Sandra Sarmento

Preliminary assessment of the dose to the interventional radiologist in fluoro-CT-guided procedures.

A preliminary assessment of the occupational dose to the intervention radiologist received in fluoroscopy computerised tomography (CT) used to guide the collection of lung and bone biopsies is presented. The main aim of this work was to evaluate the capability of the reading system as well as of the available whole-body (WB) and extremity dosemeters used in routine monthly monitoring periods to measure per procedure dose values. The intervention radiologist was allocated 10 WB detectors (LiF: Mg, Ti, TLD-100) placed at chest and abdomen levels above and below the lead apron, and at both right and left arms, knees and feet. A special glove was developed with casings for the insertion of 11 extremity detectors (LiF:Mg, Cu, P, TLD-100H) for the identification of the most highly exposed fingers. The H(p)(10) dose values received above the lead apron (ranged 0.20-0.02 mSv) depend mainly on the duration of the examination and on the placement of physician relative to the beam, while values below the apron are relatively low. The left arm seems to receive a higher dose value. H(p)(0.07) values to the hand (ranged 36.30-0.06 mSv) show that the index, middle and ring fingers are the most highly exposed. In this study, the wrist dose was negligible compared with the finger dose. These results are preliminary and further studies are needed to better characterise the dose assessment in CT fluoroscopy.

Assessment of clinically relevant dose distributions in pelvic IOERT using Gafchromic EBT3 films

PURPOSE In IOERT a single dose of radiation is delivered to the tumour site during surgery. Manual dose calculations are used and the irradiation target volume, electron energy and applicator are decided on site by the radiation oncologist. This work assesses the effect that irregular and curved surfaces, typical of pelvic IOERT, may have on the expected dose distribution. METHODS The feasibility of using Gafchromic EBT3 films and a slab phantom to obtain 2D dose distributions was investigated. Different set-ups were tested by comparison with water tank measurements, applying the gamma function analysis with 2% and 2 mm criteria. The validated set-up was then used to obtain reference dose distributions, which were converted to colour-coded graphical representations. Phantoms with step-like and curved surfaces were created to simulate typical pelvic IOERT irradiation surfaces, and the dose distributions were obtained and compared with the reference distributions. RESULTS Good agreement with water tank measurements was obtained for all applicators below 2 mm, using the chosen setup in reference conditions. In non-reference conditions, the presence of a step-like surface creates an adjacent hotspot, followed by a quick reduction of the dose in depth. With curved surfaces, the dose distribution is shifted forward, becoming curved and deeper, but when the applicator is larger than the hole, hotspots are also observed. CONCLUSIONS The shape of the irradiation surfaces alters the dose distribution. Visualization of these effects is important to assess target coverage and interpret in vivo measurements in pelvic IOERT.

Feasibility of setting up generic alert levels for maximum skin dose in fluoroscopically guided procedures

PURPOSE The feasibility of setting-up generic, hospital-independent dose alert levels to initiate vigilance on possible skin injuries in interventional procedures was studied for three high-dose procedures (chemoembolization (TACE) of the liver, neuro-embolization (NE) and percutaneous coronary intervention (PCI)) in 9 European countries. METHODS Gafchromic® films and thermoluminescent dosimeters (TLD) were used to determine the Maximum Skin Dose (MSD). Correlation of the online dose indicators (fluoroscopy time, kerma- or dose-area product (KAP or DAP) and cumulative air kerma at interventional reference point (Ka,r)) with MSD was evaluated and used to establish the alert levels corresponding to a MSD of 2 Gy and 5 Gy. The uncertainties of alert levels in terms of DAP and Ka,r, and uncertainty of MSD were calculated. RESULTS About 20-30% of all MSD values exceeded 2 Gy while only 2-6% exceeded 5 Gy. The correlations suggest that both DAP and Ka,r can be used as a dose indicator for alert levels (Pearson correlation coefficient p mostly >0.8), while fluoroscopy time is not suitable (p mostly <0.6). Generic alert levels based on DAP (Gy cm2) were suggested for MSD of both 2 Gy and 5 Gy (for 5 Gy: TACE 750, PCI 250 and NE 400). The suggested levels are close to the lowest values published in several other studies. The uncertainty of the MSD was estimated to be around 10-15% and of hospital-specific skin dose alert levels about 20-30% (with coverage factor k = 1). CONCLUSIONS The generic alert levels are feasible for some cases but should be used with caution, only as the first approximation, while hospital-specific alert levels are preferred as the final approach.

Effects of shielding on pelvic and abdominal IORT dose distributions

PURPOSE To study the impact of shielding elements in the proximity of Intra-Operative Radiation Therapy (IORT) irradiation fields, and to generate graphical and quantitative information to assist radiation oncologists in the design of optimal shielding during pelvic and abdominal IORT. METHOD An IORT system was modeled with BEAMnrc and EGS++ Monte Carlo codes. The model was validated in reference conditions by gamma index analysis against an experimental data set of different beam energies, applicator diameters, and bevel angles. The reliability of the IORT model was further tested considering shielding layers inserted in the radiation beam. Further simulations were performed introducing a bone-like layer embedded in the water phantom. The dose distributions were calculated as 3D dose maps. RESULTS The analysis of the resulting 2D dose maps parallel to the clinical axis shows that the bevel angle of the applicator and its position relative to the shielding have a major influence on the dose distribution. When insufficient shielding is used, a hotspot nearby the shield appears near the surface. At greater depths, lateral scatter limits the dose reduction attainable with shielding, although the presence of bone-like structures in the phantom reduces the impact of this effect. CONCLUSIONS Dose distributions in shielded IORT procedures are affected by distinct contributions when considering the regions near the shielding and deeper in tissue: insufficient shielding may lead to residual dose and hotspots, and the scattering effects may enlarge the beam in depth. These effects must be carefully considered when planning an IORT treatment with shielding.

Gafchromic XR-QA2 film as a complementary dosimeter for hand-monitoring in CTF-guided biopsies

Computed tomography fluoroscopy (CTF) is a useful imaging technique to guide biopsies, particularly lung biopsies, but it also has the potential for very high hand exposures, despite use of quick-check method and needle holders whenever feasible. Therefore, reliable monitoring is crucial to ensure the safe use of CTF. This is a challenge, because ring dosimeters monitor exposure only at the base of one finger, while the fingertips may be exposed to the highly collimated CT beam. In this work we have explored the possibility of using Gafchromic XR-QA2 self-developing film as a complementary dosimeter to quantify hand exposure during CTF-guided biopsies. A glove used in a previous study and designed to contain 11 TLDs was adapted to include Gafchromic strips 7 mm wide, covering the fingers. A total of 22 biopsies were successfully performed wearing this GafTLD glove under sterile gloves, and the IR reported no difficulty or reduction of dexterity while wearing it. Comparison of dose distributions obtained from digitization of the Gafchromic film strips and absolute Hp(0.07) readings from TLDs showed good agreement, despite some positional uncertainty due to relative movement. Per procedure, doses at the base of the ring finger can be as low as 3%-8% of hand dose maximum. Accumulated dose at the base of the ring finger was four times lower than the dose maximum. PACS numbers: 07.57.Kp, 29.40.-n, 85.25.Pb, 87.57.qp.Computed tomography fluoroscopy (CTF) is a useful imaging technique to guide biopsies, particularly lung biopsies, but it also has the potential for very high hand exposures, despite use of quick‐check method and needle holders whenever feasible. Therefore, reliable monitoring is crucial to ensure the safe use of CTF. This is a challenge, because ring dosimeters monitor exposure only at the base of one finger, while the fingertips may be exposed to the highly collimated CT beam. In this work we have explored the possibility of using Gafchromic XR‐QA2 self‐developing film as a complementary dosimeter to quantify hand exposure during CTF‐guided biopsies. A glove used in a previous study and designed to contain 11 TLDs was adapted to include Gafchromic strips 7 mm wide, covering the fingers. A total of 22 biopsies were successfully performed wearing this GafTLD glove under sterile gloves, and the IR reported no difficulty or reduction of dexterity while wearing it. Comparison of dose distributions obtained from digitization of the Gafchromic film strips and absolute Hp(0.07) readings from TLDs showed good agreement, despite some positional uncertainty due to relative movement. Per procedure, doses at the base of the ring finger can be as low as 3%–8% of hand dose maximum. Accumulated dose at the base of the ring finger was four times lower than the dose maximum. PACS numbers: 07.57.Kp, 29.40.‐n, 85.25.Pb, 87.57.qp

Attenuation measurements show that the presence of a TachoSil surgical patch will not compromise target irradiation in intra-operative electron radiation therapy or high-dose-rate brachytherapy

BackgroundSurgery of locally advanced and/or recurrent rectal cancer can be complemented with intra-operative electron radiation therapy (IOERT) to deliver a single dose of radiation directly to the unresectable margins, while sparing nearby sensitive organs/structures. Haemorrhages may occur and can affect the dose distribution, leading to an incorrect target irradiation. The TachoSil (TS) surgical patch, when activated, creates a fibrin clot at the surgical site to achieve haemostasis. The aim of this work was to determine the effect of TS on the dose distribution, and ascertain whether it could be used in combination with IOERT. This characterization was extended to include high dose rate (HDR) intraoperative brachytherapy, which is sometimes used at other institutions instead of IOERT.MethodsCT images of the TS patch were acquired for initial characterization. Dosimetric measurements were performed in a water tank phantom, using a conventional LINAC with a hard-docking system of cylindrical applicators. Percentage Depth Dose (PDD) curves were obtained, and measurements made at the depth of dose maximum for the three clinically used electron energies (6, 9 and 12MeV), first without any attenuator and then with the activated patch of TS completely covering the tip of the IOERT applicator. For HDR brachytherapy, a measurement setup was improvised using a solid water phantom and a Farmer ionization chamber.ResultsOur measurements show that the attenuation of a TachoSil patch is negligible, both for high energy electron beams (6 to 12MeV), and for a HDR 192Ir brachytherapy source. Our results cannot be extrapolated to lower beam energies such as 50 kVp X-rays, which are sometimes used for breast IORT.ConclusionThe TachoSil surgical patch can be used in IORT procedures using 6MeV electron energies or higher, or HDR 192Ir brachytherapy.

[P193] Improved method for in vivo dosimetry in pelvic intra-operative radiation therapy using Gafchromic films and customized digitization templates

Purpose In vivo measurements using three 1.5 × 1.5 cm2 pieces of Gafchromic EBT3 film are part of our routine quality assurance during electron intra-operative radiation therapy (IOERT) for rectal cancer. The complexity of the pelvic irradiation geometry makes it desirable to increase the number of film samples, to ensure complete coverage of the irradiated length. The positioning of small films for digitization is time consuming and challenging, because of the need to minimize undesirable scanning artifacts. The aim of this work is to implement a practical method to digitize several small film pieces in routine practice, reducing time consumption and measuring uncertainties. Methods Three sample sizes were tested, with the same width (1.5 cm) and different length (0.75, 1.0 and 1.5 cm). Combinations of equal-sized, equally spaced samples were tested in a sacrum model for typical IOERT applicators. Film samples were digitized using a flatbed scanner Epson 10000XL. Several opaque and transparent templates for digitization were developed and tested. The uncertainties related to the films and digitization process were quantified in a previous work, and helped to determine which variations needed to be minimized (e.g. film-to-light source distance), and which could be treated more flexibly. Results The use of samples smaller than 1 × 1.5 cm2 was discarded as impractical. Film samples of 1.5 × 1.0 cm2; 1 cm apart, proved the best option for adequate coverage of the irradiated length, using strips of 3–8 samples according to applicator size. The selected template allows simultaneous digitization of a non-irradiated film piece (reference) and up to eight irradiated samples, reducing inter-scan fluctuations and total digitization time. For films irradiated with 10 to 15 Gy (dose interval of interest), this template minimizes the lateral discontinuity effect previously observed on the measured response at the films’ edges. A glass plate is used for compression, to ensure uniform film-to-light source distance. Conclusions The methodology for in vivo measurements was optimized by using smaller samples and an opaque template to read up to eight films in a single scan. These changes allow a better characterization of the irradiated area with a simpler and faster digitization method.

The use of needle holders in CTF guided biopsies as a dose reduction tool

Abstract Purpose The purpose of this study was to evaluate the efficacy of needle holders in reducing staff hand exposure during biopsies guided by computed tomography fluoroscopy (CTF), through the analysis of data acquired during a detailed monitoring study, undertaken in parallel with an ongoing optimization process to reduce hand irradiation. Methods Hand monitoring was performed with 11 extremity detectors, two per finger (base and tip) and one on the back of the wrist, for the left (dominant) hand, during two series of biopsies with comparable characteristics. The first series (47 biopsies) were performed with only quick‐check method (QC) and occasional side‐handle (SH) manipulation of the needle. The second series (63 biopsies) were performed after introducing needle holders (NH) in the course of an optimization process. Results Choice of technique (QC, QC + NH, QC + SH) by the interventional radiologist (IR) was related to biopsy difficulty. Measured hand exposure was low (< 1 mSv) for all QC‐only procedures, and for most of the QC + NH procedures. Occasional side‐handle manipulation still occurred during challenging biopsies, so that 8% of biopsies in the second series accounted for ~70% of total fingertip dose (~90 mSv). The methodology used allowed a detailed insight into the dose reduction achievable with needle holders during real procedures, without the limitations of phantom measurements. Conclusions Needle holders proved effective in reducing mean hand exposure during clinical procedures where real‐time manipulation was necessary. Occasional side‐handle manipulation was found to contribute disproportionately to hand exposure. This highlights the importance of individual hand monitoring during CTF guided procedures.

Automatic calculation of patient size metrics in computed tomography: What level of computational accuracy do we need?

Abstract Objectives To compare the effectiveness of two different patient size metrics based on water equivalent diameter (D w), the mid‐scan water equivalent diameter D w_c, and the mean (average) water equivalent diameter in the imaged region, D w_ave, for automatic detection of accidental changes in computed tomography (CT) acquisition protocols. Methods Patient biometric data (height and weight) were available from a previous survey for 80 adult chest examinations, and 119 adult single‐acquisition chest–abdomen–pelvis (CAP) examinations for two 16 slice scanners (GE LightSpeed and Toshiba Aquilion RXL) equipped with automatic tube current modulation (ATCM). D w_c and D w_ave were calculated from the archived CT images. Size‐specific dose estimates (SSDE) were obtained from volume CT dose index (CTDI vol), using the conversion factors for a patient diameter of D w_c. Results CTDI vol and SSDE correlate better with D w_ave than with D w_c. R‐squared values of linear fits to CTDI vol of CAP examinations were 0.81–0.89 for D w_c and 0.93–0.94 for D w_ave (SSDE: 0.69–080 for D w_c, 0.87–0.92 for D w_ave). Percentage differences between D w_c and D w_ave were −4 ± 4% for chest and +5 ± 4% for CAP examinations (in % of D w_ave). However, small D w variations translated as larger variations in CTDI vol for these ATCM systems (e.g., a 24% increase in D w doubled CTDI vol). The dependence of CTDI vol on D w_ave was similar for chest and CAP examinations performed with similar ATCM parameters, while use of D w_c resulted in a clear separation of the same data according to examination type. Maximum D w variation in the imaged region was 5.6 ± 1.6 cm for chest and 6.5 ± 1.4 cm for CAP examinations. Conclusions D w_ave is a better metric than D w_c for binning similar‐sized patients in dose comparison studies, despite the additional computational effort required for its calculation Therefore, when implementing automatic determination of D w for SSDE calculations, automatic calculation of D w_ave should be considered.

The advantages of using average attenuation metrics to express patient size in computed tomography dose optimization

Introduction Dose comparisons are essential to the process of dose optimization in computed tomography (CT). Large-scale databases, and size-specific dose estimates (SSDE), made comparisons less dependent on patient size. However, quantification of patient size would still be useful, to allow comparisons between different patient populations and different scanners. Moreover, when optimizing acquisition protocols after installation of a new scanner, dose data will be collected first for a small number of patients, and it is important that this can be compared with an established standard. Purpose To assess the feasibility of using the water equivalent diameter (Dw) instead of patient weight to quantify patient size. Materials and methods Existing data from a previous survey was re-analyzed using Dw metrics. Two different values of Dw were obtained for each exam: the midscan or central Dw (Dw_c) used for calculation of SSDE, and the average value of Dw in the scanned region (Dw_a). The data pertained to adult chest and chest-abdomen-pelvis (CAP) exams, in two 16 slice CT scanners (a GE Lightspeed and a Toshiba Aquilion RXL). Results The volume CT dose index (CTDIvol) and the SSDE were plotted as a function of patient weight, body mass index (BMI), Dw_c and Dw_a. The use of Dw_a was found to reduce the dispersion of the data, relative to all the other metrics used, allowing a clearer visualization. Conclusion Dw_a proved a robust and useful metric to characterize patient size, which can be used for comparisons of both small and large data sets. Disclosure The authors have no relevant financial or non-financial relationships to disclose.