Prostate Cancer Contouring Guidelines for Radiation Therapy
Essential Imaging and Planning Requirements
Clinicians must utilize CT-based treatment planning with MRI fusion for optimal prostate contouring, as this combination significantly reduces inter-observer variation and improves target delineation accuracy compared to CT alone. 1, 2
Imaging Modalities and Simulation
CT simulation is mandatory for treatment planning, providing the electron density data necessary for dose calculation and serving as the primary dataset for contouring 3
MRI fusion to CT is strongly recommended using intraprostatic fiducial markers for co-registration rather than bony landmarks, as this technique reduces inter-observer contouring variation (mean SEV/SCV ratio of 1.37 for MRI vs 1.58 for CT, p=0.036) 2, 4
Multiparametric MRI (combining T2-weighted imaging, diffusion-weighted imaging, and dynamic contrast-enhanced imaging) provides superior visualization of the prostate capsule, extraprostatic extension, and seminal vesicle invasion compared to CT 1, 4
Target Volume Delineation
The clinical target volume (CTV) should include the entire prostate gland with precise attention to common contouring errors at the apex, mid-gland, and base. 1, 5
Specific Anatomic Considerations by Region:
At the Prostatic Apex:
- The most common error is overestimation due to inclusion of the genitourinary diaphragm, rectum, and anterior fascia 5
- The posterior apical border should be defined 3.6 mm more posteriorly on MRI than typically contoured on CT to avoid geographic miss 2
- Careful identification of the levator ani muscles is essential to avoid including these structures 1
At the Mid-Gland:
- Overestimation commonly occurs due to inclusion of anterior and lateral fasciae 5
- The prostate should maintain a globular form when viewed in lateral projection; deviation from this suggests contouring error 5
At the Prostatic Base:
- Anterior overestimation occurs from inclusion of bladder wall and anterior fascia 5
- Transition zone hypertrophy and bladder neck variability contribute to both over- and underestimation at the superior base 5
- Variable prostate-to-seminal vesicle relationships require careful attention, particularly with prostate hypertrophy 5
Seminal Vesicle Contouring
For low-risk disease: Seminal vesicles typically do not require inclusion in the CTV 3
For intermediate-risk disease: Consider including the proximal 1-2 cm of seminal vesicles, particularly with Gleason 4+3 or other adverse features 6
For high-risk disease: Include seminal vesicles in the treatment volume, recognizing that seminal vesicle volumes can vary by up to 100% during treatment 7
Planning Target Volume (PTV) Margins
Dose escalation to 78-79 Gy using 3-D conformal radiation technique requires margins of no more than 10 mm at the prostatic-rectal interface. 3
Image-guided radiation therapy (IGRT) is mandatory for doses ≥78 Gy to enable margin reduction and improve treatment accuracy 3
Acceptable IGRT techniques include: daily cone-beam CT, ultrasound localization, implanted fiducial markers with orthogonal imaging, or electromagnetic tracking 3
Prostate motion during treatment averages <1 mm left-right, but ranges 0 to ±1 cm in anterior-posterior and superior-inferior directions, necessitating daily localization 7
Organs at Risk (OAR) Delineation
The rectum must be carefully contoured to optimize the therapeutic ratio, as it is the primary dose-limiting structure. 1
Rectal contouring should extend from the anorectal junction to the rectosigmoid flexure 1
Bladder contouring should include the entire organ, recognizing that bladder volumes vary by ±30% during treatment 7
Consider rectal spacer placement during simulation to increase the distance between prostate and rectum, particularly for dose-escalated treatments 3
Technical Planning Requirements
Modern radiation therapy must employ intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) as the minimum standard. 3
3D conformal RT using at least four fields is the minimum acceptable technique, though IMRT/VMAT is now considered state-of-the-art and required 3
High-energy photon beams (≥6 MV) should be used 3
Multileaf collimator technology is essential for conformal dose delivery 3
Quality Assurance
Post-treatment dosimetry verification is essential to document adequate target coverage. 3
For brachytherapy, an excellent implant is defined as ≥90% of prostate volume receiving ≥100% of prescription dose 3
For EBRT, verify that dose constraints for rectum and bladder are met according to published normal tissue dose objectives 3
Common Pitfalls to Avoid
Do not rely on bony anatomy alone for CT-MRI fusion; use intraprostatic fiducial markers for superior accuracy 2
Inspect lateral view of contours to detect deviation from expected globular prostate form, which indicates likely contouring error 5
Recognize that bladder and rectal filling significantly affects prostate position; implement consistent bladder/rectum filling protocols during simulation and treatment 7
Account for organ motion by using daily IGRT rather than relying solely on initial setup, as prostate position varies by up to 1 cm during treatment course 7
Avoid including genitourinary diaphragm at the apex, which is the most common source of overestimation 5