Clinical investigation: prostate
Feasibility and acute toxicities of radioimmunoguided prostate brachytherapy

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Abstract

Purpose: We present a technique that fuses pelvic CT scans and ProstaScint images to localize areas of disease within the prostate gland to customize prostate implants. Additionally, the acute toxicity results from the first 43 patients treated with this technique are reviewed.

Methods and Materials: Between 2/97 and 8/98, 43 patients with clinical stage II prostate adenocarcinoma received ultrasound-guided transperineal implantation of I-125 or Pd-103 seeds. The median patient age was 70 years (range 49–79). Prior to treatment, the median Gleason score and prostate-specific antigen (PSA) were 6 (range 3–8) and 7.5 (range 1.8–16.6 ng/mL), respectively. The median follow-up was 10 months (range 2.9–20.4 months).

Results: The median PSA value at 10 months is 0.7 ng/mL. Significant acute complications within the first month following implantation included 13 Grade I urinary symptoms, 24 Grade II urinary symptoms, 6 Grade III symptoms, and no Grade IV complications. Beyond 4 months, complications included 12 Grade I urinary symptoms, 17 Grade II urinary symptoms, 1 Grade III, and 1 Grade IV complications.

Conclusions: The image fusion of the pelvic CT scan and ProstaScint scans helped identify regions within the prostate at high risk of local failure, which were targeted with additional seeds during implantation.

Introduction

Prostate brachytherapy is gaining widespread acceptance as an alternative to either external beam radiotherapy or surgery for patients with localized prostate adenocarcinoma. As the technique evolves, significant advancements have been made to refine the procedure. The earliest attempts at prostate brachytherapy utilized high-energy sources placed transperineally, resulting in excess radiation exposure to the surgeon and other personnel involved in the patient’s care (1). As external beam radiation techniques improved, brachytherapy fell out of favor as a treatment modality until radioactive gold seeds became available. Implantation techniques used with gold seeds improved, but, due to their high energy, concerns over staff exposure to radiation remained an issue 2, 3. Modern techniques have obviated the exposure problem by using low-energy permanent sources or remote afterloading temporary brachytherapy applicators. Other technical advances include the use of ultrasound and fluoroscopic guidance during interstitial implantation, as well as the use of postoperative CT scan–based dosimetry and ultrasound-based preplanning (4).

Despite the numerous advances in prostate brachytherapy, there is still not an accepted technique to plan an implant procedure based on the distribution of tumor within the gland, rather than on the size and shape of the prostate itself. Various imaging techniques, including ultrasound, CT, and MRI, all give information about the size and shape of the gland but are lacking the anatomic imaging sensitivity and specificity required to differentiate normal prostate tissue from malignancy. Of the three modalities available, MRI appears to delineate tumor from normal prostate tissue effectively. Most of the reported literature on MRI evaluation of the prostate relies on either endorectal or whole body coil MRI techniques to determine capsular penetration or seminal vesicle invasion to help select patients for prostatectomy, rather than for planning brachytherapy. Whereas MRI may be able to help detect macroscopic disease, its ability to detect lesions less than 5 mm appears limited. 5, 6, 7, 8.

The use of a radiolabeled antibody, indium-111 capromab pendetide (ProstaScint), specific for prostate-specific membrane antigen (PSMA) allows the use of immunoscintigraphy to detect metastatic disease beyond the prostate (9). When we began our prostate brachytherapy program in February 1997, we decided to use a ProstaScint scan on all patients as part of the routine work-up in order to help us select appropriate candidates for the procedure. Additionally, all patients had a pelvic CT scan using a Picker CT scanner and AcQSim software to determine the prostate volume and to rule out pubic arch interference utilizing three-dimensional reconstruction capabilities. Over the course of the first year, we began exploring the possibility of using the information we had collected to improve our implant technique by determining where the highest areas of tumor burden were within the prostate gland. The concept of radioimmunoguided brachytherapy for colorectal carcinoma had already been explored, and we attempted to translate this technique to prostate brachytherapy 10, 11.

Section snippets

Methods and materials

Between 2/97 and 8/98, 43 patients with clinical stage II prostate adenocarcinoma were treated with ultrasound-guided transperineal implantation of I-125 or Pd-103 at MetroHeath Medical Center, Cleveland, OH. The image fusion technique was developed and refined during this study period. Preoperative evaluation generally included a complete history and physical examination, ProstaScint scan, a thin slice pelvic CT scan with oral and IV contrast, prostate volume study, pubic arch study,

Results

The median PSA value at 10 months for the entire group of patients is 0.7 ng/mL, with a median PSA in the patients with and without adjuvant hormonal therapy of 0.33 ng/mL and 0.7 ng/mL, respectively. Acute complications within the first month after implantation included Grade I urinary symptoms in 13 patients (30%), and Grade II urinary symptoms in 24 patients (56%) requiring an alpha-blocker. Acute high-grade complications include Grade III symptoms in 6 patients (14% total, including 1

Discussion

The feasibility of radioimmunoguided prostate brachytherapy appears promising. We are now in the process of obtaining further data to validate the accuracy of the fusion study by correlating histopathologic findings with the computer images. In a previous publication, we demonstrated a semiquantitative correlation with prostate biopsy results utilizing prostate to muscle ROI ratios for specific ROIs within the prostate gland compared to background muscle ROIs placed over the external obturator

Acknowledgements

These data were presented at the 85th annual meeting of the Radiological Society of North America in Chicago, IL, Nov. 1999. We thank Ridgely Conant, C.N.R.T. for the time and effort he provided to develop the fusion technique, and for the many hours he has given to the project.

References (12)

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