R Cardan

3T magnetic resonance imaging accurately depicts radiofrequency ablation zones in a blood-perfused bovine liver model.

PURPOSE To determine if noncontrast T1-weighted (T1W) images from 3T magnetic resonance (MR) imaging accurately depict radiofrequency (RF) ablation zones as determined macroscopically and microscopically in a blood-perfused bovine liver model. MATERIALS AND METHODS Three-dimensional (3D) gradient-recalled echo (GRE) T1W images were obtained on a 3T MR imaging scanner after RF ablations (n = 14) of in vitro blood-perfused bovine livers. The resulting central hypointense and peripheral hyperintense signal regions were measured and compared with the inner tan and outer red zones of the gross specimen. Corresponding ablated hepatic tissue samples were examined microscopically and stained with nicotinamide adenine dinucleotide phosphate (NADPH) to assess for the presence or absence of NADPH diaphorase activity. Bootstrap two-sample hypothesis tests were used to compare MR imaging, gross, and histopathologic measurements. RESULTS The MR imaging inner ablation zone had a mean radius of 0.80 cm (range 0.33-1.14 cm); the inner zone plus the outer ablation zone had a mean radius of 1.40 cm (range 1.01-1.74 cm). Comparison of the measurements of the inner ablation zone on MR imaging versus the gross specimen showed equivalence (95% confidence interval [CI] -0.122 cm, 0.223 cm). Comparison of the measurements of the outer ablation zone on MR imaging versus the gross and histologic specimens also showed equivalence (95% CI -0.095 cm, 0.244 cm, and -0.146 cm, 0.142 cm). CONCLUSIONS Noncontrast 3D GRE T1W 3T MR imaging accurately depicts the RF ablation zones in a blood-perfused bovine liver model and can be used as a noninvasive means to assess the 3D morphologic characteristics of RF ablation lesions in the model.

CT Measures of Bone Mineral Density and Muscle Mass Can Be Used to Predict Noncancer Death in Men with Prostate Cancer

Purpose To determine if computed tomographic (CT) metrics of bone mineral density and muscle mass can improve the prediction of noncancer death in men with localized prostate cancer. Materials and Methods Institutional review board approval was obtained, with waiver of informed consent. All patients who underwent radiation therapy for localized prostate cancer between 2001 and 2012 with height, weight, and past medical history documented and who underwent CT that included the L4-5 vertebral interspace were included. On a single axial CT section obtained at the mid-L5 level, the mean CT attenuation of the trabecular bone of the L5 vertebral body (L5HU) was measured. The height-normalized psoas cross-sectional area (PsoasL4-5) was measured on a single CT section obtained at the L4-5 vertebral interface. Multivariable Cox proportional hazards models were used to assess effects on noncancer death. By using parameter estimates from an adjusted model, a prognostic index for prediction of noncancer death was generated and compared with age-adjusted Charlson Comorbidity Index (CCI) by using the Harrell c statistic. Results Six hundred fifty-three men met the inclusion criteria. Prostate cancer risk grouping, androgen deprivation, race, age-adjusted CCI, L5HU, and PsoasL4-5 were included in a multivariable model. Age-adjusted CCI (hazard ratio [HR] = 1.36, P < .001), L5HU (HR = 2.88 for L5HU < 105 HU, HR = 1.42 for 105 HU ≤ L5HU ≤ 150 HU, P < .001), PsoasL4-5 (HR = 1.95 for PsoasL4-5 < 7.5 cm2/m2, P = .003), and race (HR = 1.68 for African American race, HR = 1.77 for other nonwhite race, P = .019) were independent predictors of noncancer death. The prognostic index yielded a c value of 0.747 for the prediction of noncancer death versus 0.718 for age-adjusted CCI alone. Conclusion L5HU and PsoasL4-5, which are surrogates for bone mineral density and muscle mass, respectively, were independent predictors of noncancer death. The prognostic index that incorporated these measures with the CCI was associated with improved accuracy for prediction of noncancer death.

Increased radiation dose heterogeneity within the prostate predisposes to urethral strictures in patients receiving moderately hypofractionated prostate radiation therapy.

PURPOSE The purpose of this study was to determine whether radiation dose inhomogeneity within the prostate predisposes to late urinary strictures after moderately hypofractionated definitive external beam radiation therapy for prostate cancer. METHODS AND MATERIALS One hundred seventy-three men with clinically localized prostate cancer met the inclusion criteria for this analysis. All patients received 70 Gy to the prostate delivered over 28 fractions, had at least 2 years of clinical follow-up, and had dose-volume histogram information available for review. The endpoint of this study was the development of a urethral stricture that required a procedural intervention such as urethral dilation or suprapubic catheterization. Dosimetric parameters were evaluated for effect on the rate of urethral stricture formation by univariate Cox proportional hazards modeling. RESULTS The median follow-up was 49.5 months (range, 24.6-108 months). At 5 years, the actuarial rate of intervention for urethral strictures across all patients was 4.9%. The maximum point dose within the prostate (P = .034, hazard ratio = 1.006) and the mean prostate dose (P = .039, hazard ratio = 1.004) were the only parameters predictive of urethral stricture formation. All patients who developed a urethral stricture were treated by a plan with a maximum prostate dose of >75 Gy (median, 77.67 Gy). CONCLUSIONS For patients receiving moderately hypofractionated prostate radiation therapy over 28 fractions, a maximum point dose of 75 Gy within the prostate was associated with an increased probability of developing a urethral stricture that required procedural intervention. The hypothesis that hypofractionation increases susceptibility to toxicity from heterogeneity within the prostate should be confirmed by analyzing data from randomized trials with a conventionally fractionated control arm for comparison.

A priori patient‐specific collision avoidance in radiotherapy using consumer grade depth cameras

Purpose In this study, we demonstrate and evaluate a low cost, fast, and accurate avoidance framework for radiotherapy treatments. Furthermore, we provide an implementation which is patient specific and can be implemented during the normal simulation process. Methods Four patients and a treatment unit were scanned with a set of consumer depth cameras to create a polygon mesh of each object. Using a fast polygon interference algorithm, the models were virtually collided to map out feasible treatment positions of the couch and gantry. The actual physical collision space was then mapped in the treatment room by moving the gantry and couch until a collision occurred with either the patient or hardware. The physical and virtual collision spaces were then compared to determine the accuracy of the system. To improve the collision predictions, a buffer geometry was added to the scanned gantry mesh and performance was assessed as a function of buffer thickness. Results Each patient was optically scanned during simulation in less than 1 min. The average time to virtually map the collision space for 64, 800 gantry/couch states was 5.40 ± 2.88 s. The system had an average raw accuracy and negative prediction rate (NPR) across all patients of 97.3% ± 2.4% and 96.9% ± 2.2% respectively. Using a polygon buffer of 6 cm over the gantry geometry, the NPR was raised to unity for all patients, signifying the detection of all collision events. However, the average accuracy fell from 95.3% ± 3.1% to 91.5% ± 3.6% between the 3 and 6 cm buffer as more false positives were detected. Conclusions We successfully demonstrated a fast and low cost framework which can map an entire collision space a priori for a given patient during the time of simulation. All collisions can be avoided using polygon interference, but a polygon buffer may be required to account for geometric uncertainties of scanned objects.

Combining Computed Tomography-Based Bone Density Assessment with FRAX Screening in Men with Prostate Cancer

To investigate the addition of a computed tomography (CT)-based method of osteoporosis screening to FRAX without bone mineral density (BMD) fracture risk assessment in men undergoing radiotherapy for prostate cancer, we reviewed the records of all patients with localized prostate cancer treated with external beam radiotherapy at our institution between 2001 and 2012. The 10-yr probability of hip fracture was calculated using the FRAX algorithm without BMD. The CT attenuation of the L5 trabecular bone (L5CT) was assessed by contouring the trabecular bone on a single CT slice at the level of the midvertebral body and by averaging the Hounsfield units (HU) of all included voxels. L5CT values of 105 and 130 HU were used as screening thresholds. The clinical characteristics of additional patients identified by each L5CT screening threshold value were compared to patients whose estimated 10-yr risk of hip fracture was 3% or greater by FRAX without BMD. A total of 609 patients treated between 2001 and 2012 had CT scans available for review and complete clinical information allowing for FRAX without BMD risk calculation. Seventy-four (12.2%) patients had an estimated 10-yr risk of hip fracture of 3% or greater. An additional 22 (3.6%) and 71 (11.6%) patients were identified by CT screening when thresholds L5CT = 105 HU and L5CT = 130 HU were used, respectively. Compared to the group of patients identified by FRAX without BMD, the additional patients identified by CT screening at each L5CT threshold level tended to be younger and heavier, and were more likely to be African-American or treated without androgen deprivation therapy. These results suggest that the addition of CT-based screening to FRAX without BMD risk assessment identifies additional men with different underlying clinical characteristics who may be at risk for osteoporosis and may benefit from pharmacological therapy to increase BMD and reduce fracture risk.

Subcutaneous adipose tissue characteristics and the risk of biochemical recurrence in men with high-risk prostate cancer

PURPOSE/OBJECTIVE(S) To assess subcutaneous adipose tissue characteristics by computed tomography (CT) as potential imaging biomarkers predictive of biochemical recurrence in men with high-risk prostate cancer receiving radiotherapy (RT). MATERIALS AND METHODS This retrospective study included men with high-risk prostate cancer (PSA>20ng/ml, Gleason score ≥8, or clinical extraprostatic extension) treated between 2001 and 2012. All patients received definitive, dose-escalated external beam RT along with a course of neoadjuvant, concurrent, and adjuvant androgen deprivation therapy (ADT). Each patient also had a treatment planning CT that included the L4-L5 vertebral interface and prostate specific antigen (PSA) measurements for at least 2 years following RT. The subcutaneous adipose tissue was contoured on a single axial CT slice at the level of L4-L5. The average CT attenuation, in Hounsfield units (HU), of the structure was calculated and defined as SATHU. SATAREA was defined as the cross-sectional area of the structure (in cm2) that was then normalized by the square of patient height. Biochemical failure (BF) was defined as a PSA rise of 2ng/ml from the nadir. Freedom from BF (FFBF) was calculated from start time of ADT using the Kaplan-Meier method. Estimates of FFBF were stratified by SATHU and SATAREA quartiles. RESULTS A total of 171 men met the inclusion criteria with a median follow-up of 5.6 years. The mean SATHU (±standard deviation) was -99.2HU (±6.1HU), and the mean SATAREA was 93.2cm2/m2 (±39.4cm2/m2). The 5- and 8-year rates of FFBF across all patients were 81.5% and 73.5%, respectively. Patients in the lowest quartile of SATHU experienced significantly higher FFBF compared to the other quartiles (Q4 vs. Q1, P = 0.017; Q4 vs. Q2, P = 0.045; Q4 vs. Q3, P = 0.044). No other differences in FFBF were observed between quartiles of SATAREA or other quartiles of SATHU. CONCLUSION Lower subcutaneous adipose tissue density was associated with a lower rate of BF following RT with ADT for men with high-risk prostate cancer. Further research is needed to elucidate the biological underpinnings of this clinical finding and the role adipose tissue plays in modulating oncologic behavior and outcomes.

SU‐E‐T‐227: Feasibility of Collision Avoidance Using In Room 3D Camera

Purpose: To determine the feasibility of detecting potential collisions between hardware and patient without the need of a CT scan. Methods: A 3D camera (Kinect for Windows, Microsoft) was mounted onto an on board imaging arm and 3 depth images at different angles were taken of an anthropomorphic phantom in a treatment position. A convex hull was then calculated from the depth vertices for use in a computer collision simulation. Positive collisions were then determined by manually moving the couch and gantry over a sample of their motion ranges. The results were compared with a computer simulation that calculated the positive collisions from the scanned geometry. Results: The simulation calculated 64,620 collision potential configurations of the phantom/couch and gantry at 1 degree increments in 4.36 sec using an Intel Core i3 processor with 8GB of ram. The accuracy of the calculation was assessed by dividing up the results into four different groups: true positive (correctly predicted collision), true negative (correctly predicted no collision), false positive (wrongly predicted collision), false negative (wrongly predicted no collision). There were 3949 measured data points, which resulted in TP = 1131, TN = 2437, FN = 275, and FP = 106 points. The receiver operating curve accuracy metric (TP+TN)/(TP+TN+TN+FN)) was then calculated from these groups as 90.35 %. Conclusion: We have successfully demonstrated a framework that could be used to scan patient geometry on the treatment table for use in collision avoidance. This could also be used in the simulation step of treatment to avoid planning geometrically infeasible setups. Collision prediction inaccuracies are attributed to inadequate point cloud data to accurately describe phantom geometry. Better sampling with improved reconstruction software should enable the ability to completely avoid planning of non‐deliverable beams or collisions during treatment.

SU‐E‐P‐14: A Practical Approach of Small Field Dosimetry Measurement for Patient‐Specific IMRT/SRS/SBRT

PURPOSE To design and implement a small-field IMRT/SRS/SBRT dosimetry measurement with a regular-size ion chamber and films. METHODS An acrylic phantom was constructed and commissioned to sandwich a Kodak EDR2 radiographic film. After a patient QA plan was delivered, the phantom was shifted superiorly by 10 cm and a reference plan was delivered on the same film. The reference plan has four-field-box beam geometry with 4 × 4 cm2 field size. Since the dose distribution of the reference plan was uniform and large, the absolute dose of the reference plan can be accurately measured separately with a regular-size ion chamber (diameter: 0.6 cm). After normalization, a two-dimensional absolute dose distribution can be obtained and compared to that calculated by Eclipse Treatment Planning System. An in-house software written in MATLAB was used to analyze films. Two-dimensional gamma indexes were calculated to evaluate patient QA plans. RESULTS Three patient-specific IMRT/SRS/SBRT QA plans were used to verify the feasibility of the method. The prescription dose of the reference plan was 3.5 Gy. The scatter dose from one plan to the other plan (which is 10 cm or more away) is very small (<1 cGy) and can be ignored. Compared with Eclipse calculation, the measured dose errors of the reference plan were -0.1-, -0.3%, and -0.8%, respectively. For the three patient-specific QA plans, the point dose measurement errors were 2.1%, 0.8% and 0.2%, respectively, and the γ>1 failure rates were 0.2%, 0.2%, and 0.1%, respectively. CONCLUSION Since the scatter dose from another plan 10 cm (or more) away was very small, this method allows one to measure the dose of any small target with a regular-size ion chamber and films without volume limitation.

MO‐D‐BRB‐06: JUNIOR INVESTIGATOR WINNER ‐ Fast and Accurate Patient Specific Collision Detection for Radiation Therapy

PURPOSE To develop a fast and generalizable method which can identify all possible hardware collisions specific to a given patient setup before treatment planning. METHODS An anthropomorphic phantom placed in a typical breast setup using a wingboard was simulated on a CT scanner and the phantom body contour, table, and gantry geometry were made into polygon meshes using 3D modeling software. In the treatment room, a limited physical search of the collision positive zones was performed using the positioned phantom. A software tool that incorporated a generalized hierarchical bounding box (HBB) collision detection algorithm was developed and used to virtually map out the entire collision space by transforming the positions of the polygonal geometry over a given parameter range. RESULTS The geometry containing 47K polygons was mapped over a space of 6480 states with an average transform/collision check of 5.5ms, for a total time of 35.6s on a 3.14GHz dual core computer with 4GB memory. The computed collision space, using receiver operating curve analysis had an accuracy of 96.35%, and a positive predictive value of 91.2%. CONCLUSIONS This work demonstrates a framework that can provide a fast and accurate map of the collision free space specific to any patient setup. Differences in physical and simulated collision space is attributed to inaccuracies of the geometrical models used. Future work includes improving the efficiency of the algorithm, enhancing the geometrical models and increasing the dimensions of the search.

Prostate Stereotactic Body Radiotherapy with a Focal Simultaneous Integrated Boost: Acute Toxicity and Dosimetry Results from a Prospective Trial

Purpose This study aimed to report the early toxicity results of a prospective clinical trial of prostate stereotactic body radiation therapy (SBRT) to the entire prostate with a simultaneous integrated boost (SIB) to magnetic resonance imaging (MRI)-defined focal lesions. Methods and materials Eligible patients included men with biopsy-proven prostate stage T1c to T2c adenocarcinoma, a Gleason score ≤7, and prostate-specific antigen values of ≤20 ng/mL, who had at least 1 focal lesion visible on MRI and a total prostate volume no greater than 120 cm3. SBRT consisted of a dose of 36.25 Gy to the entire prostate with an SIB of 40 Gy to the MRI-defined lesions, delivered in 5 fractions. The primary purpose of the study was to confirm the feasibility of treatment planning/delivery and to estimate the rate of urinary retention requiring placement of a Foley catheter within 90 days of treatment. This study was to be considered successful if urinary retention occurred in no more than 15% of cases, with a planned enrollment of at least 25 patients. Results A total of 26 men were enrolled, and all underwent SBRT as planned. Twenty patients (77%) had intermediate-risk features, and the remainder were low risk. A treatment plan that met the protocol-defined goals for all cases was developed. Two patients (7.7%) developed acute urinary symptoms that required the temporary placement of a Foley catheter. No grade 3+ toxicity events were observed. Conclusions Planning and delivery of prostate SBRT with a whole prostate dose of 36.25 Gy and a focal 40 Gy SIB is feasible. Early follow-up suggests that this treatment is not associated with undue morbidity.