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Prostate adenocarcinoma target delineation works only when IMRT is paired with exact contouring and daily image guidance. That is the central thread of this chapter: define the prostate, seminal vesicles, prostate bed, and pelvic nodal volumes slice by slice, then deliver treatment with a setup process tight enough to preserve tumor coverage without expanding toxicity.
After pathologic confirmation, the initial workup includes digital rectal examination, urinary and erectile function scores, and relevant laboratory studies, including PSA and any additional testing needed if androgen deprivation therapy is planned. At MSKCC, mpMRI is recommended for essentially all patients unless contraindicated because it helps identify undersampled high-grade disease, measure prostate volume, localize the dominant lesion, assess extra-prostatic extension and seminal vesicle invasion, and detect gross postoperative disease that may justify dose escalation. For the broader context across other anatomical sites, see the Target Volume Delineation and Field Setup – Complete Clinical Guide.
When external beam treatment is combined with a brachytherapy boost, urethral avoidance becomes even more consequential. The chapter reflects that by recommending a Foley catheter for selected simulations; if you want a separate dosimetric discussion around combined brachytherapy workflows, see our article on TSET and Brachytherapy: Monte Carlo in Practice.
Simulation, MRI, and daily preparation
The simulation workflow carries much of the geometric burden of treatment. The institutional preference described in the chapter is MR-only simulation and planning for definitive treatment and for postoperative gross local recurrence. When that is not available, the fallback is a 2 mm slice CT simulation fused with a 3 T MRI obtained in treatment position.
For definitive treatment, three fiducial markers are placed at the base, mid-gland, and apex at least five days before simulation, with optional rectal spacer placement when there is no posterior extra-prostatic extension. If the plan involves dose escalation for suspected gross postoperative local recurrence, fiducials can be placed at the time of biopsy. Rectal preparation is standardized: low-fat, low-residue diet, fiber supplementation, and simethicone start one week before simulation and continue through treatment, while an enema is given three hours before simulation and before treatment as needed.
Bladder filling is handled just as deliberately. The protocol uses 16 ounces of water 45 minutes before simulation and treatment. For urethral avoidance, a Foley catheter is used in SBRT simulations and when EBRT is combined with a brachytherapy boost, although the chapter also cites MR-based urethral delineation as described by Zakian and colleagues. Patients are positioned supine with pelvic and leg immobilization. SBRT setups should be validated, whether frame-based or frameless. At MSKCC, a thermoplastic mold extends from mid-abdomen to mid-thigh, conforms to the inner legs, and is paired with a knee cushion for additional stability.
Simulation borders extend from L2 to the mid-femur. Isocenter is placed at the prostate or prostate bed, or at the top of the femoral heads if pelvic nodes are treated. MRI-CT fusion is improved by matching the bladder-prostate interface, which is often shifted by pitch differences between scans, and by checking bony anatomy and fiducials on sequences such as T1 SPGR or 3D BFFE. That kind of registration discipline is what keeps a few millimeters from turning into a miss at the apex, posterior capsule, or seminal vesicle base.
Definitive CTV delineation
In the definitive setting, the prostate CTV includes the entire gland and covers the seminal vesicles according to invasion risk. Diagnostic mpMRI is not decorative here. It is used to make sure gross tumor and extra-prostatic extension are not left outside the clinical target volume.
The chapter points to axial T2 MRI as the most useful sequence for prostate definition. Contouring generally begins at the mid-gland, where the posterior capsule and seminal vesicles are best visualized. From there, the boundaries are described with practical anatomic landmarks: laterally the volume stays within the levator ani; anteriorly the key structure is the anterior fibromuscular stroma; posteriorly the contour follows the rectal interface or the rectal spacer in SBRT; inferiorly the apex should be identified relative to the “hourglass” appearance of the genitourinary diaphragm described by McLaughlin et al.; superiorly the reference is the bladder.
Seminal vesicle coverage also has to be disciplined. The text describes including 5-10 mm of the proximal seminal vesicles or the entire vesicles depending on invasion risk, and it explicitly recommends checking sagittal and coronal planes as well as a three-dimensional rendering of the structure. That 3D review has a diagnostic purpose. In Fig. 25.2, the expected appearance is a relatively globular gland with a wing-like structure superiorly representing the seminal vesicles. If the inferior pedestal becomes too long, the contour probably extends too far into the genitourinary diaphragm. If the shape becomes grossly irregular from slice to slice, the problem is usually overcorrection of organ deformation or motion rather than anatomy.
The caption of Fig. 25.1 adds one more practical detail: hydrogel spacer is best seen on T2 MRI, but iodinated contrast can facilitate spacer delineation even without MRI dependence. In a hybrid imaging workflow, that kind of detail can prevent a false posterior boundary between prostate and rectum.
Practical table of targets, doses, and margins
Table 25.1 is the operational center of the chapter. Instead of treating dose, fractionation, and margin as separate decisions, it shows how each regimen changes the PTV and the target that must actually be covered.
Table 25.1 – Suggested EBRT targets and contouring concepts
The chapter organizes the main scenarios into definitive treatment, postoperative treatment, and elective pelvic nodal coverage, always linking dose, margin, and anatomic target definition.
| Setting | Protocol | Fraction dose | MSKCC regimen | PTV margin | CTV description / practical notes |
|---|---|---|---|---|---|
| Definitive | Conventional | 180-200 cGy | 81-86.4 Gy in 45-48 fractions | 6 mm except 5 mm posteriorly | CTVpros = entire prostate with seminal vesicle coverage based on invasion risk. Diagnostic mpMRI is used to ensure inclusion of gross tumor and EPE. The chapter notes that this regimen is used rarely at MSKCC. |
| Definitive | Moderately hypofractionated | 240-300 cGy | 70.2 Gy in 26 fractions | 5 mm except 3 mm posteriorly | Same intact-prostate target logic, beginning at the mid-gland where the posterior capsule and seminal vesicles are most clearly defined. This is the default EBRT option at MSKCC when the patient is not a brachytherapy or SBRT candidate. If pelvic nodes are treated, the chapter directs the reader to the dedicated nodal volume below. |
| Definitive | Ultra-hypofractionated (SBRT/SABR) | >500 cGy | 40 Gy in 5 fractions delivered every other day | 5 mm except 3 mm posteriorly | Used for low- and intermediate-risk patients with good urinary function. The posterior limit follows the rectal interface or spacer; the inferior limit depends on accurate apex identification relative to the genitourinary diaphragm; the superior reference is the bladder; seminal vesicle coverage ranges from 5-10 mm proximally to full inclusion depending on risk. |
| Post-op | Conventional | 180 cGy | 72 Gy in 40 fractions, with boost to 78 Gy for gross disease | 6 mm except 5 mm posteriorly | Prostate fossa CTV within RTOG guidance. The inferior limit is about 10 mm below the vesicourethral anastomosis on the last slice containing urine. Anteriorly, the landmark is the pubic symphysis and the CTV should not extend into the penile bulb. Posteriorly, the volume follows the anterior rectal wall or tapers off the bladder over four slices. Laterally it uses the levator ani and obturator internus fascia. Superiorly it extends about 1-2 cm above the pubic symphysis to include seminal vesicle remnants without chasing every hemostatic clip. |
| Elective pelvic nodes | Conventional with SIB | 180 cGy with simultaneous boost to gross disease | Definitive: 45 Gy in 25 fractions with SIB to 56.25 Gy. Post-op: 46.8 Gy in 26 fractions with SIB to 57.2 Gy to nodal GTV. | 8 mm for elective pelvis; 5 mm on nodal GTV | Modified RTOG nodal volume. Target regions include common, external, and internal iliac chains, obturators, and pre-sacral nodes. Coverage starts at the aortic bifurcation. External iliacs end at the top of the femoral heads; internal iliacs and obturators extend to the superior pubic symphysis; pre-sacral coverage runs from the top of S1 to S2. The text also specifies no bowel or muscle in the CTV and recommends fusing diagnostic MRI or PET if gross nodal disease requires a boost. |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 25.1)
The authors are explicit that these regimens reflect current MSKCC practice rather than a universal prescription template. Dose selection should stay tied to schedules already validated for safety and efficacy at each institution. The same applies to margins: daily pre-treatment kV imaging is matched to fiducials in definitive treatment; in hypofractionated courses, daily kV and CBCT are used and intra-fraction motion is monitored or corrected during delivery; in the postoperative setting, at least weekly CBCT is used for soft-tissue evaluation.
Prostate bed after prostatectomy
After surgery, the target is no longer an organ. It is a risk space. That changes the contouring problem completely. The chapter uses Fig. 25.3 to show a caudal-to-cranial walkthrough that begins about 10 mm below the vesicourethral anastomosis on the last slice containing urine and proceeds upward on a 2 mm CT acquired with a full-bladder protocol.
The CTV should not extend into the penile bulb. Anteriorly, the pubic symphysis is the anchor. Posteriorly, the anterior rectal wall and the posterior bladder interface define a target that has to cover microscopic risk while avoiding unnecessary rectal overdose. That is why the figure shows manual PTV modification after geometric expansion: the goal is to prevent the cranial “dumbbell” drape from projecting too aggressively over the anterolateral rectum.
Fig. 25.4 explains why postoperative volumes need routine three-dimensional review. Unlike an intact prostate plan, a postoperative plan necessarily approaches the bladder and rectum to cover the anterior perirectal space, the vesicourethral anastomosis, and the new spaces formed between the posterior bladder wall, pelvic floor, and anastomosis. In this setting, PTV overlap with bladder and rectum is not automatically a contouring error. The real mistake is failing to taper the anterior boundary gradually above the pubic symphysis. If that transition is abrupt, the dose distribution becomes more vulnerable to errant targeting when bladder volume changes from day to day despite a full-bladder protocol.
Pelvic nodes, IG-IMRT, and 3D quality checks
When nodal coverage is indicated, the chapter recommends a modified RTOG elective pelvic nodal volume. It begins at the aortic bifurcation and includes common, external, and internal iliac regions, obturator chains, and the pre-sacral space. The inferior limit of the external iliacs is the top of the femoral heads; for the internal iliacs and obturators, the inferior landmark is the superior pubic symphysis; the pre-sacral strip runs from the top of S1 to S2. One practical instruction stands out because it prevents unnecessary geographic expansion: do not include bowel or muscle in the CTV.
If gross nodal disease is present, diagnostic MRI or PET should be fused to guide the GTV boost. Fig. 25.5 gives a concrete example: a patient with T1cN1M0 disease, Gleason score 4 + 4, and PSA 22 treated with moderately hypofractionated IG-IMRT, two years of Lupron and Abiraterone, and dose painting over 26 fractions. Elective pelvis received 4680 cGy, a gross right pelvic node received 5720 cGy, and prostate plus seminal vesicles received 7020 cGy.
That case captures the chapter’s overall logic. First, every CTV should be drawn on every planning CT or MRI slice. Second, PTV is not a fixed number; it depends on fractionation, image guidance, and institutional delivery standards. Third, quality review cannot stay limited to axial slices. The figures repeatedly push the reader toward combined axial, sagittal, coronal, and 3D inspection because the most consequential prostate contouring mistakes often look small on one slice and obvious once the target is viewed as a whole structure.
If there is one clinical takeaway from the chapter, it is this: tight margins are defensible only when preparation, daily guidance, and motion management are equally tight. For the rest of the anatomical sites covered in the same book, return to the complete guide.
