Brachytherapy implant characteristics

Patients received one implant and one fraction of HDR. Fraction dose is 20.5 Gy because it is considered to correspond biologically (biologic effective dose) to > 90 Gy administered at 2 Gy/fraction according to the linear quadratic model, assuming an a/p of 1.2 Gy (7, 8, 9, 10).

Brachytherapy procedure is done under spinal anesthesia with the patient in the lithotomy position (Fig. 1). A Foley catheter is placed, and the bladder is partially filled with 100 cm3 of sterile water. The needles are positioned (Fig. 2) by transperineal placement under real time TRUS guidance using a template. Axial cross-sections is captured in 5mm steps and transferred to the Treatment Planning Software. Prostate gland, normal structures (urethra and rectum) and needle positions are identified and mapped based on the ultrasound image. Dose optimization is done on the reconstructed applicator geometry using dose point and manual optimization algorithms to determine dwell positions and times (Fig. 3).

Figure 1. Lithotomy position

Figure 2. The needles are positioned

Figure 3. Dose optimization

The prostate without safety margins is then defined as the planning target volume (PTV) to be treated (Fig. 4) with the prescribed dose (PD).

Figure 4. Treatment

Based on the dose volume histograms (DVH) data, the quality of plans and implants is evaluated using following indicators:

• The rectal dose is calculated at the anterior edge of the TRUS probe and is limited to < 75% of the prescription dose.

• The dose to any segment of the urethra is limited to < 110% of the prescription dose. V120 and D100 of the prostatic urethra are determined (volume that received a dose of 120% and dose delivered to 100% of the urethra).

• The PTV V90, V100, V150 and V200 (% of PTV receiving 90%, 100%, 150% and 200% of the PD) are recorded.

• D90 (dose delivered to 90% of the PTV) is calculated.

All patients are discharged from the center on the same day of the procedure between 6-8 hours of implantation.

To decrease rectal toxicity, transperineal hyaluronic acid (HA) injection into the peri-rectal fat is used to consistently displace the rectal wall away from the radiation sources in all patients. We believe that the increase in distance (mean 2 cm along the length of the prostate) will be enough to provide a significant radiation dose reduction from HDR brachytherapy [11, 12].

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