Evolving imaging methods of prostate cancer and the emergence of magnetic resonance imaging guided ablation techniques

Abstract

Established therapies for prostate cancer (PCa), surgery and radiotherapy, treat the entire gland regardless of the location of the cancerous lesion within the prostate. Although effective, these methods include a significant risk of worsening genitourinary outcomes. Targeted image-guided cancer therapy has gained acceptance through improved PCa detection, localization, and characterization by magnetic resonance imaging (MRI). Minimally-invasive ablative techniques aim to achieve comparable oncological outcomes to radical treatment while preserving genitourinary function. Transurethral ultrasound ablation (TULSA) and next-generation transrectal high-intensity focused ultrasound (HIFU) utilize MRI guidance to thermally ablate prostate tissue under real-time MRI monitoring and active temperature feedback control. Previous trials performed by our group and others, including a large multicenter study in men with localized favorable-risk disease, have demonstrated that TULSA provides effective prostate ablation with a favorable safety profile and low impact on quality of life. Recently, MRI-guided HIFU focal therapy was also shown as a safe and effective treatment of intermediate-risk PCa. Here we review the current literature on ablative techniques in the treatment of localized PCa with a focus on TULSA and HIFU methods.

Keywords: HIFU; MRI; Tulsa; ablation therapy; high-intensity focused ultrasound; magnetic resonance imaging; prostate cancer; transurethral ultrasound ablation.

Figure 1

Description of the TULSA technology. MRI-guided TULSA is a minimally invasive ablation technique delivering directional high-intensity ultrasound energy (*) to the prostate (yellow circle) using a transurethral rotational UA comprised of 10 independently controlled ultrasound elements (A). By actively cooling both the urethra and rectum throughout the ablation, TULSA protects these structures from thermal injuries (B, C). Real-time MRI-thermometry is continuously acquired during the ablation to automatically control the delivered lethal thermal energy by adjusting each ultrasound element’s frequency and power and the UA’s rotation rate (D). On the axial maximum temperature image of a patient undergoing lesion-targeted TULSA of a posterior peripheral zone tumor (D), a minimum lethal temperature of 55°C reaches the drawn (black) boundary. Due to prostate swelling caused by the ablation, the catheter is kept in place for weeks after the procedure (see Figure 2 patient case with a suprapubic catheter). MRI, magnetic resonance imaging; TULSA, transurethral ultrasound ablation; UA, ultrasound applicator.

Figure 2

An example of a successful salvage TULSA patient case. The patient had a rising PSA of up to 13 ng/ml within six years after primary external beam radiotherapy. Screening T2-weighted (A) and diffusion-weighted (B) MRI showed a distinct anterior lesion with early enhancement on gadolinium-enhanced imaging (C) graded as PI-RR 5 lesion. The same lesion was also clearly visible in ¹⁸F-PSMA-1007 PET-CT (maximum standardized uptake value 23) (D). Two residual gold fiducial markers implanted before image-guided radiotherapy are also visible next to the PSMA-positive lesion (D). The MRI-targeted biopsy from the lesion revealed vital carcinoma resembling ISUP GG 5 disease (E). The patient underwent whole-gland TULSA. On the sagittal T1-weighted image (F), a transurethrally inserted ultrasound applicator, endorectal cooling device, and suprapubic catheter are in place. The targeted region reached a lethal minimum temperature of 55°C (G). The non-perfused volume can be visualized immediately after treatment, demonstrating the acute ablation effect covering the targeted lesion (H). At 12 months, the patient underwent follow-up imaging with multiparametric MRI and ¹⁸F-PSMA-1007 PET-CT (I, J), both negative for cancer. The prostate volume decreased from 20 cm³ to less than 1 cm³ at 12 months. The 12-month post-TULSA biopsy agreed with imaging findings and showed only a treatment effect with no signs of cancer (K). At the recent follow-up visit two years after TULSA, PSA is still low (PSA 0.067 ng/ml) and stable, and the patient has leak- and pad-free continence and erections sufficient for intercourse. The TULSA treatment report of the patient case, including treatment planning, thermal mapping, and post-treatment gadolinium-enhanced images, is provided in Supplementary Figure S1 . CT, computed tomography; ISUP GG, International Society of Urological Pathology grade group; MRI, magnetic resonance imaging; PET, positron emission tomography; PI-RR, Prostate Imaging for Recurrence Reporting; PSA, prostate-specific antigen; PSMA, prostate-specific membrane antigen; TRUS, transrectal ultrasound; TULSA, Transurethral ultrasound ablation.