A predictive model to guide management of the overlap region between target volume and organs at risk in prostate cancer volumetric modulated arc therapy.
OBJECTIVE
The goal of this study is to determine whether the magnitude of overlap between planning target volume (PTV) and rectum (Rectumoverlap) or PTV and bladder (Bladderoverlap) in prostate cancer volumetric-modulated arc therapy (VMAT) is predictive of the dose-volume relationships achieved after optimization, and to identify predictive equations and cutoff values using these overlap volumes beyond which the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) dose-volume constraints are unlikely to be met.


METHODS
Fifty-seven patients with prostate cancer underwent VMAT planning using identical optimization conditions and normalization. The PTV (for the 50.4 Gy primary plan and 30.6 Gy boost plan) included 5 to 10 mm margins around the prostate and seminal vesicles. Pearson correlations, linear regression analyses, and receiver operating characteristic (ROC) curves were used to correlate the percentage overlap with dose-volume parameters.


RESULTS
The percentage Rectumoverlap and Bladderoverlap correlated with sparing of that organ but minimally impacted other dose-volume parameters, predicted the primary plan rectum V45 and bladder V50 with R(2) = 0.78 and R(2) = 0.83, respectively, and predicted the boost plan rectum V30 and bladder V30 with R(2) = 0.53 and R(2) = 0.81, respectively. The optimal cutoff value of boost Rectumoverlap to predict rectum V75 >15% was 3.5% (sensitivity 100%, specificity 94%, p < 0.01), and the optimal cutoff value of boost Bladderoverlap to predict bladder V80 >10% was 5.0% (sensitivity 83%, specificity 100%, p < 0.01).


CONCLUSIONS
The degree of overlap between PTV and bladder or rectum can be used to accurately guide physicians on the use of interventions to limit the extent of the overlap region prior to optimization.
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
Malcolm D
MD
Mattes
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
Jennifer C
JC
Lee
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
Sara
S
Elnaiem
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
Adel
A
Guirguis
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
N C
NC
Ikoro
Department of Radiation Oncology, New York Methodist Hospital, Brooklyn, NY, USA.
Hani
H
Ashamalla
Computer assisted radiotherapy planning; Intensity-modulated radiotherapy; Organs at risk; Prostate cancers; Radiation injuries