Radiation and Cancer Physics

PD 13 - Physics 8 - Poster Discussion - Outcome Analysis and Response Imaging

1108 - Can Modeling of Acute Rectal Toxicity in IMRT Treatments for Prostate Cancer be Used for Quality Assurance?

Tuesday, October 23
4:45 PM - 4:51 PM
Location: Room 217 C/D

Can Modeling of Acute Rectal Toxicity in IMRT Treatments for Prostate Cancer be Used for Quality Assurance?
M. Fatyga1, S. E. Schild2, M. Schild3, S. A. Vora1, W. W. Wong1, M. G. Herman4, X. Liu5, and J. Li5; 1Mayo Clinic Arizona, Phoenix, AZ, 2Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, AZ, 3Duke University Medical Center, Durham, NC, 4Mayo Clinic, Rochester, MN, 5Arizona State University, Tempe, AZ

Purpose/Objective(s): Intensity Modulated Radiation Therapy (IMRT) allows significant dose reductions to organs at risk in prostate cancer patients. However, accurate delivery of IMRT plans can be compromised by patient positioning errors. The purpose of this study was to determine if modeling of grade>=2 acute rectal toxicity could be used to monitor the quality of patient positioning protocols in IMRT treatments for prostate cancer.

Materials/Methods: 79 patients treated with Image Guided IMRT with implanted fiducial markers (FMIGRT) and 302 patients treated with trans-abdominal ultrasound guided IMRT (USGRT) were selected for this study. Treatment plans were available for the FMIGRT group and hand recorded dosimetric indices were available for both groups. We fit LKB and Univariate Logistic Regression (ULR) models to planning dosimetry in the FMIGRT group and the ULR model to recorded indices in both groups. We used the ULR model which was obtained in the FMIGRT group to predict the expected incidence of acute rectal toxicity in the USGRT group, assuming that the toxicity was determined by the planning dosimetry alone. We compared predicted to observed incidence of toxicity in the USGRT group. We performed Receiver Operating Characteristics (ROC) analysis on all models and compared areas under the ROC curves (AUC). We modeled a hypothetical change in toxicity for a systematic posterior shift of the isocenter by 5mm in all FMIGRT patients.

Results: Incidence of grade >= 2 rectal toxicity was 20% in FMIGRT patients and 54% in USGRT patients. LKB model parameters in FMIGRT group were , TD_50 =56.8Gy, slope m=0.093, and exponent n=0.131. The most predictive indices in the ULR model for FMIGRT group were D_25% and v_50Gy. AUC for both models in the FMIGRT group was similar (AUC = 0.67), which strongly suggests that planning dosimetry was correlated with rectal toxicity in the FMIGRT group. Fits of the ULR model to dosimetric indices in USGRT patients did not yield predictive models (AUC = 0.5) in spite of higher patient numbers, which strongly suggests lack of correlation between planning dosimetry and rectal toxicity in the USGRT group. The predicted incidence of grade >= 2 acute rectal toxicity in the USGRT group was 27% on average, while the incidence of 54% was actually observed. A simulated posterior shift of the isocenter in FMIGRT patients by 5mm estimated doubling of the incidence of acute rectal toxicity, which means that our data is consistent with a systematic setup error in the USGRT patient cohort. Literature search revealed one published study which detected a systematic posterior shift of the isocenter in ultrasound-guided patient positioning.

Conclusion: A correlation between planning dosimetry and acute rectal toxicity can be measured in a relatively small patient cohort. Our study suggests that an unusually weak correlation between planning dosimetry and acute rectal toxicity should be concerning because it may be an indication of a systematic patient positioning error.

Author Disclosure: M. Fatyga: None. S.E. Schild: Research Grant; Alliance. Travel Expenses; Alliance. M. Schild: None. W.W. Wong: Research Grant; Arizona State University. M.G. Herman: None. X. Liu: None. J. Li: None.

Mirek Fatyga, PhD

Disclosure:
No relationships to disclose.

Presentation(s):

Send Email for Mirek Fatyga


Assets

1108 - Can Modeling of Acute Rectal Toxicity in IMRT Treatments for Prostate Cancer be Used for Quality Assurance?



Attendees who have favorited this

Please enter your access key

The asset you are trying to access is locked. Please enter your access key to unlock.

Send Email for Can Modeling of Acute Rectal Toxicity in IMRT Treatments for Prostate Cancer be Used for Quality Assurance?