What is the Optimal Clinical Target Volume for Patients Receiving Postprostatectomy Radiotherapy?
Defining the Prostatic Fossa Clinical Target Volume
With the increasing use of high-precision RT techniques, it is important to clearly define the clinical target volume (CTV) for patients who undergo RT. The EORTC and SWOG trials treated patients with 2D treatment planning techniques where the prostatic fossa was targeted using large treatment portals in which precise definitions of target volumes were not required. This is in contrast to modern radiation treatment techniques such as IMRT, which is currently in use by >80% of practices around the USA. Unlike 2D based planning, IMRT allows for significant normal tissue sparing, but also requires a clearly defined target volume.
When defining the optimal CTV, it is important to understand the local failure patterns of patients in the post-RP setting. Pathology studies have shown that the most common site of failure in the post-RP setting occurs at the vesicourethral anastomosis (VUA) (66%), followed by the bladder neck (16%) and retrotrigone area (13%). A more recent study utilized endorectal MRI to detect local relapse patterns following RP in order to further define the optimal CTV. Based on the results of this study, the authors recommended that a cylindrical shaped CTV that was centered 5 mm posterior and 3 mm inferior to the VUA, keeping in line with the previously mentioned pathological studies.
To address any uncertainties in delineating a CTV, the Radiation Therapy Oncology Group (RTOG) and other cooperative groups have created consensus guidelines for how target volumes for postprostatectomy patients should be created.[35–37] The RTOG recommends that the CTV should extend superiorly from the level of the caudal vas deferens remnant (or 3–4 cm superior to the pubic symphysis, whichever is higher) and inferiorly to 8–12 mm inferior to VUA. The VUA is defined as the retropubic region that can be visualized one slice below the most inferior urine-containing image of the bladder (often best seen on a sagittal reconstruction). Below the superior border of the pubic symphysis, the anterior border is at the posterior aspect of the pubis and extends posteriorly to the rectum. At this level, the lateral border extends to the levator ani muscles. Above the pubic symphysis, the anterior border should encompass the posterior 1–2 cm of the bladder wall and should extend posteriorly to the mesorectal fascia.
Use of Image Guidance to Improve Prostatic Fossa Localization
Several new methods are now employed to improve daily prostatic fossa localization. Techniques currently utilized in most practices in the USA include daily portal imaging with implanted gold seed fiducials, daily cone-beam imaging and calypso beacon localization. Such image-guidance techniques allow for a minimal (7–10 mm) expansion from a CTV to a planning target volume, thereby further minimizing RT dose to the rectum and bladder.
Should the CTV be Expanded to Include Pelvic Lymph Nodes?
An area currently under investigation is whether the CTV for patients treated with postoperative RT should include the prostatic fossa alone or the prostatic fossa and pelvic lymph nodes. There is currently no level I evidence supporting pelvic nodal irradiation in the postprostatectomy setting. This issue has, however, been previously examined in a single institutional retrospective analysis. A review of patients treated at Stanford University with ART or SRT aimed at the prostatic fossa or prostatic fossa, and whole pelvis, demonstrated a biochemical relapse-free survival benefit for patients treated with pelvic irradiation for the subset of patients with high-risk PC (D'Amico Classification). The 5-year biochemical relapse-free survival for patients treated with whole pelvic irradiation was 47% as compared with 21% for patients treated with prostatic fossa irradiation alone (p = 0.008). In a multivariate analysis, whole pelvic RT (p = 0.02) and a pretreatment PSA level of <1 ng/ml (p = 0.002) were the only factors found to be independently correlated with an improved biochemical relapse-free survival. Also, for the subset of patients treated with ADT, there was only a benefit to ADT if given concurrently with whole pelvic RT (p = 0.04). The RTOG has recently initiated a randomized controlled clinical trial (RTOG 0534) to investigate this issue further.
Future Oncol. 2011;7(12):1429-1440. © 2011 Future Medicine Ltd.