Moataz N. El-Ghamry

Tumor volume change with stereotactic body radiotherapy (SBRT) for early-stage lung cancer: evaluating the potential for adaptive SBRT.

Objectives:To quantify gross tumor volume (GTV) change during stereotactic body radiotherapy (SBRT) and on first follow-up, as well as to evaluate for any predictive prognostic risk factors related to GTV decrease. An attempt was also made to identify the potential timing for adaptive SBRT. Methods:Twenty-five tumors in 24 consecutive patients were treated with SBRT to total dose of 50 Gy in 5 fractions. Median age was 72.5 years. Tumor stage was T1, 68%; T2, 20%; and other, 12%. The GTVs of on the 5 cone-beam computed tomographies (CBCT1-5) obtained before each fraction and the first follow-up CT (CTPOST) were analyzed. Results:Median time from diagnosis to initiation of radiotherapy was 64 days. GTV on CBCT1 was the baseline for comparison. GTV decreased by a mean of 7% on CBCT2 (P=0.148), 11% on CBCT3 (P=0.364), 19% on CBCT4 (P=0.0021), and 32% on CBCT5 (P=0.0004). Univariate analyses of GTV shrinkage was significantly associated with “time from CBCT5 to CTPOST” (P=0.027) and “T-stage” (P=0.002). In multivariate analyses, “T-stage” remained significant with T1 tumors showing greater GTV shrinkage than T2 tumors. Conclusions:Significant decrease in GTV volume based on daily CBCT was demonstrated during SBRT treatment. Adaptive SBRT has the potential to minimize integral dose to the surrounding normal tissues without compromising GTV coverage.

Computed tomography–planned interstitial brachytherapy for locally advanced gynecologic cancer: Outcomes and dosimetric predictors of urinary toxicity

PURPOSE To identify dosimetric predictors of outcome and toxicity in patients receiving CT-planned interstitial brachytherapy (ISBT) for gynecologic cancers. METHODS AND MATERIALS Patients who received ISBT between 2009 and 2014 were reviewed. Demographic, disease specific, treatment, and toxicity data were collected. Logistic regression was used to model toxicity. A least absolute shrinkage and selection operator penalty was used to identify relevant predictors. Receiver operating characteristic curves were used to analyze the relation between dosimetric factors and urinary toxicity. RESULTS Seventy-three patients received ISBT (21 at time of cancer recurrence and 52 at the first presentation). Thirty-six patients had cervical cancer, 16 had vaginal cancer, 13 had uterine cancer, and 8 had vulvar cancer. ISBT was performed using both high-dose-rate and low-dose-rate 192Ir sources (27 low dose rate and 46 high dose rate). With a median followup of 12 months, Grade 3 vaginal, urinary, and rectal toxicity occurred in 17.8%, 15.1%, and 6.8% of patients, respectively. No patients experienced Grade 4 or 5 toxicity. Dose to 0.1cc of urethra predicted for development of Grade 3 urinary toxicity (area under the curve of 0.81; 95% confidence interval: 0.66, 0.96). A 10% probability of a Grade 3 urinary toxicity associated with a dose of 23.1 equivalent dose in 2 Gy fractions (95% confidence interval: 9.51, 36.27 equivalent dose in 2 Gy fractions). CONCLUSIONS ISBT is a safe treatment for gynecologic malignancies. The dose to 0.1cc significantly predicts for severe urinary toxicity. Our data suggests that dose to a small urethral volume may be the most significant predictor of urinary toxicity in patients receiving ISBT for gynecologic cancer.

SU-E-T-96: An Analysis of VMAT SBRT Lung Treatment Plans.

Purpose: To present an analysis of 218 lung sbrt treatment plans delivered using Varian Eclipse RapidArc software and delivered with Varian linear accelerators. Methods: A retrospective analysis of treatment plans generated using the Varian Eclipse PRO RapidArc VMAT optimization and AAA photon beam calculation algorithms was done for 218 plans delivered using Varian Trilogy and TrueBeam linear accelerators. Some of the patients were enrolled in the RTOG 0813 and RTOG 0915 lung protocols. All of the patients’ plans were optimized using the guidelines outlined by these protocols. The plans presented were normalized so that 95% of the PTV volume received 100% of the prescribed dose. Results: The dose co formality constraints from the RTOG protocols and the number of plans within those constraints were:Tumor dose between 60% of maximum dose and 90% of maximum: 218/218 = 100%.Volume of 105% tumor dose outside put less than 15% of the PTV volume: 218/218 = 100%.Maximum dose 2 cm away from PTV within protocol table guidelines: No minor deviation: 188/218 = 86.2%. No major deviation: 216/218 = 99.1%.Ratio of 50% dose volume to PTV volume less than R50 in protocol table: No minor deviation 148/218 = 67.9%. No major deviation: 213/218 = 97.7%.Percent of lung receiving 20 Gy less than 10% (no minor violation): 218/218 = 100%.99% of PTV receiving at least 90% of the tumor dose: 214/218 = 98.2%. Conclusion: Varian RapidArc VMAT can successfully deliver lung SBRT treatments as outlined in the RTOG 0813 and 0915 protocols. The largest deviations were seen in the R50 constraints for the larger PTV volumes.