20 Prostate Cancer

Prostate Cancer

4

THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II: Clinical Practice Version 1 - 01/12/2014

3. ANATOMICAL TOPOGRAPHY

The prostate surrounds the urethra from the bladder base to its apex, which is near to the external sphincter. It is composed of four regions, the peripheral zones, central zone, transitional zones and anterior fibro muscular stroma zone as shown in fig- ure 21.1. Structures reviewed in the literature all include prostate zonal anatomy (transition zone, peripheral zone, central zone). The central zone surrounds the ejaculatory ducts; the anterior fi- bro muscular stroma is an anterior band of fibro muscular tissue contiguous with the bladder muscle and the external sphincter. The seminal vesicles arise from the superolateral aspects of the gland, and extend posterior to the gland wrapping around the rectum. The normal gland volume is 20 to 30 cm 3 but in mid- dle age benign prostatic hypertrophy (BPH) will develop and the gland volume will slowly increase. BPH typically affects the median transition zone which can cause a compression of the surrounding prostate structures and extending superiorly will indent the bladder base where parts of the bladder muscle converge and merge with the inner longitudinal muscle of the internal preprostatic sphincter. The bladder, bladder neck and internal sphincter are continuous and therefore no demarcation line is visible. This has a very important role in brachytherapy planning, since too high dose in this region may cause sphincter function problems due to late fibrosis. The important anatomical relations of the prostate gland in brachytherapy are to the urethra and rectum. The urethra passes through the gland from the bladder base typically moving at first inferiorly and then looping anteriorly to enter the base of the pe- nis through the urogenital diaphragm. This is illustrated in figure 21.2 which is taken from a transrectal ultrasound volume study using aerated gel to identify the urethra. The external sphincter surrounds the urogenital diaphragm and is easily seen with MR tomography. The cross sectional profile of the urethra also varies along its length. When catheterised it appears a uniform circular tube but in fact it is a crescentic shape expanding as it passes through the gland to the veru montanum where the prostatic ducts enter and then reducing in size as it approaches the uro- genital diaphragm. The rectum lies immediately posterior to the prostate along its entire length. The distance between the pos- terior capsule and anterior rectal wall varies and is a critical di- mension when planning prostate brachytherapy. The rectal wall thickens inferiorly as it becomes the anal canal and approaches the internal anal sphincter. There are three major vascular and neural pathways around the prostate. The first is located posterolaterally and includes the neurovascular bundles (NVB) and courses over the lateral rectal surface and then posteriorly and laterally adjacent to the seminal vesicle and superior prostate. The second pathway is inferolateral and has no contact with the prostate. The anteroinferior pathway is the third, consisting of the dorsal venous plexus coursing over the anterior prostate from the inferior direction. The majority of tumours start in the peripheral zones of the prostate in the posterior and lateral regions, however close his- tological examination of prostatectomy specimens often shows scattered foci of tumour throughout the gland and transurethral resection specimens suggest that up to 85% of cancers will be found in the periurethral region also. The issue of prognostic rel- evance of the zonal anatomy has been reviewed [3]. This identi- fied 64% of cancer involvement only in the peripheral zone and

Fig 21.1: Zonal anatomy of prostate

Fig 21.2: TRUS showing urethral anatomy

8% only transitional zone tumours. Of the remaining 28% of the patients with involvement in both the peripheral and transition- al zones, one third had dominant involvement of the peripheral zone and two thirds of the transitional zone, so the study con- firms the frequent transitional zone involvement in early tumour stages. As the tumour grows it extends into and through the loose cap- sule of fibrous connective tissue, which surrounds the prostate (extracapsular extension-ECE). This may also spread into the seminal vesicles and into adjacent pelvic nodes. This pattern of local extension is reflected in the T staging classification shown in table 21.1. The probability of finding disease outside the pros- tate capsule based on prostatectomy specimens can be related to clinical stage, PSA and Gleason grade as shown in table 21.2. This is an important consideration when evaluating a patient for brachytherapy; those with a high risk of ECE are considered unsuitable for permanent LDR seed brachytherapy and the risk of ECE and seminal vesicle invasion may be taken into account when designing the clinical target volume (CTV) for HDR brachytherapy.