Choice of optimal margins in prostate conformal radiotherapy

The planning CT allows the delineation of the prostate (clinical target volume, CTV) but with an imprecision at the apex and the inability to visualize the intraprostatic cancer. MRI enables accurate visualization of the prostate outlines and in some extent the intraprostatic tumour (gross tumour volume [GTV]). The integration of MRI data within the CT remains still complex. Analysis of prostatectomy specimen has guided the definition of a CTV beyond the capsule, depending on pretreatment factors. In practice, the CTV can be defined as follows: prostate-only, for good prognosis tumours; prostate (+0 to 5mm margins, excluding the rectum) and seminal vesicles (possibly limited to 2 cm of their proximal region) for intermediate-risk tumours; prostate (+5mm margins) and seminal vesicles for high-risk tumours. The planning target volume (PTV) should be between 5mm and 10mm depending on the space directions, in the absence of image-guidance (IGRT). It could be reduced to 5mm in case of IGRT. In the adjuvant setting after prostatectomy, the definition of the CTV should follow the recommendations from cooperative groups. It takes into account both the analysis of pattern of local recurrence after prostatectomy, but also the specific histological analysis of the surgical specimen of the patient. The corresponding PTV margin is 6 to 8mm.