20 Prostate Cancer

Prostate Cancer

8

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

are different from that of the catheter; whilst this is an acceptable estimate when radiation exposure is completed with the catheter in situ, as in HDR brachytherapy, it will not represent the urethra during prolonged exposure after LDR seed insertion [20]. Bladder base and penile bulb may also be outlined for reporting but at present there is not sufficient evidence to define dose con- straints for these structures. CTV and PTV Whilst previously the CTV was considered to be the same as the PTV it is now recognised that there are a number of uncer- tainties which are not accounted for in definition of the CTV ranging from 5% for HDR to 11% for LDR [21]. These relate to physical parameters such as image definition and registration, catheter reconstruction and source positioning and also changes in the patient with prostatic and periprostatic oedema, varia- tion in bladder filling and bowel position which should be taken into account when planning treatment. This will vary from pa- tient to patient and is also dependent on technique. There will be less uncertainty during HDR, particularly when undertaken as a single step procedure with radiation delivery immediately after implantation in contrast to LDR when radiation delivery occurs over several months. No specific recommendations for this effect can be made; usually the addition of 2-3mm beyond the capsule to define the CTV is considered adequate to account for this but if this is considered the margin required to include all microscopic disease then a further expansion of 2-3mm should be considered for the PTV. Pre-implant preparation: 1. The rectum is cleared with an enema before implantation. Lax- atives may also be given in the preceding days. 2. Evaluation for anaesthetic is required. In some centres spinal anaesthesia will be used whilst others prefer general anaesthe- sia. 3. Where there is an element of outflow obstruction the use of an alpha blocking drug may improve flow both before and after implant. The patient is catheterised to drain the bladder during the treat- ment and to visualise the urethra. It may also be helpful to place air filled gel in the catheter or urethra so that it is more clearly visible on ultrasonography. The transrectal ultrasound probe is inserted and attached to a stepping unit and template (Fig 21.4) as for the volume study (see below). Traditionally image acquisi- tion is by stepping in 5.0 mm Intervals from base to apex of the gland.. Modern devices and planning systems now allow higher resolution with up to 1mm continuous movement of the probe without stepping and with rotational volume acquisitions. In this way a higher accuracy in volume definition and needle place- ment can be achieved. Implant procedure The patient is placed in the dorsal lithotomy position. 8. TECHNIQUE

a. MR image showing CTV 1 (entire prostate); CTV2 (peripheral zone) and CTV 3 (GTV boost)

CTV 1

CTV 2

CTV 3

b. CT image showing CTV 1 (entire prostate); CTV2 (peripheral zone) and CTV 3 (GTV boost)

Fig 21.3: CTV subvolumes for HDR brachytherapy

nated CTV2, CTV3 etc. as in the GEC ESTRO guidelines [17] and shown in Figure 21.3.

Organs at risk The critical organs, urethra and rectum, will also be outlined. The urethra will be defined by the visible intraurethral catheter which remains in situ throughout the procedure; when ultra- sound is used visibility may be enhanced using aerated sterile ultrasound gel. The natural shape and dimensions of the urethra