In prostate brachytherapy, the Clinical Target Volume (CTV) most commonly includes the entire prostate and areas of local extra-prostatic extension. A 2-3 mm margin around the prostate may be added to encompass microscopic disease. Failure to accurately identify the prostate can result in underdosing of the cancer and overdosing of adjacent organs with unnecessary toxicity. The Gross Tumour Volume (GTV) comprises clinically and radiographically identified gross disease. This may not always exist in early stage disease. Visualizing the GTV helps ensure accurate delineation of the CTV e.g. to identify disease extension into seminal vesicles, or anterior extension into periprostatic tissue. Identifying the GTV assists in determining catheter placement and plan optimization. The clinician may alter standard catheter insertion approach according to GTV location by for example placing extra catheters in and around the GTV, into the seminal vesicles, or outside the prostatic capsule. Localizing the GTV aids plan optimization, ensuring dose coverage particularly when it is close or adjacent to urethra or rectum. Finally, identifying the GTV can be used for focal dose escalation, or focal treatment.
The CTV for prostate HDR is identified using CT, trans-rectal ultrasound (TRUS), MRI, or a combination of the above. Accurately defining the apex is particularly important to avoid over-treatment of the external sphincter, penile bulb and bulbar urethra. CT scanning has the advantage of wide availability and practitioner familiarity with this modality. Although implanted catheters are easily identified, it lacks soft-tissue discrimination to accurately identify the prostate boundary. This is particularly so at the apex, bladder neck and anteriorly, commonly leading to an overestimation of prostate volume. Prostate identification can be improved with implantation of fiducial markers. Furthermore, CT is unable to identify early extraprostatic extension of disease. TRUS provides superior resolution to CT, providing superior anatomic delineation. In contrast to CT, prostate volumes defined by TRUS have high inter-observer reliability and closely match that of the prostatectomy specimen. Prostate volume on TRUS is highly correlated with prostate volume on endorectal MRI. It is inexpensive, portable, and provides real-time imaging. Image quality can be degraded by calcification, or artefact and shadowing from implanted catheters. T2-weighted MRI on 1.5 or 3T systems provides accurate and reproducible anatomical visualization of the prostate. An endorectal coil is not necessary. When combined with diffusion-weighted imaging and dynamic contrast enhancement, MRI enables identification of the GTV within the prostate and delineates extraprostatic extension. MR stage is better correlated with outcome than clinical stage.
Multiparametric MRI is commonly used to identify the GTV, or Dominant Intraprostatic Lesion (DIL) within the prostate. It has a sensitivity of 70-80% for detecting clinically significant disease, and a specificity of 80-95%. MR is less well able to accurately define the borders of the lesion, however. Hence a 3-5 mm margin is generally added to the MR defined DIL for treatment planning. Molecular imaging with anti-Prostate Specific Membrane Antigen (PSMA) antibodies is emerging as a powerful imaging modality for prostate cancer detection, both within the prostate and at metastatic sites. PSMA PET and MRI appear to have comparable and complementary roles in GTV detection. Studies comparing imaging results with whole mount prostatectomy specimens show high concordance with presence and location of disease. PSMA PET is not as accurate as MRI for detection and delineation of extraprostatic disease.
A quality HDR brachytherapy program requires high quality imaging, using modern ultrasound, MRI and access to molecular imaging. Multiparametric MRI and PSMA-PET provide best available local staging information, allowing CTV and GTV delineation to optimize patient selection, HDR catheter insertion and treatment planning.