within �10%). At our institution and over the last 3 years, the mostcommonly used plaque was the notched 20-mm COB (n� 20), followed by the circular 20-mm CCB (n� 15) and the 15-mmcircular CCA (n� 10). Relative to I-125 COMS, our results indicate that Ru-106 delivers lower dose to surrounding structuresdue to its steep depth dose curve; while reasonable doses are delivered to the sclera if tumor apical height within 5 mm isindicated. For tumors close to the edge of the optical disk, we found the notched COB plaque to deliver lower dose to the opticaldisk than I-125.

Conclusions: We performed an independent verification of BEBIG Ru-106 plaques dosimetry. Implementation of this qualityassurance program insures accurate Ru-106 radiotherapy treatment planning and delivery. We find Ru-106 plaque preparationlogistics to be easier than I-125 COMS due to reusability over its one-year half-life and the allowable 50 sterilization cycles.Our surgeons also prefer Ru-106 for its thinner profile with easier insertions and less trauma to surrounding tissue.

Author Disclosure: F. Mourtada, Eckert & Ziegler BEBIG Gmbh, C. Other Research Support; J. Horton, None; D. Gombos,None; A. Garden, None.

2862 Dosimetric Impact of Partial Gland Irradiation Utilizing Functional Image Guidance in Men UndergoingBrachytherapy for Localized Prostate Cancer

B. Wang1, Y. Zhu1, S. J. DiBiase1, R. Parra1, J. Mammone1, Y. Feng2, C. Yu2, R. P. Gullapalli2

1Cooper University Hospital, Camden, NJ, 2University of Maryland, Baltimore, MD

Purpose/Objective(s): The objective of this simulation study was to provide proof of the dosimetric efficacy of using MRSIto guide partial gland irradiation in order to minimize the side effects and produce equivalent outcome.

Materials/Methods: Eight subjects with localized prostate cancer treated on prospective Phase II study utilizing MagneticResonance Spectroscopic Imaging (MRSI) to guide prostate brachytherapy (PB) were simulated in this dosimetric impact study.All patients had histological confirmed adenocarcinoma of the prostate, clinical stage T1c or T2a, PSA �� 10, and Gleasonscore of 6 or less. Biologic tumor volumes (BTV) that were previously utilized in a Phase II prospective study were utilizedfor this dosimetric simulation study of partial gland irradiation. In this simulation study, only the BTV within the prostate glandwas treated with I-125 radioactive seed implantation to a dose of 160 Gy.

Results: The simulated dosimetry and DVH were analyzed and compared to whole gland brachytherapy. We found the doseto both urethra and rectum much reduced. As shown in Table 1, the median D90 for urethra was only 23.6 Gy for partial glandirradiation, while 159.1 Gy for whole gland irradiation. The median V100 for rectum was 21.6% for whole gland irradiationand it decreased to 2.0% for partial gland irradiation.

S696 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006