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. 2011 May 17;19(7):1353–1359. doi: 10.1038/mt.2011.89

Feasibility of Adenovirus-Mediated hNIS Gene Transfer and 131I Radioiodine Therapy as a Definitive Treatment for Localized Prostate Cancer

Kenneth N Barton 1, Hans Stricker 2, Mohamed A Elshaikh 1, Jan Pegg 1, Jingfang Cheng 1, Yingshu Zhang 1, Kastytis C Karvelis 3, Mei Lu 4, Benjamin Movsas 1, Svend O Freytag 1
PMCID: PMC3129572  PMID: 21587209

Abstract

We have developed a replication-competent adenovirus (Ad5-yCD/mutTKSR39rep-hNIS) armed with two suicide genes and the human sodium iodide symporter (hNIS) gene. In this context, hNIS can be used as a reporter gene in conjunction with nuclear imaging and as a potentially therapeutic gene when combined with 131I radioiodine therapy. Here, we quantified the volume and magnitude of hNIS gene expression in the human prostate following injection of a high Ad5-yCD/mutTKSR39rep-hNIS dose using a standardized injection algorithm, and estimated the radiation dose that would be delivered to the prostate had men been administered 131I with curative intent. Six men with clinically localized prostate cancer received an intraprostatic injection of Ad5-yCD/mutTKSR39rep-hNIS under transrectal ultrasound guidance. All men received 2 × 0.5 ml deposits (5 × 1011 vp/deposit) in each of the four base and midgland sextants and 2 × 0.25 ml deposits (2.5 × 1011 vp/deposit) in each of the two apex sextants for a total of 12 deposits (5 × 1012 vp) in 5 ml. On multiple days after the adenovirus injection, men were administered sodium pertechnetate (Na99mTcO4) and hNIS gene expression in the prostate was quantified by single photon emission computed tomography (SPECT). hNIS gene expression was detected in the prostate of six of six (100%) men. On average, 45% (range 18–83%) of the prostate volume was covered with gene expression. Had men been administered 200 mCi 131I, we estimate that the mean absorbed dose to the prostate would be 7.2 ± 4.8 Gy (range 2.1–13.3 Gy), well below that needed to sterilize the prostate. We discuss the obstacles that must be overcome before adenovirus-mediated hNIS gene transfer and 131I radioiodine therapy can be used as a definitive treatment for localized prostate cancer.

Introduction

Prostate cancer is the second leading cause of cancer death in men in the United States and ~32,000 men will die from it in 2011. Management options for newly diagnosed, nonmetastatic prostate cancer include expectant management, surgery, and radiotherapy ± androgen suppression therapy (AST). For men with low-risk disease (Stage ≤T2a and Gleason score ≤6 and PSA ≤10 ng/ml), surgery and radiation therapy result in excellent 10-year progression-free survival.1 However, these treatments alone are less effective against more advanced forms of the disease (Gleason score ≥7 or PSA >10 ng/ml) and a significant fraction of men will develop biochemical recurrence, an event that often triggers the implementation of salvage therapies. Novel therapies that can improve outcome without increasing morbidity in these more advanced patients need to be developed.

We have developed a multi-modal, gene therapy–based approach for the treatment of cancer (refs. 2,3 and references therein). Our original “three-pronged approach” utilized a replication-competent, oncolytic adenovirus to deliver a pair of therapeutic suicide genes to the tumor. The oncolytic adenovirus itself generates an antitumor effect by replicating in, and destroying, human cancer cells (oncolytic viral therapy). The therapeutic effect of the oncolytic adenovirus is enhanced significantly by invoking two suicide gene systems [cytosine deaminase (CD)/5-fluorocytosine (5-FC) and herpes simplex virus thymidine kinase (HSV-1 TK)/valganciclovir (vGCV)], which render malignant cells sensitive to specific pharmacological agents (suicide gene therapy) and sensitize them to ionizing radiation (radiotherapy). The combined effects of these three modalities result in significant tumor cell destruction and release of tumor antigens, which, when coupled with the robust immune response elicited by the adenovirus, may generate an environment that fosters tumor antigen processing and cross-presentation culminating in the generation of long-lasting antitumor immunity (antitumor immunity).

We have evaluated the toxicity and efficacy of our multi-modal approach in five clinical trials of nonmetastatic prostate cancer using three different (but related) adenoviruses.4,5,6,7,8,9 These studies have demonstrated that our approach is safe and may have the potential to improve tumor control. With regards to safety, there have been no treatment-related dose-limiting toxicities or serious adverse events in 79 patients treated to date. Only 7% of the adverse events have been ≥ grade 3 with the vast majority being transient and asymptomatic. With respect to efficacy, in the locally recurrent setting following radiation failure, patients who received the gene therapy exhibited a significant lengthening of PSA doubling time,6 a non-validated surrogate endpoint that is highly prognostic for the development of distant metastases and prostate cancer-specific mortality.10,11,12,13 At 10 years, there was an improvement in disease-specific survival (DSS) relative to well-matched historical controls (ref. 9 and S.O. Freytag, unpublished results). When combined with prostate radiotherapy in the newly diagnosed setting, better-than-expected 2-year negative biopsy results were obtained in men with intermediate-risk (Gleason 7 or PSA 10–20 ng/ml) features (refs. 3,7,9 and S.O. Freytag, unpublished results). Although these preliminary results await confirmation from randomized controlled trials and no conclusions can be drawn at the present time, they raise the possibility that replication-competent adenovirus-mediated suicide gene therapy may have the potential to impact clinically meaningful endpoints.

One adenovirus (Ad5-yCD/mutTKSR39rep-hNIS) that we have evaluated clinically expresses the yeast CD (yCD) and mutant SR39 HSV-1 TK (mutTKSR39) suicide genes as well as the human sodium iodide symporter (hNIS) gene.8 In the context of Ad5-yCD/mutTKSR39rep-hNIS, hNIS can be used as a reporter gene in conjunction with nuclear imaging and as a potentially therapeutic gene when combined with 131I radioiodine therapy.14,15,16,17,18,19,20,21 Here, we assess the feasibility of Ad5-yCD/mutTKSR39rep-hNIS-mediated 131I radioiodine therapy as a definitive treatment for localized prostate cancer.

Results

Patient baseline characteristics and treatment plan

Patient baseline characteristics are depicted in Table 1. All men had clinically localized high-risk disease (Stage T1/T2 and Gleason ≥8 or PSA >20 ng/ml). The mean prostate volume prior to the adenovirus injection was 34 cm3 (range 22–45 cm3). Two men had bilateral disease whereas the remaining four had adenocarcinoma in only one sextant (see below, Table 3).

Table 1. Patient baseline characteristics.

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Regardless of the distribution of cancer within the prostate, all men received 2 × 0.5 ml deposits (5 × 1011 vp/deposit) of Ad5-yCD/mutTKSR39rep-hNIS in each of the four base and midgland sextants and 2 × 0.25 ml deposits (2.5 × 1011 vp/deposit) in each of the two apex sextants for a total of 12 deposits (5 × 1012 vp) in 5 ml. Deposits were placed in each sextant using two needle passes (medial and lateral relative to midline) on each side of the prostate. Two days later, men received 2 weeks of 5-FC + vGCV prodrug therapy concomitant with a standard course of image-guided radiotherapy (IGRT) and AST. On multiple days after the adenovirus injection, men were administered Na99mTcO4 and underwent single photon emission computed tomography (SPECT) to quantify hNIS gene expression in the prostate as described previously.8

Toxicities

Treatment-related toxicities at 5 × 1012 vp are depicted in Table 2 and compared to our previously published results at ≤1 × 1012 vp.8 There were no dose-limiting toxicities or serious adverse events at 5 × 1012 vp. As expected, there was a greater incidence and severity of flu-like symptoms, transaminitis, and fatigue at the higher adenovirus dose. The two events of grade 3 transaminitis occurred 1 day after the adenovirus injection and before initiation of the prodrug therapy indicating they were attributable to the oncolytic adenovirus. There was also a greater incidence of anemia and thrombocytopenia, and severity of lymphopenia, which may be due to greater conversion of the 5-FC + vGCV prodrugs into their toxic metabolites. All adverse events were transient. Both events of grade 3 transaminitis returned to within normal limits 3 weeks after the adenovirus injection. Hematological events (except lymphopenia, see Table 2 legend) resolved within 2 weeks after completion of the prodrug therapy course.

Table 2. Treatment-related adverse events.

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Quantification of hNIS gene expression in the prostate

Since there was no representative patient, fused computerized tomography/SPECT images of the worst (patient 4), median (patient 1), and best (patient 5) patient with respect to total gene expression in the prostate are shown in Figure 1. The results of all patients are summarized in Table 3. hNIS gene expression (i.e., Ad5-yCD/mutTKSR39rep-hNIS-mediated 99mTcO4− uptake) was detected in the prostate of six of six (100%) patients and varied over a 7.5-fold range. In the three patients shown, hNIS gene expression was detected on both sides of the prostate. In patient 4 (worst), a significant amount of radionuclide was trapped in the bowel, which may have limited its availability to hNIS-expressing cells in the prostate. In patients 1 and 5, the mean pixel intensity in the prostate approached that of the urinary bladder, underscoring the robust gene expression observed in these patients. In four of six patients (patients 1, 2, 3, and 5), more hNIS gene expression was observed on the side of the prostate that harbored more cancer (Table 3). However, this relationship was not observed consistently across all patients. In two patients (patients 2 and 6), no hNIS gene expression was detected on one side of the prostate even though 2.5 × 1012 vp of Ad5-yCD/mutTKSR39rep-hNIS was deposited there and cancer was present. In the four patients that had cancer on only one side (patients 3–6), hNIS gene expression was detected on the side that lacked cancer in all, demonstrating that Ad5-yCD/mutTKSR39rep-hNIS is capable of infecting normal human prostate tissue.

Figure 1.

Figure 1

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Ad5-yCD/mutTKSR39rep-hNIS-mediated hNIS gene expression in the human prostate. Shown are fused CT/SPECT images for the worst (patient 4, a–c), median (patient 1, d–f) and best (patient 5, g–i) patient with respect to total human sodium iodide symporter (hNIS) gene expression in the prostate. All images represent the day when hNIS gene expression peaked. Activity in the prostate, urinary bladder (UB), bowel (Bo) and penis (Pe) are indicated. Activity in the prostate is due to Ad5-yCD/mutTKSR39rep-hNIS-mediated hNIS gene expression. Activity in the other tissues is due to route of excretion (UB), trapping of the radioactive substrate (Bo), or blood flow (Pe). Only the activity in the prostate can be seen in the transverse images. The color scale with pixel intensities for patient 1 is shown on the right. For reference, the mean pixel intensity in a non-Ad5-yCD/mutTKSR39rep-hNIS-injected human prostate 3 hours after administration of 17 mCi Na99mTcO4 is 37. A, anterior; L, left; P, posterior; R, right.

Table 3. Summary of SPECT imaging results.

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The mean gene expression volume at its maximum (GEVmax) was 14.6 cm3 (range 6.6–28.9 cm3), representing, on average, 45% (range 18–83%) of the prostate volume (Table 3). As was observed previously at lower adenovirus doses,8 gene expression in the prostate was short-lived (<7 days) and no extraprostatic hNIS gene expression was detected in any patient.

Absorbed dose to the prostate when combined with 131I radioiodine therapy

hNIS gene transfer has been shown to induce significant tumor regression in preclinical models of cancer when coupled with 131I radioiodine therapy.14,15,16,17,18 Using our SPECT imaging results, we estimated the absorbed dose that would be delivered to the prostate had men been administered 200 mCi 131I, the highest dose recommended for patients with metastatic thyroid cancer. These estimations were conducted by relating the mean pixel intensity in the GEV to that of the iliac artery in transverse images yielding a GEV:blood mean pixel intensity ratio, and knowing the clearance kinetics of 99mTcO4 in human blood,22 we could estimate the radionuclide activity in the GEV after correcting for tissue attenuation of the signals (see Materials and Methods). Once the radionuclide activity in the prostate was known, the absorbed dose could be calculated by applying the absorbed dose constant and mean life for 131I. The absorbed dose constant is specific to each radionuclide and is the absorbed dose rate per unit of activity.23 The mean life reflects the radionuclide dwell time in the target organ, and takes into account both biological and physical decay. Our calculations are summarized in Table 4.

Table 4. Absorbed dose calculations.

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Had men been administered 200 mCi 131I, the mean absorbed dose to the prostate would be 7.2 ± 4.8 Gy (range 2.1–13.3 Gy). Had the entire prostate been covered with hNIS gene expression, the mean absorbed dose would be 15.4 ± 3.3 Gy (range 12.0–20.6 Gy). These estimates represent an upper limit, for they assume that hNIS gene expression was constant and maximal over the in vivo life of 131I, which it was not. In most cases, hNIS gene expression peaks 2 days after the adenovirus injection and declines rapidly thereafter becoming undetectable by SPECT by day 8 (Table 3). These absorbed dose estimates are well below the standard doses used with external beam prostate radiotherapy (70–80 Gy) and permanent prostate brachytherapy (145 Gy and 120 Gy for 125I and 103Pd, respectively).

Discussion

Owing to its ability to transport radioactive anions such as pertechnetate and iodide, the feasibility of using hNIS gene transfer for diagnostic and therapeutic purposes was first investigated in preclinical models.14,15,16,17,18,19,20,21 We have now examined these two potential uses of hNIS in humans. Using a replication-competent adenovirus expressing hNIS, we demonstrated previously8 and here that hNIS can be used diagnostically to quantify the volume, magnitude, and kinetics of adenovirus-mediated gene expression in the human prostate. At 5 × 1012 vp, hNIS gene expression was detected in six of six (100%) patients, which compares favorably to our previous experience in which hNIS gene expression was detected in seven of nine (78%) patients at 1 × 1012 vp and zero of three (0%) at 1 × 1011 vp.8 Because our previous results indicated that a 1 ml (1 cm3) adenovirus deposit expanded to 6.6 cm3 representing, on average, 18% of the prostate volume, here we injected 5 ml (12 deposits) using a standardized injection algorithm hoping to achieve complete coverage of an average human prostate (30–40 cm3). We observed that, on average, 45% of the prostate volume was covered with hNIS gene expression. This lower than expected increase in GEV per adenovirus deposit is probably due to overlap of neighboring deposits and the fact that a fraction of the deposits failed to generate any detectable gene expression (discussed more below).

A second objective was to estimate the absorbed dose to the prostate had men been administered 131I radioiodine therapy with curative intent. Following intraprostatic injection of Ad5-yCD/mutTKSR39rep-hNIS at 5 × 1012 vp using a standardized injection algorithm, we estimate that the mean absorbed dose to the prostate would be ~7 Gy had men been administered 200 mCi 131I. Assuming that 100% of the prostate could be covered with hNIS gene expression, the mean absorbed dose would be ~15 Gy. We conclude that significant improvements are needed before this approach will have applicability in the clinic. Such improvements might include (i) escalating the adenovirus dose beyond 5 × 1012 vp,24 (ii) use of an adenovirus with more robust replicative, but not oncolytic, properties (i.e., one with a wild-type E1 region but does not lyse the cell), (iii) implementation of radiosensitizing genes,2 or (iv) concomitant use of agents that prevent the repair of double stranded DNA breaks.25 However, probably the best way to increase the absorbed dose is to develop ways to trap 131I inside the cell once it is transported by hNIS. 131I radioiodine therapy is efficacious in the treatment of thyroid cancer because iodine becomes covalently fixed to thyroglobulin (via thyroid peroxidase) once inside the cell thereby preventing its rapid efflux as is observed in nonthyroid hNIS-expressing tissues. This process, called organification, causes 131I to accumulate thereby increasing both the resident activity and biological half-life in the thyroid. Hence, simultaneous delivery of thyroid peroxidase is worthy of consideration.26,27

Dose considerations aside, our results raise two other concerns regarding the use of adenovirus-mediated hNIS gene transfer and 131I radioiodine therapy as a definitive treatment for localized prostate cancer. First is the fact that a fraction of the adenovirus deposits failed to generate any detectable hNIS gene expression. We estimate this fraction to be about 17% (one side of the prostate in two patients, representing 12 deposits, out of a total of 72 deposits made in six patients). Specifically, in two patients, no hNIS gene expression was detected on one side of the prostate even though 2.5 × 1012 vp of Ad5-yCD/mutTKSR39rep-hNIS was deposited there and cancer was present. Had this approach been used by itself as a definitive treatment, it would have failed to eradicate the cancer in these two patients (33% of total) even if the absorbed dose to the contralateral side was tumoricidal. Indeed, the 2-year post-treatment biopsy (12 core) of patient 2 showed residual adenocarcinoma only on the left side where no hNIS gene expression was detected. By contrast, the right side, which exhibited hNIS gene expression, was negative for cancer. The 2-year post-treatment biopsy (12 core) of patient 1, who exhibited hNIS gene expression on both sides, was completely negative. Although only two of six patients have undergone the 2-year biopsy and no conclusions can be drawn, thus far, there is concordance between detection of adenovirus-mediated gene expression in the prostate and eradication of the prostatic cancer. Our results demonstrate that direct injection of therapeutic adenoviruses into human tissue comes with a degree of uncertainty, which is likely to reduce efficacy in the clinic particularly when used as a single modality.

The second concern is more conceptual. An important consideration in cancer radiotherapy is control of the radiation dose distribution within the target volume. Prior to treatment, individualized treatment plans are generated for each patient to ensure that the tumor gets the maximal planned radiation dose while sparing nearby critical structures. For example, in prostate brachytherapy, the entire prostate is the clinical target volume and the radioactive seeds are arranged so that most of the prostate (plus a 3–5 mm margin, except posterior) gets a sterilizing radiation dose while critical structures such as the urethra and rectum get a lower dose. Because the seeds are embedded in solid tissue in a precise manner and remain (largely) in place, the radiation dose distribution within the prostate can be both simulated and controlled. This high level of precision may not be possible with adenovirus-mediated gene transfer. Because the adenovirus is injected as a liquid and spreads unpredictably throughout the prostate, it may not be possible to control (or simulate) the radiation dose distribution within the prostate. Hence, some regions of the prostate will get too low of a dose (i.e., tumor) whereas others will get too high of a dose (i.e., urethra, rectum). Although our previous studies demonstrated that a 0.5 ml adenovirus deposit could result in a well-defined, almost spherical, region of gene expression in the prostate,8 it is unclear whether this definition can be obtained consistently with every deposit. Thus, rather than using hNIS gene transfer and 131I radioiodine therapy alone as a definitive treatment, it may make more sense to combine this novel approach with external beam radiotherapy as is done with prostate brachytherapy. Whereas brachytherapy provides only a radiation boost, Ad5-yCD/mutTKSR39rep-hNIS-mediated 131I radioiodine therapy would provide additional therapeutic effects via the oncolytic adenovirus, suicide genes, and possibly antitumor immune response. Men with bulky T2 tumors would be good candidates to evaluate this novel combined approach.

Materials and Methods

Description of Ad5-yCD/mutTKSR39rep-hNIS adenovirus. Ad5-yCD/ mutTKSR39rep-hNIS has been described previously.8,20 Briefly, it is a 55 kDa E1B-deleted replication-competent adenovirus that contains a yeast cytosine deaminase (yCD)/mutant SR39 herpes simplex virus thymidine kinase (mutTKSR39) fusion gene in the E1 region and the hNIS gene in the E3 region. Both the yCD/mutTKSR39 fusion and hNIS genes are under the transcriptional control of the human cytomegalovirus promoter. It contains wild-type Ad5 E1A and 19 kDa E1B genes, and is devoid of all E3 genes.

Study design and patient selection. This study is an extension of a previous phase 1 trial.8 The primary purpose was to determine the safety of administering the Ad5-yCD/mutTKSR39rep-hNIS adenovirus at a dose of 5 × 1012 vp concomitant with 5-FC + vGCV prodrug therapy and a standard course of IGRT and AST. Secondary endpoints were (i) quantification of the volume, magnitude, and kinetics of hNIS gene expression in the prostate, and (ii) estimation of the absorbed dose to the prostate had men been administered 131I. The study called for six men with localized prostate cancer to receive an intraprostatic injection of Ad5-yCD/mutTKSR39rep-hNIS at 5 × 1012 vp on day 1 followed by 2 weeks of 5-FC and vGCV prodrug therapy concomitant with 76 Gy IGRT and ≥ 2 years of AST. Because our previous results indicated that an adenovirus dose of 1 × 1012 vp was sufficient to improve tumor control (2-year biopsy) in men with intermediate-risk, but not high-risk features,7 eligibility in the current study was restricted to men with clinically localized high-risk disease (Stage T1/T2 and Gleason score ≥8 or PSA >20 ng/ml). Other inclusion and exclusion criteria were identical to the original protocol and have been described previously.8 On multiple days after the adenovirus injection, patients were administered Na99mTcO4 and underwent whole body and pelvic planar scans to quantify hNIS gene expression (i.e., Ad5-yCD/mutTKSR39rep-hNIS-mediated 99mTcO4− uptake) in the prostate as described previously.8 All patients were treated in the Department of Radiation Oncology at the Henry Ford Health System. All patients met the eligibility requirements of the protocol and signed the informed consent. The protocol received all required federal and institutional regulatory approvals before initiation and was conducted under BB-IND 12786 (RAC protocol 0501-690). Good clinical practices were used throughout.

Injection of Ad5-yCD/mutTKSR39rep-hNIS adenovirus. Pretreatment planning included transrectal ultrasound-guided needle biopsy of the prostate and computerized tomography simulation. The intraprostatic Ad5-yCD/mutTKSR39rep-hNIS injection was performed on an outpatient basis as described previously.8 All men received 2 × 0.5 ml deposits (5 × 1011 vp/deposit) in each of the four base and midgland sextants and 2 × 0.25 ml deposits (2.5 × 1011 vp/deposit) in each of the two apex sextants for a total of 12 deposits (5 × 1012 vp) in 5 ml. Deposits were placed in each sextant using two needle passes (medial and lateral relative to midline) on each side of the prostate. All injections were recorded and peer-reviewed.

Administration of prodrugs, IGRT, and AST. Prodrugs were administered as described previously8 with one exception. The duration of the prodrug therapy was reduced from 3 (previous study) to 2 (current study) weeks since our previous results indicated that gene expression in the prostate was short-lived (≤7 days). All patients were treated with intensity modulated radiation therapy to 76 Gy in 38 fractions as described previously8 utilizing daily IGRT. The position of the prostate was verified by on-board cone-beam computed tomography, ultrasound imaging, or fiducial placement. All men were offered ≥2 years of AST (five of six patients accepted). As AST has a dramatic effect on the prostate gland, AST commenced after the SPECT imaging sessions were completed.

Patient safety monitoring. The following evaluations were conducted at least once a week during the 7.5-week treatment course and at scheduled follow-up visits: (i) history and physical examination, (ii) blood chemistries and complete blood counts, (iii) serum PSA, and (iv) presence of Ad5-yCD/mutTKSR39rep-hNIS viral DNA in blood. Scheduled follow-up visits occurred at 1, 3, 6, 9, 12, 18, and 24 months after completion of the IGRT course and then two to four times a year thereafter. Toxicities were graded using the National Cancer Institute's Common Terminology Criteria for Adverse Events (version 3). Neutralizing antibodies to adenovirus were measured in previous studies4,5 but not here. The presence of adenoviral DNA in blood was quantified by PCR as described previously.8 Ad5-yCD/mutTKSR39rep-hNIS DNA was detected in the blood of all patients and persisted for a mean of 60 days (range 17–84 days). The study was monitored by an internal Data and Safety Monitoring Board.

Nuclear image acquisition and processing, and calculation of absorbed dose. On multiple days (days 2, 3, 4, and 8) after the Ad5-yCD/mutTKSR39rep-hNIS injection (defined as day 1), men were administered Na99mTcO4 (mean dose 16.4 mCi, range 14.7–17.0 mCi) and underwent whole body and pelvic planar scans to quantify hNIS gene expression (i.e., Ad5-yCD/mutTKSR39rep-hNIS-mediated 99mTcO4 uptake) in the prostate as described previously.8 The injected Na99mTcO4 dose was determined by measuring the activity in the syringe before and after administration of the radiotracer. The injected activity was corrected for decay between the time of calibration and injection. Nuclear imaging commenced 3 hours later.

Nuclear image acquisition and processing were performed as described previously.8 Regions of interest on the left and right sides of the prostate, and iliac artery, were identified in the SPECT images and drawn automatically using a tissue segmentation threshold function of the MIM software. To correct for tissue attenuation of the prostate and iliac artery signals, the distance from the center of the signal to the body surface was determined at 15 evenly spaced (24 degree) angles in a single transverse image. The mean distance was calculated and used to determine the attenuation factor using the equation A = Aoe−µt, where A is the signal intensity at the body surface, Ao is the signal intensity at the source, µ is the attenuation coefficient for broad beam geometry (0.12 cm−1), and t is the mean distance from the source to the surface in centimeters. The mean attenuation factors for the prostate and iliac artery were 0.165 and 0.269, respectively, meaning, on average, the source signals were attenuated 6.1-fold (prostate) and 3.7-fold (iliac artery) by body tissue. The mean pixel intensity for the left and right GEVs (prostate) and blood were corrected using the attenuation factors determined for each patient.

The specific radionuclide activity (µCi/g) in the left and right GEVs (prostate) was calculated by multiplying the corrected GEV:blood mean pixel intensity ratio times the specific activity (µCi/g) of 99mTcO4 in human blood 3 hours after its administration, the latter of which can be calculated based on its clearance kinetics.22 Assuming a human blood volume of 5 l, a blood density of 1.0 g/cm3, and knowing the injected Na99mTcO4 dose for each patient, the specific activity of 99mTcO4 in blood 3 hours after its administration was determined to be 0.24–0.26 µCi/g. The activity (µCi) in the left and right GEV (prostate) was calculated by multiplying the specific radionuclide activity (µCi/g) times the GEV mass (g), assuming a prostate tissue density of 1.0 g/cm3. The total radionuclide activity in the prostate (reported in Table 4) is the sum of the activities in the left and right GEVs. These calculations assume that the rate of 131I and 99mTcO4 uptake by NIS is identical in nonthyroid tissue, which has been documented.28

To calculate the absorbed dose to the prostate, the total radionuclide activity (µCi) in the prostate (i.e., sum of left and right GEVs) was divided by prostate mass (g) yielding a prostate specific activity (µCi/g). The absorbed dose is the prostate specific activity times the absorbed dose constant and mean life for 131I. The absorbed dose constant includes both the β- and γ-emissions.23 The mean life was calculated using a physical half-life of 193 hours (8.04 days) and assuming iodine has a biological half-life of 288 hours (12 days) in nonthyroid human tissues.29 The mean life is the effective half-life × 1.44, where the effective half-life is (physical t1/2 × biological t1/2)/(physical t1/2 + biological t1/2).

Acknowledgments

We thank Indrin Chetty and Steve Brown for their help with the absorbed dose calculations. The authors declared no conflict of interest. This work was supported by P01 CA097012 to S.O.F.

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