Urethral spectroscopic alarm (USA) probe for prostate implants
Urethral Spectroscopic Alarm (USA) Probe for Prostate Implants 1A.B. Rosenfeld, 1D.L. Cutajar, 1M.L.F. Lerch, 1G.J. Takacs, 2J.A. Bucci, 2L.J. Duggan, 3M. Zaider, 4M. Zelefsky and 3G.N. Cohen 1Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia, 2Institute for Prostate Cancer, St. George Cancer Care Centre, Kogarah, Australia, 3Department of Medical Physics and 4Department of Radiation Oncology, MSKCC, New York, NY Purpose: Prostate implants are associated with excellent biochemical outcome; however, their benefit in terms of overall survival remains unknown. For many patients with early, localized prostate cancer the decision to undergo a treatment of questionable benefit yet because of potentially significant morbidity - tangibly impacting upon their quality of life is understandably difficult. Thus, diminishing the risk of complications, whilst at the same time maintaining good dosimetric coverage of the tumor, remains the overriding concern in prostate brachytherapy. We describe a probe which reports in real time the dose delivered at selected locations in the urethra and alerts the physician if the planned dose was exceeded. Methods and Materials: In vivo dosimetry is commonly performed with miniature counters such as mosfets or diodes (1). The main disadvantages of such systems are their energy dependence and low sensitivity. The USA probe circumvents these problems by measuring the actual energy spectrum of the radiation field; the dose is evaluated in relation to the magnitude of the photopeak and this makes for an essentially noise-free system. The probe is silicon mini-detector connected to a shaping amplifier and discriminator and placed in a cable that can be inserted in a urethral catheter Results: In a previous study (2) we have shown that urethral NTCP (meaning: grade 2 urinary symptoms that remain unresolved at 12 months post implant) correlate with DU20 [meaning that 20% of the urethral volume is treated to a dose of at least DU20]. Specifically: where g= -2.60Â±0.50 and d=0.0066Â±0.0016 Gy-1. After each seed insertion the planned DU20 is calculated and compared to the probes signals. The workings of this system are illustrated with data obtained in a training phantom. Conclusions: The USA probe offers the physician-implanter the opportunity to monitor on the fly the urethral dose and modify the implantation plan should the dose surpass the pre-set DU20 value. As well, the fact that the probe operates in spectroscopic mode eliminates the need for a tissue-equivalent dosimeter. Reference List 1. A.B. Rosenfeld, D.L. Cutajar, M.L.F. Lerch, G.J. Takacs, J. Brady, T.Braddock, V.L. Perevertaylo, J. Bucci, J. Kearsley, Zaider M, Zelefsky, In vivo dosimetry and seed localization in prostate brachytherapy with permanent implants. IEEE Trans. Nucl. Science, 51: 3013-3018 (2004). 2. Zaider M, Zelefsky M, Cohen AM, Chui C, Yorke ED, Ben-Porat L et al. Methodology for biologically-based treatment planning for combined low dose-rate (permanent implant) and high dose-rate (fractionated) treatment of prostate cancer. International Journal of Radiation Oncology Biology Physics (in press, 2005).