CTV to PTV margins for prostate irradiation. Three-dimensional quantitative assessment of interfraction uncertainties using portal imaging and serial CT scans.
OBJECTIVE
To determine the magnitude of setup and organ motion errors from a subset of prostate cancer patients treated with conventional conformal radiotherapy, and to estimate the CTV-PTV margin according to published margin recipes.


METHODS
Twenty prostate cancer patients were treated with external radiotherapy using electronic portal images (EPIs). Weekly treatment EPIs and pelvic CT scans were obtained. These data allowed interfractional analysis of prostate centre of mass motion and setup error. The margins needed to compensate these uncertainties were calculated.


RESULTS
Tattoo localisation requires a margin of 9-10.5 mm (LR), 15.2-17.8 mm (anterior-posterior (AP)) and 10.6-12.4 mm (superior-inferior (S-I)). Systematic displacements due to prostatic motion, with standard deviations of 2.4 mm (LR), 4.2 mm (AP) and 3.1 mm (S-I) were found to be larger than setup errors (1.8, 3.0 and 1.7 mm respectively).


CONCLUSIONS
Customised PTV margin definition has been possible through in-house measurements of geometrical clinical uncertainties involved in the conventional conformal radiotherapy process. Uncertainty measurements in our department have proved to be larger than those used in common practice. Additional margin reduction procedures are needed in order to accomplish conformal radiotherapy goals.
Radiation Oncology, Hospital General de Ciudad Real, Ciudad Real, Spain. llpr06@telefonica.net
Luis A
LA
Pérez-Romasanta
Eva
E
Lozano-Martín
Joaquín
J
Velasco-Jiménez
Fermín
F
Mendicote-León
Miguel
M
Sanz-Martín
Javier
J
Torres-Donaire
Carmen
C
Carrascosa-Fernández
Juan Carlos
JC
Zapata-Jimínez
Jacinto
J
Arjona-Gutiérrez
Antonio
A
Gil-Agudo
D002277:Carcinoma / Q000473:pathology / Q000532:radiotherapy*; D015331:Cohort Studies; D003952:Diagnostic Imaging / Q000379:methods; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008297:Male; D019300:Medical Errors / Q000706:statistics & numerical data; D009038:Motion; D009929:Organ Size; D010388:Pelvis / Q000473:pathology; D011467:Prostate / Q000473:pathology* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D020266:Radiotherapy, Conformal / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D047368:Tumor Burden; D035501:Uncertainty