# Prostate Brachytherapy: LDR Permanent Seed Implant and HDR Boost

## Introduction

Brachytherapy is a highly effective treatment modality for prostate cancer, known for providing excellent biochemical control rates. Its key advantage lies in delivering a concentrated radiation dose directly to the prostate with rapid dose fall-off, thereby minimizing exposure to surrounding tissues. There are two primary approaches to prostate brachytherapy: low-dose-rate (LDR) permanent seed implantation, which can be used either as monotherapy or as a boost, and high-dose-rate (HDR) brachytherapy, which is typically employed as a boost in combination with external beam radiation therapy (EBRT). The success of these treatments depends heavily on careful patient selection, meticulous technique, and precise dosimetric planning.

## LDR Permanent Seed Implant

### Indications

LDR permanent seed implant is commonly used as monotherapy for patients with low-risk and select favorable intermediate-risk prostate cancer. Ideal candidates typically have a prostate-specific antigen (PSA) level below 10 ng/mL, Gleason scores of 3+3 or 3+4, and clinical stage T1c to T2a disease. The prostate volume is ideally less than 60 cc; larger glands may require downsizing with androgen deprivation therapy (ADT) prior to implantation. For patients with intermediate or high-risk disease, LDR brachytherapy is often combined with EBRT. In this setting, EBRT is delivered to a dose of approximately 45 Gy, followed by an LDR seed boost of 110 Gy using I-125 seeds.

### Isotope Selection

The choice of radioactive isotope influences dose delivery characteristics. Iodine-125 (I-125) is the most commonly used isotope, with a half-life of 59.4 days and a mean photon energy of 28 keV, providing a sustained dose over several months. Palladium-103 (Pd-103) has a shorter half-life of 17 days and a mean energy of 21 keV, resulting in faster dose delivery. Cesium-131 (Cs-131) has an even shorter half-life of 9.7 days and delivers dose rapidly but is less commonly used. I-125 is generally preferred for most cases, while Pd-103 may be considered for higher-grade tumors due to a theoretical radiobiologic advantage related to its dose rate.

### Prescription Dose

For I-125 monotherapy, the prescribed dose to the prostate is typically 145 Gy, known as the matched peripheral dose. Pd-103 monotherapy is prescribed at 125 Gy. When used as a boost following 45 Gy of EBRT, the I-125 dose is reduced to 110 Gy, while Pd-103 boost doses range from 90 to 100 Gy.

### Technique

The procedure is performed under transrectal ultrasound (TRUS) guidance, with the patient under general or spinal anesthesia positioned in dorsal lithotomy. A template guides transperineal needle placement through the perineum. Seeds are deposited using either preloaded needles or a Mick applicator. Planning can be done preoperatively or intraoperatively with real-time TRUS-based dosimetry to optimize seed placement. Typically, between 60 and 120 seeds are implanted depending on the prostate volume.

### Dosimetric Parameters (ABS Recommendations)

Dosimetric goals recommended by the American Brachytherapy Society (ABS) include achieving a V100 (the volume of the prostate receiving 100% of the prescription dose) greater than 90%, and a D90 (the dose covering 90% of the prostate volume) exceeding the prescription dose. The V150 (volume receiving 150% of the prescription dose) should ideally be less than 50% to limit hot spots. The urethral dose, measured as D30 (dose to 30% of the urethra), should remain below 130% of the prescription dose to minimize toxicity. Rectal dose, typically measured as D2cc (dose to the most exposed 2 cc of rectum), should be minimized and generally kept below the prescription dose.

![Intraoperative TRUS image showing transperineal needle placement with real-time seed dosimetry during LDR prostate brachytherapy](images/prostate-ldr-trus.jpg)

## Post-Implant Dosimetry

Post-implant dosimetry is performed using CT imaging at 4 to 6 weeks after implantation for I-125 seeds, or 2 to 3 weeks for Pd-103 seeds. Seed positions are identified on CT scans, and prostate contours are refined by fusing MRI images. Key dosimetric parameters such as D90, V100, V150, and doses to organs at risk (OARs) are assessed. Among these, the post-implant D90 is the strongest predictor of biochemical control. An acceptable D90 for I-125 monotherapy on day-30 CT is greater than 130 Gy.

## HDR Brachytherapy Boost

### Indications

HDR brachytherapy is primarily used as a boost in combination with EBRT for intermediate and high-risk prostate cancer. EBRT doses typically range from 45 to 50.4 Gy, followed by or interdigitated with an HDR boost. Some centers also use HDR brachytherapy as monotherapy, delivering two fractions of 13.5 Gy.

### Advantages Over LDR

HDR brachytherapy offers several advantages over LDR. It allows for dose optimization through modulation of dwell times, eliminating the need for permanent seeds and avoiding seed migration. The source positioning is precise and adjustable at each fraction, enabling tailored dose distribution. Additionally, patients do not require radiation precautions after treatment since no radioactive material remains implanted.

### Dose and Fractionation (Common Regimens)

Common HDR boost regimens include a single fraction of 15 Gy or two fractions of 9.5 Gy combined with EBRT. HDR monotherapy regimens often consist of two fractions of 13.5 Gy or four fractions of 9.5 Gy. Multiple fractionation schedules exist, and the ABS has published consensus guidelines to standardize practice.

| Modality | Setting | Isotope / Source | Prescription Dose | Combined EBRT |
|---|---|---|---|---|
| LDR monotherapy | Low / fav. intermediate risk | I-125 | 145 Gy | None |
| LDR monotherapy | Low / fav. intermediate risk | Pd-103 | 125 Gy | None |
| LDR boost | Intermediate / high risk | I-125 | 110 Gy | 45 Gy EBRT |
| LDR boost | Intermediate / high risk | Pd-103 | 90–100 Gy | 45 Gy EBRT |
| HDR boost (1 fx) | Intermediate / high risk | Ir-192 | 15 Gy x 1 | 45–50.4 Gy EBRT |
| HDR boost (2 fx) | Intermediate / high risk | Ir-192 | 9.5 Gy x 2 | 45–50.4 Gy EBRT |
| HDR monotherapy | Low / intermediate risk | Ir-192 | 13.5 Gy x 2 | None |

### Technique

HDR brachytherapy is performed under TRUS or MRI guidance with transperineal catheter placement. A template guides the insertion of 14 to 20 afterloading catheters into the prostate. Imaging with CT or MRI is obtained with the catheters in place for treatment planning. Inverse planning algorithms optimize dose distribution to the prostate clinical target volume (CTV) while respecting constraints for organs at risk. Treatment is delivered using an Iridium-192 HDR afterloader, and catheters are removed after the final fraction.

### Dosimetric Goals (HDR)

Dosimetric goals for HDR include achieving a prostate V100 of greater than 90-95% and a D90 exceeding the prescription dose. Urethral dose constraints aim for a V120 (volume receiving 120% of the prescription dose) less than 1 cc. Rectal dose constraints typically require the V75% (volume receiving 75% of the prescription dose) to be less than 1 cc or the D2cc to remain below tolerance thresholds.

![HDR prostate brachytherapy treatment plan showing catheter positions, isodose lines, and DVH with urethral and rectal sparing](images/prostate-hdr-plan.jpg)

## Patient Selection Considerations

Ideal candidates for brachytherapy have a prostate volume under 60 cc or can achieve downsizing with ADT. Patients should have no prior transurethral resection of the prostate (TURP), as this is a relative contraindication for LDR due to the risk of urethral defects. Adequate pubic arch clearance is necessary to allow transperineal access. Baseline urinary function should be good, with an American Urological Association (AUA) symptom score below 15 to 20.

Relative contraindications include very large prostate glands exceeding 60 cc without ADT downsizing, significant pubic arch interference that obstructs needle placement, prior TURP with a large defect which increases the risk of urinary incontinence after LDR, severe baseline obstructive urinary symptoms, and inflammatory bowel disease due to concerns about rectal dose.

## Outcomes

For LDR monotherapy, 10-year biochemical control rates for low-risk patients range from 90 to 95%. Intermediate-risk patients treated with combined EBRT and LDR boost achieve biochemical control rates of 85 to 90% at 10 years. The ASCENDE-RT trial demonstrated that an LDR boost provides superior biochemical progression-free survival compared to dose-escalated EBRT alone in intermediate and high-risk patients.

HDR boost therapy similarly shows excellent long-term outcomes. The ASCENDE-RT trial reported a 9-year biochemical progression-free survival of 83% for the LDR boost arm versus 63% for dose-escalated EBRT alone. Multiple institutional series have confirmed the efficacy of HDR boost, and emerging data suggest that HDR monotherapy yields outcomes comparable to LDR.

![Bar chart comparing biochemical control rates between LDR monotherapy, LDR boost with EBRT, HDR boost with EBRT, and dose-escalated EBRT alone](images/prostate-brachy-outcomes.jpg)

## Key Clinical Pearls

LDR permanent seed implant is an excellent monotherapy option for patients with low and favorable intermediate-risk prostate cancer, achieving 10-year biochemical control rates exceeding 90%. The ASCENDE-RT trial demonstrated that brachytherapy boost, whether LDR or HDR, is superior to dose-escalated EBRT alone for intermediate and high-risk disease. Post-implant dosimetry performed at 4 to 6 weeks is essential for quality assurance, with D90 serving as the strongest predictor of biochemical outcome. HDR brachytherapy offers superior dose optimization and is preferred as a boost in many centers for intermediate and high-risk disease. Careful patient selection is critical, with assessment of prostate volume, pubic arch anatomy, prior TURP status, and baseline urinary function guiding treatment decisions.

## References

1. Morris WJ, Tyldesley S, Rodda S, et al. Androgen Suppression Combined with Elective Nodal and Dose Escalated Radiation Therapy (the ASCENDE-RT Trial): an analysis of survival endpoints for a randomized trial comparing a low-dose-rate brachytherapy boost to a dose-escalated external beam boost for high- and intermediate-risk prostate cancer. *Int J Radiat Oncol Biol Phys*. 2017;98(2):275-285.  
2. Davis BJ, Horwitz EM, Lee WR, et al. American Brachytherapy Society consensus guidelines for transrectal ultrasound-guided permanent prostate brachytherapy. *Brachytherapy*. 2012;11(1):6-19.  
3. Hoskin PJ, Colombo A, Henry A, et al. GEC/ESTRO recommendations on high dose rate afterloading brachytherapy for localised prostate cancer: an update. *Radiother Oncol*. 2013;107(3):325-332.  
4. Yamada Y, Rogers L, Demanes DJ, et al. American Brachytherapy Society consensus guidelines for high-dose-rate prostate brachytherapy. *Brachytherapy*. 2012;11(1):20-32.