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Dosimetric Effects of Needle Tip Localization Errors in Prostate Brachytherapy

B Ruiz

B Ruiz*, S Leu, T Podder, East Carolina University, Greenville, NC

WE-A-BRB-9 Wednesday 8:00:00 AM - 9:55:00 AM Room: Ballroom B

Purpose: In prostate seed implantation, beveled tip needles are used for convenience of steering. However, needle tip localization and seed deposition errors can occur due to the bevel tip orientation. Purpose of this study is to evaluate the dosimetric consequences of needle tip localization errors due to transrectal ultrasound (TRUS) elevation beamwidth artifacts and beveled needle tip orientation in prostate seed implantation cases.

Methods: Using VariSeed brachytherapy seed planning software (Varian, version 8.0), 15 clinical cases were evaluated for dosimetry (145Gy prescription, I-125 seed each of 0.375mCi). Seed positions were adjusted to represent the localization errors of 0.8mm-6.5mm (combined errors due to elevation beamwidth artifacts (2.5mm) and needle tip orientation (4mm)) for an ultrasound setting of 50% gain, and 0.9mm-8.5mm for 100% gain. Each plan had a corresponding reference plan for comparison purposes. Plan parameters were compared using the AAPM TG-137 guidelines for the prostate, prostatic urethra, and rectum.

Results: Compared to the reference plans, due to needle tip localization errors, prostate dosimetric parameters V100, V150, and V200 deviated by -2.8%, -2.4%, and -3.6%, respectively for TRUS setting of 50% gain. D90 was decreased by 7.3Gy. Observed rectal dose increased in V100, D2cc, and D0.1cc by 0.13%, 0.96Gy, and 2.78Gy, respectively. Urethral doses D30 and D10 changed by -0.85Gy and -0.44Gy respectively. For TRUS of 100% gain, prostate V100, V150, and V200 were decreased by 2.8%, 3.0%, and 3.8%, respectively. D90 was lowered by 6.7Gy. Rectal dose increased in V100, D2cc, and D0.1cc by 0.05%, 0.37Gy and 1.44Gy, respectively. Urethral doses D30 and D10 decreased by 1.03Gy and 1.37Gy, respectively.

Conclusions: Seed deposition based on the erroneous needle tip localization can cause considerable dose deviation of the prostate, rectum, and urethra. These results may be prostate volume and shape dependent. Further investigation is warranted in this regard.

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