Joana Lencart

Experimental characterization of megavoltage beams for orthogonal ray imaging

OrthoCT (orthogonal computed tomography) is a potential new imaging technique that aims to acquire images of the volume to be irradiated immediately before or during a radiotherapy treatment. It potentially provides imaging with very low to eventually null dose, allowing to check if the morphology/anatomy of the patient and tumour are in agreement with the planned one. This technique relies on the detection of photons that are scattered in the patient and are emitted perpendicularly to the incident beam direction. To acquire the OrthoCT morphological images the scanning of the volume to be irradiated is done using pencil-like mega-voltage beams. The corresponding scan profile requires: (1) high homogeneity, so that variations can be associated only to dose/morphological alterations, and (2) high velocity, which favors multi-leaf collimator-based scans in respect to jaw-based ones. To compare the variability of a homogeneous beam with the variability of a scanned profile two scans with a cross-section of 5mm × 5mm (MLC-collimated) and 6mm × 6mm (jaw-collimated) were experimentally evaluated. The transverse profiles obtained with MLC-collimation in this work reveal a homogeneity with an intensity variability inferior to 1%, thus supporting OrthoCT imaging with morphology/anatomy changes superior to that value.

Impact of tumor contrast in orthogonal ray imaging: A prostate irradiation study

A new very-low-dose imaging technology to assist external-beam radiotherapy treatments has been proposed. This technique, called orthogonal ray imaging, does not require X-ray source rotation around the target. It is based on the detection of photons escaping the target at almost right angles with respect to the incoming photon flux. Modern image-guided radiation therapy techniques allow an accurate positioning of the patient, consequently improving the treatment accuracy. However, some of these techniques, such as electronic portal imaging and mega-voltage cone-beam computed tomography provide poor resolution in soft tissues. Other techniques like kilovoltage cone-beam computed tomography, allow a good visualization of these type of tissues but at the cost of an increase of the dose delivered to the patient, consequently increasing the risk of possible side-effects on the surrounding healthy tissues. One way to enhance the contrast of the lesion region is by injecting a contrast agent. The aim of this study was to evaluate through simulation the benefit of iodinated-contrast agent (Ultravist 370) administration in cases of prostate cancer in OrthoCT using the Geant4 simulation toolkit and the anthropomorphic phantom NCAT. We conclude that (1) in all studied scenarios it was possible to obtain OrthoCT images with good visual agreement with the simulated dose as well as the phantom pelvic structures with a maximum dose of the order of 2mGy and (2) the administration of 100mL of Ultravist 370 is not enough to allow the visualization of the tumor or the normal prostate tissue.

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

Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

BackgroundThis is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated.MethodsTen prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the Rwall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm1 to Vm5). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for Rwall including the median dose and the dose received by 10 to 60% of the Rwall, bladder wall (Bwall) and femoral heads (FHeads) were compared. The monitor units (MU) per plan were recorded.ResultsVm4 reduced by half D30%, D40%, D50%, and Dmed for Rwall and decreased by a third D60% while HIPTV, HIuPRV and DSC remained stable with and without ERB compared to Vmref. HIPTV worsened at Vm5 both with and without ERB. No statistical differences were observed between paired plans on Rwall, Bwall except a higher D2% for Fheads with and without an ERB.ConclusionsFurther optimization to the Rwall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the Rwall can be significantly reduced using a four-step sequential optimization approach.

Monitoring Tumor Lung Irradiation With Megavoltage Patient-Scattered Radiation: A Full System Simulation Study

We analyze by simulation a fully-3-D, low-dose imaging system aiming at assisting external-beam radiotherapy, either for on-board patient imaging, or for real-time radiotherapy monitoring. The system consists in detecting megavoltage patient-scattered radiation that is emitted at right angles with respect to the beam axis. Since photon scattering in the patient occurs with higher intensity in tissues of higher density, a multislice photon detection system positioned perpendicularly to the beam axis yields a signal correlated with patient morphology, including the tumor. We thus report on GEANT4 simulations carried out with an anthropomorphic phantom in order to analyze the capability of the system to detect pertinent and clinically relevant scenarios, e.g., lung tumor deviation and tumor regression/progression. The signal distribution obtained with a realistic full system (including the multislice collimator, the scintillator crystals, and the charge electronic readout mode) show a very high visual agreement both with the simulated, prescribed dose, and with the tumor location/size, as well as with the phantom structures. The capability of this system to obtain morphological images without X-ray source rotation potentially allows to highly reduce dose in healthy tissues and organs at risk in respect to other existing image-guided radiation therapy techniques, thus complementing them.

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.

Prostate SBRT: The use of ERB and MOSFET in vivo dosimetry feasibility

Introduction Hypofractionated treatment regimens are indicated for some stages of prostate cancer. In order to provide the reproducibility of the relative position between rectum and prostate and to allow that only a small volume of the rectal wall remains close to the prostate, an endorectal balloon (ERB) may be used. Purpose This work aims to assess the dose on the rectal wall in the presence of the ERB filled with water or air and evaluate the deviation between the measured and calculated doses using two different algorithms (Eclipse AAA and iPlan PencilBeam) Materials and methods Two CT scans were obtained for a modified Rando phantom where an ERB (filled with water or air) with three MOSFET detectors was inserted. A simple 4 field in box plan and an IMRT (6MV beam) clinical plan were calculated in both CT sets, using two different algorithms. The treatment plans were delivered to the phantom using a Varian Novalis linac. Four sets of measurements were obtained and the results were compared with the calculated values. Results The difference between the calculated and measured doses around the ERB is lower when it is filled with water for both algorithms. The maximum relative differences when the ERB is filled with air are 1.8% for Eclipse and 5.5% for iPlan. Conclusion Both algorithms show a better a performance in the presence of water. Water filled ERB seems to be a suitable option. MOSFET dosimetry in association with ERBs for real-time in vivo during hypofractionated treatment of the prostate is a practical and reliable procedure. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

Electronic Poster: Brachytherapy track: PhysicsEP-1606: Geometrical deviations during cervical carcinoma HDR brachytherapy procedures using vaginal cylinders

areas included eyelid (1), nasal dorsum (7), pre auricular region (1), forehead (2) and cheek (1). The planning method was identical to all lesions, although two different immobilization techniques were applied, depending on the size and location of the lesion. The first technique was applied in facial lesions (11) and large lesions of the thorax (4): first the lesion was delineated with a radiopaque marker, followed by the immobilization of the patient with a thermoplastic mask, then one bolus plaque of 2 mm thickness was applied above the mask and the plastic catheters were placed and immobilized with two bolus plaques. The second technique was applied in small central lesions of the thorax (2) and in the eyelid: after the delineation of the lesion with a radiopaque marker, bolus plaques were placed directly above and the plastic catheters were placed and immobilized with two bolus plaques. In these cases, the limits of the mold were tattooed in the patient for reproducibility between fractions. The distance between catheters was always 10 mm. A planning CT with 1,25 (facial) and 2,5 mm (thoracic) slice thickness was acquired. The treatment planning was developed by the treatment planning system (TPS) Oncentra MasterPlan v4.1 taking into account the prescription points, dose to the skin and doses to the organs at risk (OAR). The dose was prescribed to points at 0 (eyelid) and 3 mm depth. All patients were treated by a microSelectron v3 (Elekta) that used a Ir source. Eye protectors were used when necessary and the standard fractionation was 40 Gy in 10 fractions twice weekly. Dosimetric plans were analyzed for the following parameters: dose to the OAR, dose to the skin and dose to the prescribing points. The QA procedure was done using mosfets and gafchromic films. Results: The doses to the OAR were converted to Biologically Effective Dose (BED) and were always under their maximum tolerance dose. The dose verification results were accepted within a range of 10 % deviation of the planned dose to the prescribing points and to the skin. Conclusions: This technique showed to be a viable option to facial and thoracic skin lesions. Due to the steep dose gradient of the Ir sources used in brachytherapy, adjacent healthy tissue and deep structures are spared to irradiation. Furthermore, as masks and applicators are flexible, it is possible to treat lesions in irregular or curved surfaces assuring catheter fixation and treatment reproducibility between fractions.

EP-1453: Urethra-sparing prostatic SBRT: extreme dosimetric optimization on rectal wall using an endorectal balloon

Materials and Methods: Using the CBCT of the XRAD225Cx preclinical irradiator, 8 tissue equivalent cylinders of known composition and density (Gammex RMI, Middelton, WI) were imaged at 40kVp. The HU variation was plotted versus the product ρ times Zeff that yielded to a monotonically increasing curve. Based on this relationship and the tissues defined in the ICRU-44 report, interpolated tissues were created for ρZeff varying from 2 up to 27 with a 0.2 step. Tissue equivalent cylinders were irradiated with the XRAD225Cx (225kVp). Exit dose was measured with EBT3 films and compared to Monte Carlo (MC) calculations from our GATE model of the irradiator. On the CT images, tissue segmentation was performed either by manual assignation of the elemental composition provided by the manufacturer or by using the (HU, ρ, EC) method. Dosimetric impact of the (HU, ρ, EC) method was evaluated on mice CT comparing with manual segmentation for brain and femoral head irradiations. Results: Tissue equivalent exit dose measurements relative to solid water varied from 1.13 (AP6 adipose) down to 0.36 (SB3 cortical bone). Max 2% deviation was found with MC dose calculation performed with manufacturer data and 4.3% with calculation performed with the (HU, ρ, EC) method. Mean deviations were respectively 1.1% and 1.8%. It must be noticed that the segmentation method was based on real human tissues defined in ICRU-44 whereas measurements were performed with substitutes with elemental composition slightly different from human tissue elemental composition. The (HU, ρ, EC) method applied on mice CT allowed the automatic definition of 125 tissues. Dosimetric impact of the (HU, ρ, EC) was significant for bony tissues (>25%). Conclusions: A robust tissue segmentation method was developed for dose calculation in preclinical radiation therapy based on the (HU, ρZeff) relationship. Our method was successfully tested by comparing exit dose measurements from materials of known composition with MC dose calculation. The method was applied on mice CT for brain and femoral head irradiation with significant dosimetric impact.