On the necessity of specialized knowledge-based models for SBRT prostate treatments plans.

PURPOSE: Test whether a well-grounded KBP model trained on moderately hypo-fractionated prostate treatments can be used to satisfactorily drive the optimization of SBRT prostate treatments. MATERIALS AND METHODS: A KBP model (SBRT-model) was developed, trained and validated using the first forty-seven clinically treated VMAT SBRT prostate plans (42.7 Gy/7fx or 36.25 Gy/5fx). The performance and robustness of this model were compared against a high-quality KBP-model (ST-model) that was already clinically adopted for hypo-fractionated (70 Gy/28fx and 60 Gy/20fx) prostate treatments. The two models were compared in terms of their predictions robustness, and the quality of their outcomes were evaluated against a set of reference clinical SBRT plans. Plan quality was assessed using DVH metrics, blinded clinical ranking, and a dedicated Plan Quality Metric algorithm. RESULTS: The plan libraries of the two models were found to share a high degree of anatomical similarity. The overall quality (APQM%) of the plans obtained both with the ST- and SBRT-models was compatible with that of the original clinical plans, namely (93.7 ± 4.1)% and (91.6 ± 3.9)% vs (92.8.9 ± 3.6)%. Plans obtained with the ST-model showed significantly higher target coverage (PTV V95%): (97.9 ± 0.8)% vs (97.1 ± 0.9)% (p < 0.05). Conversely, plans optimized following the SBRT-model showed a small but not-clinically relevant increase in OAR sparing. ST-model generally provided more reliable predictions than SBRT-model. Two radiation oncologists judged as equivalent the plans based on the KBP prediction, which was also judged better that reference clinical plans. CONCLUSION: A KBP model trained on moderately fractionated prostate treatment plans provided optimal SBRT prostate plans, with similar or larger plan quality than an embryonic SBRT-model based on a limited number of cases.

Effectiveness of body size stratification for patient exposure optimization in Computed Tomography.

PURPOSE: To establish size-dependent DRL and to estimate the effectiveness of the size-dependent DRLs over size-independent DRLs for a CT exposure optimization process. METHODS: The study included 16,933 adult CT body examinations of the most common CT protocols. Acquisitions were included following an image quality assessment. Patients were grouped into five different classes by means of the water equivalent diameter (Dw): 21 ≤ Dw < 25, 25 ≤ Dw < 29, 29 ≤ Dw < 33,33 ≤ Dw < 37 (in cm). CTDIvol, DLP, DLPtot. and SSDE median values were provided both for the sample as a whole (size-independent approach) and for each Dw class (size-dependent approach). The performance of the two approaches in classifying sub-optimal examinations was evaluated through the confusion matrix and Matthews Correlation Coefficient (MCC) metric. The 75th percentile of the CTDIvol distribution was arbitrarily chosen as a threshold level above which the acquisitions are considered sub-optimal. RESULTS: CTDIvol, DLP, DLPtot and SSDE typical values (median values) are statistically different across Dw groups. The confusion matrix analysis suggests that size-independent DRL could not mark potential suboptimal protocols for small and large patients. The agreement between the size-dependent and size-independent methods is strong only for the most populous classes (MCC > 0.7). For small and large patients size-independent approach fails to identify as sub-optimal around 20 % of the acquisition (MCC≪0.2). CONCLUSIONS: It was proven by means of the confusion matrix and MCC metric that stratifying DRLs by patient size, size-dependent DRL can be a powerful strategy in order to improve the dose optimization process shown that a size-independent DRL fails to identify sub-optimal examinations for small and large patients.

Size-specific dose estimates for pediatric head CT protocols based on the AAPM report TG-293.

PURPOSE: To establish the Size Specific Dose Estimate (SSDE) typical values for pediatric head CT examinations based on the AAPM report TG-293; to provide a new stratification based on the water-equivalent diameter (Dw), given that SSDE is related to the head size. METHODS: 296 Head CT scans of pediatric patients collected using a dose monitoring software were retrospectively analysed. Typical values were derived stratifying data by age in three methods: the first proposed by the European Guidelines on Diagnostic Reference Levels for Pediatric Imaging (RP185), the second by the National Istisan Report 20/22 and a local one related to the clinical protocols (LStrata). For each scan, a self-developed Matlab routine calculated the water-equivalent diameter (Dw) and related SSDE values with the conversion factors fH16and fB16provided by the AAPM reports TG-293 and TG-204, respectively. Eventually, a Dwstratification was introduced starting from a measure of the lateral dimension of the head. RESULTS: SSDE based on TG-204 overestimatesthe dose up to 12%. Four Dwgroups were identified thanks to the good correlation between the head lateral dimension andDw: Dw < 14 cm, 14 ≤ Dw < 16 cm, 16 ≤ Dw< 17 cm, Dw≥ 17 cm. The Dw-stratified dosimetric indices presentgreater variability than those grouped by age because of the large variability of the size of the infant's head. CONCLUSIONS: The variability of the SSDE metric underlines that age-optimized protocols are not when size is considered.

Radiological aspects of CO2 peripheral DSA: Preliminary analysis on the dedicated protocols.

OBJECTIVES: Thanks to its lack of allergic reactions and renal toxicity, CO2 represents an alternative to iodine as a contrast medium for peripheral subtraction angiography. Since CO2 has a lower and negative contrast than iodine, postprocessing DSA and stacking are mandatory. So, it seems that higher doses than traditional iodine angiography are required. We addressed the dosimetric aspects of CO2 angiography for two different commercial DSA-apparatus. MATERIALS AND METHODS: Two different radiological suites were analyzed by recreating the same setup on all the apparatuses: we used a PMMA slabs phantom with a MPD Barracuda dosimeter on its side to collect all radiological parameters. RESULTS: Results show that the irradiation parameters were left completely unchanged between the traditional and CO2 angiographic programs. CONCLUSIONS: This leads to thinking that these CO2 protocols do not operate on the X-ray emission, but only differ on image manipulation. The possibility of improvements by changing radiological parameters are still not explored and really promising.

Automated CO2 angiography: Injection pressure and volume settings.

The purpose of this work was to outline some practical rules for pressure and volume settings in automatic CO2 injection angiographic procedures focusing on the iliac arterial system, since, in current clinical practice, each operator uses his personal experience to obtain imaging results which are not always easy to compare. A theoretical model was thus developed and then verified by a mechanical simulator of the aortoiliac vascular system, with constant and pulsatile blood flow. The conditions of forward and reverse flows have been described, both for constant and pulsatile regimens and pressures, flows, and optical images of the bubbles in glass vessels were simultaneously acquired, analyzed and compared. Our results demonstrated that "good" radiological images (adequate to patient's conditions and clinical need) are strictly related to appropriate settings of gas injection pressure and flow, in accordance with two simple operative rules. These rules prescribe that the patient's pressure, the blood flow in the vessel, and the hydraulic resistance of the gas injection line be known: the first two parameters may be estimated, while the third must be experimentally measured. By following these rules, it is possible to obtain the best results for each clinical setting, a more standardized approach and better imaging during angiographic procedures with carbon dioxide as contrast medium.