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

PurposeIn 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.MethodsThe 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.ResultsGood 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.ConclusionsThe 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.     

Electron radiotherapy (IOERT) for applications outside of the breast: Dosimetry and influence of tissue inhomogeneities

PurposeThe purpose of study is to investigate the dosimetry of electron intraoperative radiotherapy (IOERT) of the Intraop Mobetron 2000 mobile LINAC in treatments outside of the breast. After commissioning and external validation of dosimetry, we report in vivo results of measurements for treatments outside the breast in a large patient cohort, and investigate if the presence of inhomogeneities can affect in vivo measurements.Methods and materialsApplicator factors and profile curves were measured with a stereotactic diode. The applicators factors of the 6 cm flat and beveled applicators were also confirmed with radiochromic films, parallel-plate ion chamber and by an external audit performed with ThermoLuminescent Dosimeters (TLDs). The influence of bone on dose was investigated by using radiochromic films attached to an insert equivalent to cortical bone, immersed in the water phantom. In vivo dosimetry was performed on 126 patients treated with IOERT using metal oxide-silicon semiconductor field effect transistors (MOSFETs) placed on the tumor bed.ResultsRelatively small differences were found among different detectors for measurements of applicator factors. In the external audit, the agreement with the TLD was mostly within ±0.2%. The largest increase of dose due to the presence of cortical bone insert was +6.0% with energy 12 MeV and 3 cm applicator. On average, in vivo dose was significantly (+3.1%) larger than prescribed dose.ConclusionIOERT in applications outside the breast results in low discrepancies between in vivo and prescribed doses, which can be also explained with the presence of tissue inhomogeneity.     

Shielding disk position in intra-operative electron radiotherapy (IOERT): A Monte Carlo study

PurposeIn IOERT breast treatments, a shielding disk is frequently used to protect the underlying healthy structures. The disk is usually composed of two materials, a low-Z material intended to be oriented towards the beam and a high-Z material. As tissues are repositioned around the shield before treatment, the disk is no longer visible and its correct alignment with respect to the beam is guaranteed. This paper studies the dosimetric characteristics of four possible clinical positioning scenarios of the shielding disk. A new alignment method for the shielding disk in the beam is introduced. Finally, it suggests a new design for the shielding disk.MethodsAs the first step, the IOERT machine “Mobetron 1000” was modeled by using Monte Carlo simulation, tuning the MC model until an excellent match with the measured PDDs and profiles was achieved. Four possible shielding disk positioning scenarios were considered, determining the dosimetric impact. Furthermore, in our center, to prevent beam misalignment, we have developed a shielding disk equipped with guiding rods. Having ascertained a correct alignment between the disk and the beam, we can propose a new internal design of the shielding disk that can improve the dose distribution with a better coverage of the treated area.ResultsAll MC simulations were performed with a 12 MeV beam, the maximum energy of Mobetron 1000 and a 5.5 cm diameter flat tip applicator, this applicator being the most clinically used. The simulations were compared with measurements performed in a water phantom and showed good results within 2.2% of root mean square difference (RMSD). The misplacement positions of the shielding disk have dosimetric impacts in the treatment volume and a small translation could have a significant influence on healthy tissues. The D-scenario is the worst which could happens when the shielding disk is flipped upside down, giving up to 144% dose instead of 90% at the surface of the Pb/Al shielding disk. A new shielding design used, together with our alignment tool, is able to give a more homogeneous dose in the target area.ConclusionsThe accuracy of shielding disk position can still be problematic in IOERT dosimetry. Any method that can ascertain the good alignment between the shielding disk and the beam is beneficial for the dose distribution and is a prerequisite for an optimized shield internal design that could improve the coverage of the treated area and the protection of healthy tissues.     

Dosimetric comparison of analytic anisotropic algorithm and Acuros XB algorithm in VMAT plans for high-grade glioma

PurposeTo investigate the dosimetric impact between the anisotropic analytical algorithm (AAA) and the Acuros XB (AXB) algorithm in volumetric-modulated arc therapy (VMAT) plans for high-grade glioma (HGG).MethodsWe used a heterogeneous phantom to quantify the agreement between the measured and calculated doses from the AAA and from the AXB. We then analyzed 14 patients with HGG treated by VMAT, using the AAA. We newly created AXB plans for each corresponding AAA plan under the following conditions: (1) re-calculation for the same number of monitor units with an identical beam and leaf setup, and (2) re-optimization under the same conditions of dose constraints. The dose coverage for the planning target volume (PTV) was evaluated by dividing the coverage into the skull, air, and soft-tissue regions.ResultsCompared to the results obtained with the AAA, the AXB results were in good agreement with the measured profiles. The dose differences in the PTV between the AAA and re-calculated AXB plans were large in the skull region contained in the target. The dose difference in the PTV in both types of plan was significantly correlated with the volume of the skull contained in the target (r = 0.71, p = 0.0042). A re-optimized AXB plan's dose difference was lower vs. the re-calculated AXB plan's.ConclusionsWe observed dose differences between the AAA and AXB plans, in particular in the cases in which the skull region of the target was large. Considering the phantom measurement results, the AXB algorithm should be used in VMAT plans for HGG.     

Dosimetric impact of different multileaf collimators on prostate intensity modulated treatment planning

AimThe main purpose of this study is to perform a dosimetric comparison on target volumes and organs at risks (OARs) between prostate intensity modulated treatment plans (IMRT) optimized with different multileaf collimators (MLCs).BackgroundThe use of MLCs with a small leaf width in the IMRT optimization may improve conformity around the tumor target whilst reducing the dose to normal tissues.Materials and methodsTwo linacs mounting MLCs with 5 and 10 mm leaf-width, respectively, implemented in Pinnacle³ treatment planning system were used for this work. Nineteen patients with prostate carcinoma undergoing a radiotherapy treatment were enrolled. Treatment planning with different setup arrangements (7 and 5 beams) were performed for each patient and each machine. Dose volume histograms (DVHs) cut-off points were used in the treatment planning comparison.ResultsComparable planning target volume (PTV) coverage was obtained with 7- and 5-beam configuration (both with 5 and 10 mm MLC leaf-width). The comparison of bladder and rectum DVH cut-off points for the 5-beam arrangement shows that 52.6% of the plans optimized with a larger leaf-width did not satisfy at least one of the OARs’ constraints. This percentage is reduced to 10.5% for the smaller leaf-width. If a 7-beam arrangement is used the value of 52.6% decreases to 21.1% while the value of 10.5% remains unchanged.ConclusionMLCs collimators with different widths and number of leaves lead to a comparable prostate treatment planning if a proper adjustment is made of the number of gantry angles.     

Validation and practical implementation of seated position radiotherapy in a commercial TPS for proton therapy

PurposeThis work aims to validate new 6D couch features and their implementation for seated radiotherapy in RayStation (RS) treatment planning system (TPS).Materials and methodsIn RS TPS, new 6D couch features are (i) chair support device, (ii) patient treatment option of “Sitting: face towards the front of the chair”, and (iii) patient support pitch and roll capabilities. The validation of pitch and roll was performed by comparing TPS generated DRRs with planar x-rays. Dosimetric tests through measurement by 2D ion chamber array were performed for beams created with varied scanning and treatment orientation and 6D couch rotations. For the implementation of 6D couch features for treatments in a seated position, the TPS and oncology information system (Mosaiq) settings are described for a commercial chair. An end-to-end test using an anthropomorphic phantom was performed to test the complete workflow from simulation to treatment delivery.ResultsThe 6D couch features were found to have a consistent implementation that met IEC 61712 standard. The DRRs were found to have an acceptable agreement with planar x-rays based on visual inspection. For dose map comparison between measured and calculated, the gamma index analysis for all the beams was >95% at a 3% dose-difference and 3 mm distance-to-agreement tolerances. For an end-to end-testing, the phantom was successfully set up at isocenter in the seated position and treatment was delivered.ConclusionsChair-based treatments in a seated position can be implemented in RayStation through the use of newly released 6D couch features.     

Imaging opportunities for treatment planning in intraoperative electron beam radiotherapy (IOERT): Developments in the context of RADIANCE system

AimThe purpose of this report is to store the information of the pre-planning and compare this data with the actual data of the procedure.BackgroundCurrently, intraoperative electron beam radiotherapy clinical practice lacks a treatment planning system.Materials and methodsThe RADIANCE concept approaches treatment planning by providing the user with a navigation platform based on a three-dimensional imaging system in which the radiation oncologist can target the tumor and risk areas in different sections (axial, coronal, sagittal), while a volume rendering engine displays a 3D image that is automatically updated as we make any changes of the space. Finally, the user may select the parameters of the applicator, energy and dose of treatment to optimize the procedure. Six cases are clinically described and illustrated.ResultsRADIANCE is a useful tool in planning IOERT. Tumor segmentation and risk areas with minimal guide in the selection of parameters for the applicator. Complex locations are challenging, where the experience and skill of the radiation oncologist is necessary to optimize the process. New developments include imaging innovated uses. Intraoperative imaging will approach reality and allow real time, dosimetry estimations, stereotactic recognition of patient and tumor bed position, will guide automatization of radiation beam recognition and pre-robotic arrangements with linear accelerator movements.ConclusionsRADIANCE offers a new imaging expansion for IOERT, in the context of a multidisciplinary approach to optimize and define the treatment parameters to approximate the actual treatment radiotherapy procedure.     

Performance of the irregular surface compensator compared with four-field box and intensity modulated radiation therapy for gynecologic cancer

PurposeA retrospective planning study was undertaken to evaluate the dosimetric advantages of the irregular surface compensator (ISC) technique, a forward planning technique with electronic compensation algorithm available on Varian Eclipse treatment planning system. This was extensively compared to the conventional four-field box (4FB) and intensity modulated radiation therapy using 5 fields (IMRT5F) on gynecologic cancer patients.MethodsTwenty-two patients were enrolled. The prescribed dose was 50.4 Gy in 28 fractions to the primary target including pelvic lymph nodes. 4FB treatment plans were generated, then fluence of anterior and posterior fields were modified to generate ISC plans. IMRT5F were inversely optimized with equally spaced five coplanar fields. Dose-volume parameters were evaluated for the comparison of three planning techniques. The MU and delivery time were also estimated.ResultsIn terms of target coverage, the conformity and homogeneity index of ISC (1.67 and 1.03, respectively) were superior to those of 4FB (2.43 and 1.06, respectively) but slightly inferior to those of IMRT5F (1.10 and 1.02, respectively). ISC also illustrated an overall improvement in normal organ saving. Compared to 4FB, the mean dose of the rectum was reduced by about 4.0–5.0 Gy with ISC and IMRT5F. The volume receiving large doses was reduced for bladder with statistical significance with ISC and more with IMRT5F relative to 4FB. The mean number of MU per fraction were 200.86 (4FB), 446.09 (ISC) and 895.59 (IMRT5F).ConclusionThe ISC technique has the superior target coverage and healthy tissue sparing in comparison with conventional 4FB and comparable normal organ saving compared to IMRT5F. The ISC can be an available option for gynecologic radiotherapy.     

VMAT-like plans for magnetic resonance guided radiotherapy: Addressing unmet needs

PurposeVMAT delivery technique is currently not applicable to Magnetic Resonance-guided radiotherapy (MRgRT) hybrid systems. Aim of this study is to evaluate an innovative VMAT-like (VML) delivery technique.Material and methodsFirst, planning and dosimetric evaluation of the MRgRT VML treatment have been performed on 10 different disease sites and the results have been compared with the corresponding IMRT plans. Then, in the second phase, 10 of the most dosimetrically challenging locally advanced pancreas treatment plans have been retrospectively re-planned using the VML approach to explore the potentiality of this new delivery technique. Finally, VML robustness was evaluated and compared with the IMRT plans, considering a lateral positioning error of ± 5 mm.ResultsIn phase one, all VML plans were within constraint for all OARs. When PTV coverage is considered, in the 50% of the cases VML PTV coverage is equal or higher than in IMRT plan. In the remaining 50%, the highest target under coverage difference in comparison with IMRT plan is −1.71%. The mean and maximum treatment time differences (VML-IMRT) is 0.2 min and 3.1 min respectively. In phase two, the treatment time variation (VML-IMRT), shows a mean, maximum and minimum variations of 1.3, 4.6 and −0.6 min respectively. All VML plans have a better target coverage if compared with IMRT plans, keeping in any case the OARs constraints within tolerance. VML doesn’t increase plan robustness.ConclusionVMAT-like treatment approach appeared to be an efficient planning solution and it was decided to clinically implement it in daily practice, especially in the frame of hypo fractionated treatments.     

Assessment of intra-fraction motion during automated linac-based SRS treatment delivery with an open face mask system

Purpose/objectiveTo evaluate intra-fraction target shift during automated mono-isocentric linac-based stereotactic radiosurgery with open-face mask system and optical real-time tracking.Materials/methodsNinety-five patients were treated using automated linac-based stereotactic radiosurgery in 1–5 fractions with single isocenter for a total of 195 fractions. During treatment, patient positioning was tracked real-time with optical surface guidance and immobilized with a rigid open-face mask. Patients were re-positioned if optical surface guidance error exceeded 1 mm magnitude or 1°. Translational and rotational intra-fractional changes were determined by post-treatment CBCT matched to the planning CT. Target specific error was calculated by translation and rotation matrices applied to isocenter and target spatial coordinates.ResultsFor 132 fractions with isocenter within a single target, the median shift magnitude was 0.40 mm with a maximum shift of 1.17 mm. A total of 398 targets treated for plans having multiple or single targets that lied outside isocenter, resulted in a median shift magnitude of 0.46 mm, with median translational shifts of 0.20 mm and 0.20° rotational shifts. A 1 mm PTV margin was insufficient in 18% of targets at a distance greater than 6 cm away from isocenter, but sufficient for 96% of targets within 6 cm.ConclusionsThe findings of this study support 1 mm PTV expansion due to intra-fraction motion to ensure target coverage for plans with isocenter placement less than 6 cm away from the targets.     

Personalized automation of treatment planning in head-neck cancer: A step forward for quality in radiation therapy?

PurposeTo perform a comprehensive dosimetric and clinical evaluation of the new Pinnacle Personalized automated planning system for complex head-and-neck treatments.MethodsFifteen consecutive head-neck patients were enrolled. Radiotherapy was prescribed using VMAT with simultaneous integrated boost strategy. Personalized planning integrates the Feasibility engine able to supply an “a priori” DVH prediction of the achievability of planning goals. Comparison between clinically accepted manually-generated (MP) and automated (AP) plans was performed using dose-volume histograms and a blinded clinical evaluation by two radiation oncologists. Planning time between MP and AP was compared. Dose accuracy was validated using the PTW Octavius-4D phantom together with the 1500 2D-array.ResultsFor similar targets coverage, AP plans reported less irradiation of healthy tissue, with significant dose reduction for spinal cord, brainstem and parotids. On average, the mean dose to parotids and maximal doses to spinal cord and brainstem were reduced by 13–15% (p < 0.001), 9% (p < 0.001) and 16% (p < 0.001), respectively. The integral dose was reduced by 16% (p < 0.001). The dose conformity for the three PTVs was significantly higher with AP plans (p < 0.001). The two oncologists chose AP plans in more than 80% of cases. Overall planning times were reduced to <30 min for automated optimization. All AP plans passed the 3%/2 mm γ-analysis by more than 95%.ConclusionComplex head-neck plans created using Personalized automated engine provided an overall increase of plan quality, in terms of dose conformity and sparing of normal tissues. The Feasibility module allowed OARs dose sparing well beyond the clinical objectives.     

The influence of respiratory motion on dose distribution in accelerated partial breast irradiation using volumetric modulated arc therapy

PurposeAccelerated partial breast irradiation (APBI) is alternative treatment option for patients with early stage breast cancer. The interplay effect on volumetric modulated arc therapy APBI (VMAT-APBI) has not been clarified. This study aimed to evaluate the feasibility of VMAT-APBI for patients with small breasts and investigate the amplitude of respiratory motion during VMAT-APBI delivery that significantly affects dose distribution.MethodsThe VMAT-APBI plans were generated with 28.5 Gy in five fractions. We performed patient-specific quality assurance using Delta⁴ phantom under static conditions. We also measured point dose and dose distribution using the ionization chamber and radiochromic film under static and moving conditions of 2, 3 and 5 mm. We compared the measured and calculated point doses and dose distributions by dose difference and gamma passing rates.ResultsA total of 20 plans were generated; the dose distributions were consistent with those of previous reports. For all measurements under static conditions, the measured and calculated point doses and dose distributions showed good agreement. The dose differences for chamber measurement were within 3%, regardless of moving conditions. The mean gamma passing rates with 3%/2 mm criteria in the film measurement under static conditions and with 2 mm, 3 mm, and 5 mm of amplitude were 95.0 ± 2.0%, 93.3 ± 3.3%, 92.1 ± 6.2% and 84.8 ± 7.8%, respectively. The difference between 5 mm amplitude and other conditions was statistically significant.ConclusionsRespiratory management should be considered for the risk of unintended dose distribution if the respiratory amplitude is >5 mm.     

Film-based delivery quality assurance for robotic radiosurgery: Commissioning and validation

PurposeRobotic radiosurgery demands comprehensive delivery quality assurance (DQA), but guidelines for commissioning of the DQA method is missing. We investigated the stability and sensitivity of our film-based DQA method with various test scenarios and routine patient plans. We also investigated the applicability of tight distance-to-agreement (DTA) Gamma-Index criteria.Methods and materialWe used radiochromic films with multichannel film dosimetry and re-calibration and our analysis was performed in four steps: 1) Film-to-plan registration, 2) Standard Gamma-Index criteria evaluation (local-pixel-dose-difference ≤2%, distance-to-agreement ≤2 mm, pass-rate ≥90%), 3) Dose distribution shift until maximum pass-rate (Max_γ) was found (shift acceptance <1 mm), and 4) Final evaluation with tight DTA criteria (≤1 mm). Test scenarios consisted of purposefully introduced phantom misalignments, dose miscalibrations, and undelivered MU. Initial method evaluation was done on 30 clinical plans.ResultsOur method showed similar sensitivity compared to the standard End-2-End-Test and incorporated an estimate of global system offsets in the analysis. The simulated errors (phantom shifts, global robot misalignment, undelivered MU) were detected by our method while standard Gamma-Index criteria often did not reveal these deviations. Dose miscalibration was not detected by film alone, hence simultaneous ion-chamber measurement for film calibration is strongly recommended. 83% of the clinical patient plans were within our tight DTA tolerances.ConclusionOur presented methods provide additional measurements and quality references for film-based DQA enabling more sensitive error detection. We provided various test scenarios for commissioning of robotic radiosurgery DQA and demonstrated the necessity to use tight DTA criteria.     

Investigating the spatial accuracy of CBCT-guided cranial radiosurgery: A phantom end-to-end test study

PurposeTo evaluate the spatial accuracy of a frameless cone-beam computed tomography (CBCT)-guided cranial radiosurgery (SRS) using an end-to-end (E2E) phantom test methodology.Methods and materialsFive clinical SRS plans were mapped to an acrylic phantom containing a radiochromic film. The resulting phantom-based plans (E2E plans) were delivered four times. The phantom was setup on the treatment table with intentional misalignments, and CBCT-imaging was used to align it prior to E2E plan delivery. Comparisons (global gamma analysis) of the planned and delivered dose to the film were performed using a commercial triple-channel film dosimetry software. The necessary distance-to-agreement to achieve a 95% (DTA95) gamma passing rate for a fixed 3% dose difference provided an estimate of the spatial accuracy of CBCT-guided SRS. Systematic (∑) and random (σ) error components, as well as 95% confidence levels were derived for the DTA95 metric.ResultsThe overall systematic spatial accuracy averaged over all tests was 1.4 mm (SD: 0.2 mm), with a corresponding 95% confidence level of 1.8 mm. The systematic (Σ) and random (σ) spatial components of the accuracy derived from the E2E tests were 0.2 mm and 0.8 mm, respectively.ConclusionsThe E2E methodology used in this study allowed an estimation of the spatial accuracy of our CBCT-guided SRS procedure. Subsequently, a PTV margin of 2.0 mm is currently used in our department.     

Proton or photon radiosurgery for cardiac ablation of ventricular tachycardia? Breath and ECG gated robust optimization

PurposeVentricular tachycardia (VT) is a life-threatening heart disorder. The aim of this preliminary study is to assess the feasibility of stereotactic body radiation therapy (SBRT) photon and proton therapy (PT) plans for the treatment of VT, adopting robust optimization technique for both irradiation techniques.MethodsECG gated CT images (in breath hold) were acquired for one patient. Conventional planning target volume (PTV) and robust optimized plans (25GyE in single fraction) were simulated for both photon (IMRT, 5 and 9 beams) and proton (SFO, 2 beams) plans. Robust optimized plans were obtained both for protons and photons considering in the optimization setup errors (5 mm in the three orthogonal directions), range (±3.5%) and the clinical target volume (CTV) motion due to heartbeat and breath-hold variability.ResultsThe photon robust optimization method, compared to PTV-based plans, showed a reduction in the average dose to the heart by about 25%; robust optimization allowed also reducing the mean dose to the left lung from 3.4. to 2.8 Gy for 9-beams configuration and from 4.1 to 2.9 Gy for 5-beams configuration. Robust optimization with protons, allowed further reducing the OAR doses: average dose to the heart and to the left lung decreased from 7.3 Gy to 5.2 GyE and from 2.9 Gy to 2.2 GyE, respectively.ConclusionsOur study demonstrates the importance of the optimization technique adopted in the treatment planning system for VT treatment. It has been shown that robust optimization can significantly reduce the dose to healthy cardiac tissues and that PT further increases this gain.     

Commissioning and performance testing of the first prototype of AlignRT InBore™ a Halcyon™ and Ethos™-dedicated surface guided radiation therapy platform

PurposeTo commission and assess the performance of AlignRT InBore™, a Halcyon™ and Ethos™-dedicated Surface Guided Radiation Therapy (SGRT) platform which combines ceiling-mounted cameras for patient setup and bore-mounted cameras for in-bore tracking.MethodsTo check the potential impact of InBore™ cameras on dose delivery, 16 SRS, H&N, breast and pelvis patients’ quality assurance (QA) treatment plans were measured with/without AlignRT InBore™ and using ArcCHECK® and SRS MapCHECK®. Impact on image quality was determined using Catphan® 540 phantom and considering all available MV and CBCT protocols (head, breast, chest and pelvis). The stability, accuracy and overall performance of AlignRT InBore™ was assessed using an MV Cube and anthropomorphic phantoms.ResultsComparison of 2D dose distributions with/without AlignRT InBore™ showed no impact on treatment delivery for all 16 QA checks (p-value > 0.25). 2D and CBCT images showed no artefacts or change in the contrast-to-noise ratio, resolution and noise values measured with Catphan® 540. Anti-collision sensors were unaffected by the bore-mounted cameras. Additionally, AlignRT InBore™ cameras allowed for motion detection with sub-0.5 mm accuracy and sub-0.4 mm stability with surface coverage of >50 × 60 × 35 cc. Accurate transition (sub-0.3 mm) from virtual to treatment isocentres was achieved. Finally, Halcyon™ rotations during CBCT and beam delivery resulted in limited camera vibrations with translation uncertainty <0.5 mm in left-right and anterior-posterior directions and <0.1 mm in head-feet direction.ConclusionAlignRT InBore™ provides SGRT setup and intrafraction monitoring capabilities with a performance comparable to standard SGRT solutions while having no adverse effect on Halcyon™.     

Accuracy of dose calculation on iterative CBCT for head and neck radiotherapy

PurposeTo evaluate the feasibility of the use of iterative cone-beam computed tomography (CBCT) for dose calculation in the head and neck region.MethodsThis study includes phantom and clinical studies. All acquired CBCT images were reconstructed with Feldkamp–Davis–Kress algorithm-based CBCT (FDK-CBCT) and iterative CBCT (iCBCT) algorithm. The Hounsfield unit (HU) consistency between the head and body phantoms was determined in both reconstruction techniques. Volumetric modulated arc therapy (VMAT) plans were generated for 16 head and neck patients on a planning CT scan, and the doses were recalculated on FDK-CBCT and iCBCT with Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). As a comparison of the accuracy of dose calculations, the absolute dosimetric difference and 1%/1 mm gamma passing rate analysis were analyzed.ResultsThe difference in the mean HU values between the head and body phantoms was larger for FDK-CBCT (max value: 449.1 HU) than iCBCT (260.0 HU). The median dosimetric difference from the planning CT were <1.0% for both FDK-CBCT and iCBCT but smaller differences were found with iCBCT (planning target volume D_50%: 0.38% (0.15–0.59%) for FDK-CBCT, 0.28% (0.13–0.49%) for iCBCT, AAA; 0.14% (0.04–0.19%) for FDK-CBCT, 0.07% (0.02–0.20%) for iCBCT). The mean gamma passing rate was significantly better in iCBCT than FDK-CBCT (AAA: 98.7% for FDK-CBCT, 99.4% for iCBCT; AXB: 96.8% for FDK_CBCT, 97.5% for iCBCT).ConclusionThe iCBCT-based dose calculation in VMAT for head and neck cancer was accurate compared to FDK-CBCT.     

3D-printed patient-specific pelvis phantom for dosimetry measurements for prostate stereotactic radiotherapy with dominant intraprostatic lesion boost

AimDeveloping and assessing the feasibility of using a three-dimensional (3D) printed patient-specific anthropomorphic pelvis phantom for dose calculation and verification for stereotactic ablative radiation therapy (SABR) with dose escalation to the dominant intraprostatic lesions.Material and methodsA 3D-printed pelvis phantom, including bone-mimicking material, was fabricated based on the computed tomography (CT) images of a prostate cancer patient. To compare the extent to which patient and phantom body and bones overlapped, the similarity Dice coefficient was calculated. Modular cylindrical inserts were created to encapsulate radiochromic films and ionization chamber for absolute dosimetry measurements at the location of prostate and at the boost region. Gamma analysis evaluation with 2%/2mm criteria was performed to compare treatment planning system calculations and measured dose when delivering a 10 flattening filter free (FFF) SABR plan and a 10FFF boost SABR plan.ResultsDice coefficients of 0.98 and 0.91 were measured for body and bones, respectively, demonstrating agreement between patient and phantom outlines. For the boost plans the gamma analysis yielded 97.0% of pixels passing 2%/2mm criteria and these results were supported by the chamber average dose difference of 0.47 ± 0.03%. These results were further improved when overriding the bone relative electron density: 97.3% for the 2%/2mm gamma analysis, and 0.05 ± 0.03% for the ionization chamber average dose difference.ConclusionsThe modular patient-specific 3D-printed pelvis phantom has proven to be a highly attractive and versatile tool to validate prostate SABR boost plans using multiple detectors.     

Utilization of cone-beam CT for offline evaluation of target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment

AimTo assess target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment and to assess possibility of safety margin reduction.BackgroundImplementation of IGRT should influence safety margins. Utilization of cone-beam CT provides current 3D anatomic information directly in irradiation position. Such information enables reconstruction of the actual dose distribution.Materials and methodsSeventeen prostate patients were treated with daily bony anatomy image-guidance. Cone-beam CT (CBCT) scans were acquired once a week immediately after bony anatomy alignment. After the prostate, seminal vesicles, rectum and bladder were contoured, the delivered dose distribution was reconstructed. Target dose coverage was evaluated by the proportion of the CTV encompassed by the 95% isodose. Original plans employed a 1 cm safety margin. Alternative plans assuming a smaller 7 mm margin between CTV and PTV were evaluated in the same way. Rectal and bladder volumes were compared with the initial ones. Rectal and bladder volumes irradiated with doses higher than 75 Gy, 70 Gy, 60 Gy, 50 Gy and 40 Gy were analyzed.ResultsIn 12% of reconstructed plans the prostate coverage was not sufficient. The prostate underdosage was observed in 5 patients. Coverage of seminal vesicles was not satisfactory in 3% of plans. Most of the target underdosage corresponded to excessive rectal or bladder filling. Evaluation of alternative plans assuming a smaller 7 mm margin revealed 22% and 11% of plans where prostate and seminal vesicles coverage, respectively, was compromised. These were distributed over 8 and 7 patients, respectively.ConclusionSufficient dose coverage of target volumes was not achieved for all patients. Reducing of safety margin is not acceptable. Initial rectal and bladder volumes cannot be considered representative for subsequent treatment.