Background and purpose: To delineate the clinical target volume (CTV) in low dose rate (LDR) brachytherapy for prostate cancer, American Brachytherapy Society (ABS) recommends a CTV = prostate. ESTRO advocates a CTV = prostate + 3 mm excluding rectum and many authors use and recommend other different CTVs. This study aims to: (1) evaluate the appropriateness of these recommendations and (2) test the applicability of seed distributions on the different CTVs and contrast the dosimetric differences.
Materials and methods: Ninety-eight patients treated with (125)I seeds (dose 145 Gy; CTV = prostate) were studied. We established for every patient: (1) risk of extraprostatic extension (EPE), (2) adequacy of original plan to an extended CTV with 3 mm-margin (3) a new planning and seed distribution for this CTV and (4) comparison of dosimetry of both plans.
Results: Mean risk of EPE was 28.46 %. Original plan, when applied to the extended CTV, resulted in unsatisfactory dosimetry. A plan was generated for the 98 extended CTVs meeting all dosimetric specifications.
Conclusions: The risk of EPE is high enough to consider a 3 mm-margin around prostate necessary for all cases. A CTV = prostate + 3 mm except rectum as ESTRO recommends is feasible and would adjust planning to the most probable extension of the tumor.