Definition of the CTV Prostate in CT and MRI by Using CT–MRI Image Fusion in IMRT Planning for Prostate Cancer

Purpose:To determine the prostate volumes defined by using MRI and CT scans, as well as the difference between prostate delineation in MRI and CT in three dimensions (3D). A further goal was to use MRI to identify subgroups of patients in whom seminal vesicle irradiation can be avoided.Methods and Materials:A total of 294 patients with biopsy-proven prostate cancer (MRI stages: T1, 16 [5%]; T2, 84 [29%]; T3, 191 [65%]; T4, 3 [1%]) underwent pelvic CT and MRI scans before intensity-modulated radiation therapy (IMRT) planning. 3D images were used to compare the prostate volumes defined by superimposed MR and CT images. Prostate volumes were calculated in cm3.Results:The mean prostate volume defined by MRI (44.3 cm3 [range, 8.8–182.8 cm3]) was 35% smaller than that defined by CT (68.5 cm3 [range, 15.2–241.3 cm3]). The areas of nonagreement were observed predominantly in the most superior and inferior portions of the prostate. The incidence of seminal vesicle invasion (SVI) identified by MRI was 63% (n = 182 of 290). The median length of SVI was 2.6 cm (range, 1.1–4.7 cm; 62% of the median SV length). The low-risk patients (59%, n = 171 of 290) calculated by applying the Roach and Diaz formula had a SVI rate of 57% (n = 97 of 171), the high-risk patients (41%, n = 119 of 290) of 71% (n = 85 of 119).Conclusions:Compared with MRI, CT scans overestimate prostate volume by 35%. CT–MRI image fusion-based treatment planning allows more accurate prediction of the correct staging and more precise target volume identification in prostate cancer patients.ZusammenfassungZiel:Ziel war es, die Vorteile der CT/MR- Bildfusion für die IMRT-Planung des Prostatakarzinoms zu prüfen sowie die Inzidenz des MR-tomografisch nachweisbaren Samenblasenbefalls (SVI) zu ermitteln.Methodik:Von 294 Patienten mit histologisch gesichertem Prostatakarzinom (MRT-Tumorstadium: T1,, 16 [5 %]; T2, 84 [29 %]; T3, 191[65 %]; T4, 3 [1 %]), die vor IMRT-Planung eine Prostata-CT und MRT erhielten, wurden die nach Bildfusion in MRT und CT konturierten Prostatavolumina berechnet und verglichen.Ergebnisse:Das mittlere Prostatavolumen war im MRT (44,3 cm3 [8,8–182,8 cm3]) um 35 % kleiner gegenüber dem ermittelten Volumen im Planungs-CT (68,5 cm3 [15,2–241,3 cm3]). Eine Diskrepanz der Zielvolumina für das CTV Prostata bestand insbesondere im Bereich von Basis und Apex prostatae. Die MR-tomografische SVI-Häufigkeit betrug 63 %, die mittlere SVI-Länge 2,6 cm (1,1–4,7 cm; 62 % der SV Länge). Das SVI-Risiko betrug 57 % (97/171) für die entsprechend der Roach-Diaz-Formel ermittelte Niedrigrisikogruppe (59 % [171/290]) und 71 % (85/119) für die Hochrisikopatienten (41 % [119/290]).Schlussfolgerung:Unsere Ergebnisse zeigen, dass durch die MRT-Integration in die IMRT-Bestrahlungsplanung die Definition des CTV Prostata präzisiert werden kann und dass eine MR-basierte Bestrahlungsplanung eine genaue Definition des Tumorstadiums ermöglicht, da sie die exakte Beurteilung einer Tumorinfiltration der Vesiculae seminales erlaubt.

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