Radiation safety of receptive anal intercourse with prostate cancer patients treated with low-dose-rate brachytherapy

Abstract

PURPOSE

Prostate low-dose-rate (LDR) brachytherapy involves implantation of radioactive seeds permanently into the prostate gland. During receptive anal intercourse, the penis of the partner may come in close proximity to the implanted prostate gland. We estimate the potential intrarectal dose rates and suggest guidance on radiation precautions.

METHODS AND MATERIALS

One hundred two patients were included in the study. After implantation, with patients under anesthesia in the dorsal lithotomy position, a new set of ultrasound (US) images and a CT scan were obtained. The images were fused, radioactive seeds and US probe locations were determined on the CT, and prostate, bladder, and rectal contours were drawn on the US. Dose rates (cGy/h) were calculated for the portion of the US probe spanning the prostate for several dose-volume histogram parameters.

RESULTS

Twenty patients were treated with ¹²⁵I and 82 patients with ¹⁰³Pd. Average dose rates at Day 0 to the portion of the US probe spanning the prostate were 2.1 ± 1.3 cGy/h and 2.5 ± 0.8 cGy/h for patients treated with ¹²⁵I and ¹⁰³Pd, respectively. After 60 days, average calculated probe dose drops to 1.0 ± 0.6 cGy/h and 0.2 ± 0.1 cGy/h for ¹²⁵I and ¹⁰³Pd, respectively.

CONCLUSIONS

During the immediate weeks after prostate seed implant, the estimated intrarectal dose rates are higher in ¹⁰³Pd compared to ¹²⁵I. As ¹⁰³Pd decays faster than ¹²⁵I, 2 months after the implant, radiation exposure from ¹⁰³Pd becomes lower than ¹²⁵I. Receptive anal intercourse time should be kept as low as possible during 2 and 6 months after low-dose-rate brachytherapy of the prostate with ¹⁰³Pd and ¹²⁵I, respectively.

Introduction

Prostate cancer is the most common malignancy among men (1). Low-dose-rate (LDR) brachytherapy is an efficient treatment for low-risk, and selected patients with intermediate-risk disease as a single modality (2, 3), and is used in combination with external beam radiation for intermediate- and high-risk patients to achieve radiation dose escalation (4–8). LDR prostate brachytherapy is performed by permanently implanting radioactive seeds inside the prostate in a distribution that results in proper radiation coverage to the prostate and a margin around it (9, 10). The most used radionuclides in prostate LDR brachytherapy are ¹²⁵I with a radioactivity half-life of 59.4 days or ¹⁰³Pd with a half-life of 17 days (11). The International Commission on Radiological Protection (ICRP) issued guidance on radiation precautions based on potential external exposures to members of the public and/or carers for prostate seed brachytherapy (12). Prostate brachytherapy patients are provided with instructions on precaution actions recommended to maintain doses to other individuals as low as is reasonably achievable (ALARA) (13). Typical anterior skin surface dose after a prostate implant is about 37 μSv/h for ¹²⁵I and 8 μSv/h for ¹⁰³Pd (13). The time required to reach an effective dose equivalent limit of 5 mSv for caregivers was estimated to be 19 days on contact with the skin surface for ¹²⁵I, and the total doses to decay for ¹²⁵I at a distance of 30 cm from the anterior skin surface or for ¹⁰³Pd on contact with the skin surface and at 30 cm from the anterior skin surface were found to be less than 5 mSv (13). Typical radiation safety ALARA instructions to patients after permanent prostate brachytherapy include avoiding close contact, that is, within 30 cm, with others (including children and pregnant women) for extended periods of time; and avoiding sleeping in the “spoon” position (i.e., in contact) with the primary caregiver (12, 13). A more recent study from Memorial Sloan Kettering Cancer Center with radiation exposure rate measurements obtained from 1279 patients with prostate cancer who underwent transperineal ¹²⁵I or ¹⁰³Pd seed implantation found that for the typical ¹⁰³Pd patient, no radiation safety precautions are required and that for the typical ¹²⁵I patient, no precautions are required for coworkers, nonpregnant adults who do not sleep with the patient, or nonpregnant adults who sleep with the patient (14). Typical 125I patients should only avoid sleeping in the “spoon” position (i.e., in contact) with pregnant adults and avoid holding a child for long periods of time in the lap for about 2 months (14). The only precaution that was provided to patients regarding sexual activity was the use of a condom for the first two to three occasions of intercourse after implantation because it is theoretically possible for a seed to be expelled in the semen on ejaculation and in the very rare event that this happens it is usually in the first one or two ejaculations (15). Some centers advise patients who are not interested in using a condom, to masturbate on five occasions before engaging in unprotected sexual activity, so if a seed were to be expelled it would not be ejaculated into the body of the partner. No special precautions have been typically provided for patients regarding receptive anal intercourse (RAI). During RAI, the penis of the partner may come in close proximity to the implanted prostate gland (Fig. 1). Here, using the transrectal ultrasound (US) probe as a surrogate for the partners’ penis, we estimate the potential intrarectal partners’ penile dose rates during RAI with prostate cancer patients treated with LDR brachytherapy.

Fig. 1.

Schematic drawing of the internal anatomy of prostate cancer patients after low-dose-rate brachytherapy during receptive anal intercourse (RAI). During RAI, the penis of the partner may come in close proximity to the implanted prostate gland.

Methods and materials

Patients

This retrospective study was approved by Memorial Sloan Kettering Cancer Center institutional review board. Eligible patients for inclusion in the study were men with clinically localized prostate cancer, who elected to be treated with LDR prostate brachytherapy as a single treatment modality or combined with external beam radiation, and had intraoperative CT scan performed at the end of the seed implantation procedure for intraoperative dosimetry.

Brachytherapy technique

The LDR seed implantation procedure and intraoperative planning technique used in this study have been previously described in details (16). Briefly, the patients were intubated under general anesthesia, and intravenous dexamethasone (8 mg) was given at the time of induction (17). The patients were positioned in the dorsal lithotomy position using Allen Stirrups. Interstitial needles were inserted into the peripheral substance of the prostate gland through a perineal template under sagittal and axial transrectal US image guidance. Transurethral aerated gel was applied to maximize urethral visualization, and axial US images of the prostate were subsequently acquired at 5-mm increments from the base of the prostate to the apex and transferred into the brachytherapy treatment planning system (17). Contours of the target volume and organs at risk were drawn on each axial image for computer-based three-dimensional reconstruction. Needle positions were identified and reconstructed. An inverse planning system using a genetic optimization algorithm for intraoperative US-guided transperineal technique was used. A conformal treatment plan was generated to determine the optimal seed distribution to deliver a prescription dose to the target volume (prostate). Patients treated with brachytherapy as monotherapy were prescribed a dose of 144 Gy or 125 Gy for ¹²⁵I or ¹⁰³Pd seeds, respectively (11, 18). For patients treated with brachytherapy combined with external beam radiation, ¹⁰³Pd seeds were used with a prescription dose of 100 Gy to the prostate (11). Urethral dose was kept below 120% and rectal dose below 80% of prescription dose (17). Radioactive seeds were placed with a Mick applicator (Mick Radio-Nuclear Instruments, Inc, Mount Vernon, NY) under US and fluoroscopic image guidance according to the approved treatment plan.

Intraoperative dosimetry

On completion of the seed implantation according to the intraoperative treatment plan, a new set of US images and a CT scan were obtained while the patient was under anesthesia in the lithotomy position as part of an intraoperative dosimetry (19). US and CT images were registered using US probe locations determined on the CT. Radioactive seed coordinates were translated from CT to the US image space, and prostate, bladder, and rectal contours were drawn on the US. Additional seeds were added as necessary. Final intraoperative dosimetric assessment was performed at the conclusion of the seed implantation, for verification of proper radiation coverage of the prostate gland with the prescription dose.

For this study, we retrospectively calculated dose rates (cGy/h) for the portion of the US probe spanning the prostate for several dose-volume histogram parameters. Penile tissues radiation dose limit of 2 cGy was calculated applying the current US Nuclear Regulatory Commission (NRC) tissue-weighting factor of 0.25 for the gonads, to the 5 mSv whole body effective dose limit (20).

Patient recovery

On completion of the seed implantation and obtaining the final intraoperative dosimetry, urinary catheter was removed, anesthesia was reversed, and patients were transferred to the recovery room and were discharged with a prescription for tamsulosin pending a successful voiding trial (17).

Results

Patients

Between December 2012 and June 2015, 102 patients with prostate cancer were treated with brachytherapy and were eligible for inclusion in the study. Twenty patients were treated with 125I seeds as a definitive therapy (iodine-mono); 82 patients were treated with ¹⁰³Pd, 34 of them as single modality (palladium-mono), and for 48 patients, brachytherapy was followed with external beam radiation therapy as part of combined modality treatment (palladium-combo) (11).

Probe dose

Probe dose rate per hour at the end of the seed implantation procedure is described in Table 1. Probe D_max, D_1cc, D_2cc, and average dose to the part of the probe spanning the prostate are described in Fig. 2. Calculated anal receptive sex time (minutes) with prostate cancer patient 0, 2, 4, and 6 months after prostate seed implant, which will result in an average penile dose of 2 cGy are described in Table 2. Average dose rate (cGy/h) absorbed by the portion of the US probe spanning the prostate at different time points from prostate seed implant was calculated as a function of the seed radioactivity half-life and is described in Fig. 3.

Table 1.

Probe dose rate per hour at the end of the seed implantation procedure

	Dmax cGy/h	D1cc cGy/h	D2cc cGy/h	Davg cGy/h
Iodine-mono
Min	3.5	2.6	2.1	1.0
Max	23.4	17.2	14.9	7.4
Average	6.7	4.6	3.7	2.1
Median	5.2	3.7	3.1	1.8
Pd-mono
Min	5.1	2.6	1.0	1.3
Max	25.3	14.8	11.5	5.7
Average	11.6	6.8	5.1	2.8
Median	10.7	6.5	4.8	2.6
Pd-combo
Min	3.0	2.0	1.6	0.8
Max	16.8	10.8	8.9	3.9
Average	9.4	5.9	4.7	2.3
Median	9.4	6.0	4.7	2.4

D_max = maximal dose to the probe; D_1cc = dose received by 1 cm³ of the probe; D_2cc = dose received by 2 cm³ of the probe; Davg = average dose received by the part of the probe spanning the prostate.

Fig. 2.

Patients with localized prostate cancer were treated with ¹²⁵I brachytherapy as monotherapy to a dose of 144 Gy (red), or with ¹⁰³Pd as monotherapy or combined with external beam radiation to doses of 125 Gy (gray) or 100 Gy (black), respectively. D_max (circles), D_1cc (squares), D_2cc (triangles pointing up), and average (triangles pointing down) radiation dose rates (cGy/h) calculated for the portion of the US probe spanning the prostate. (a) At Day 0; (b) 60 days after the seed implantation procedure. Box plots denotes mean ± standard deviation. US = ultrasound. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 2.

Anal receptive sex time (minutes) with prostate cancer patient after seed implant which will result in an average penile dose of 2 cGy

	Iodine-mono	Pd-mono	Pd-combo
Immediately after implant
Min	16	21	31
Max	114	93	155
Average	58	43	52
Median	65	47	49
2 months after implant
Min	32	238	346
Max	229	1050	1748
Average	116	489	586
Median	131	527	558
4 months after implant
Min	65	2688	3910
Max	458	11884	19776
Average	232	5532	6625
Median	262	5962	6316
6 months after implant
Min	129	30411	44235
Max	916	134450	223740
Average	464	62587	74954
Median	524	67447	71456

Fig. 3.

Average dose rate (cGy/h) calculated for the portion of the US probe spanning the prostate as a function of time from prostate seed implant. Graphs are provided for ¹²⁵I brachytherapy as monotherapy to a dose of 144 Gy (red), or with ¹⁰³Pd as monotherapy or combined with external beam radiation, to doses of 125 Gy (gray) or 100 Gy (black), respectively. A dose rate of 0.25 cGy/h is reached at 2 and 6 months for patients treated with ¹⁰³Pd or ¹²⁵I, respectively. US = ultrasound. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Rectal wall dose

Median rectal wall volume that receives 100% of the prescribed dose (RV₁₀₀) was 0.1 cm³ (average = 0.2 cm³; range, 0–1.6 cm³). Median rectal wall dose to 1 cm³ (RD_1cc) at Day 0 was 8.1 cGy/h (range, 5.5–41.8), 25.7 cGy/h (range, 13.7–56.8), and 19.4 cGy/h (range, 11.8–99.2) for iodine-mono, palladium-mono, and palladium-combo, respectively.

Discussion

According to the recommendations of the ICRP, the National Council on Radiation Protection and Measurements, and the guidance of the NRC, a medical facility may authorize the release from its control any individual who has been administered by-product material if the whole body effective dose to any other individual is not likely to exceed 5 mSv and the whole body effective dose to a member of the general public is not likely to exceed 1 mSv (20–22).

Prostate brachytherapy patients are given instructions on actions recommended to maintain doses to other individuals ALARA (14). The safety of LDR prostate brachytherapy has been investigated in several studies (13–15), and the radiation dose emitted from the implanted seeds was shown to be negligible as far as exposure of the general public to radiation was a concern (13, 14). Applying the current NRC tissue-weighting factor of 0.25 for the gonads (as a surrogate for penile tissues) to the 5 mSv, the whole body effective dose limit, results in an equivalent organ dose limit for the penile tissue of 20 mGy (2 cGy). Our study shows that during the immediate weeks after prostate seed implant, RAI may result in exposure of the penis of the partner to a cumulative penile radiation doses of 2 cGy after limited RAI time. Two months after LDR brachytherapy, the average intrarectal dose drops to 0.2 cGy/h for patients treated with ¹⁰³Pd (monotherapy or combination therapy) and to 1 cGy/h for patients treated with ¹²⁵I monotherapy. RAI should be kept to the minimum time possible for 2 months after LDR brachytherapy of the prostate with ¹⁰³Pd seeds and for 6 months when ¹²⁵I seeds are implanted to avoid intrarectal doses of 0.25 cGy/h and cumulative penile doses of 2 cGy to the partner.

According to cancer statistics, during the year 2015, 220,800 patients were expected to be diagnosed with prostate cancer in the United States (1). Analysis of National Survey of Family Growth finds that 4.1% of men identify as gay or bisexual (23). Thus, in 2015, it could be estimated that more than 9000 homosexual or bisexual men were diagnosed with prostate cancer in the United States, and about one-third of them were probably suitable for LDR brachytherapy. Moreover, prostate brachytherapy was correlated with higher rates of preservation of erectile function compared to external beam radiation therapy or surgery (24, 25), and thus, sexually active men may prefer it over the other treatment modalities. Thus, the present study is potentially relevant to thousands of patients in the United States and around the world.

The radiation dose absorbed by the partner penis is a function of several parameters including geometry factors and the total cumulative intercourse time during which the penis could come in close proximity of the prostate.

Two months after ¹⁰³Pd implant and 6 months after ¹²⁵I implant, the radiation dose to the partner penis will be negligible even with extended intercourse times. The intercourse time which is needed to achieve an average dose to the penis of 2 cGy relates in our study to a situation in which the penis is continuously adjacent to the implanted prostate, which may tend to overestimate actual geometry conditions.

The model used in our study utilizes the US probe as a surrogate to the partner male penis, and the dorsal lithotomy position as the only position investigated. We recognize that our estimates can be associated therefore with variations in actual clinical dose exposures as well as expected variations in the penile lengths and diameters, and intercourse positions, which all may affect cumulative average doses to the penis during RAI with individuals treated with LDR prostate brachytherapy. The present study focuses only on the precautions related to the effect of radiation after LDR prostate brachytherapy. Mechanical injury to the rectum during the seed implantation procedure may be an issue during RAI in the immediate few days after LDR brachytherapy. The long-term effects of radiation on the mucosa of the rectum and its effect on RAI are beyond the scope of this report and warrant further investigation.

Conclusions

During the immediate weeks after prostate seed implant, the estimated intrarectal dose rates are higher in 103Pd compared to 125I. Both would likely exceed equivalent ICRP guidance, after limited RAI time. As ¹⁰³Pd decays faster than ¹²⁵I, 2 months after the implant, radiation exposure from ¹⁰³Pd becomes lower than ¹²⁵I. RAI time should be kept to the minimum possible during 2 and 6 months after LDR brachytherapy of the prostate with ¹⁰³Pd and ¹²⁵I, respectively. Consideration therefore should be given with regard to sexually active homosexual and bisexual men who opt for LDR brachytherapy of the prostate to use ¹⁰³Pd rather than ¹²⁵I, to shorten the period of recommended abstinence form RAI.