M Wong

Target localization of 3D versus 4D cone beam computed tomography in lipiodol-guided stereotactic radiotherapy of hepatocellular carcinomas

Background Aim of this study was to comparatively evaluate the accuracy of respiration–correlated (4D) and uncorrelated (3D) cone beam computed tomography (CBCT) in localizing lipiodolized hepatocellular carcinomas during stereotactic body radiotherapy (SBRT). Methods 4D–CBCT scans of eighteen HCCs were acquired during free–breathing SBRT following trans–arterial chemo–embolization (TACE) with lipiodol. Approximately 1320 x–ray projections per 4D–CBCT were collected and phase–sorted into ten bins. A 4D registration workflow was followed to register the reconstructed time–weighted average CBCT with the planning mid–ventilation (MidV) CT by an initial bone registration of the vertebrae and then tissue registration of the lipiodol. For comparison, projections of each 4D–CBCT were combined to synthesize 3D–CBCT without phase–sorting. Using the lipiodolized tumor, uncertainties of the treatment setup estimated from the absolute and relative lipiodol position to bone were analyzed separately for 4D– and 3D–CBCT. Results Qualitatively, 3D–CBCT showed better lipiodol contrast than 4D–CBCT primarily because of a tenfold increase of projections used for reconstruction. Motion artifact was observed to subside in 4D–CBCT compared to 3D–CBCT. Group mean, systematic and random errors estimated from 4D– and 3D–CBCT agreed to within 1 mm in the cranio–caudal (CC) and 0.5 mm in the anterior–posterior (AP) and left–right (LR) directions. Systematic and random errors are largest in the CC direction, amounting to 4.7 mm and 3.7 mm from 3D–CBCT and 5.6 mm and 3.8 mm from 4D–CBCT, respectively. Safety margin calculated from 3D–CBCT and 4D–CBCT differed by 2.1, 0.1 and 0.0 mm in the CC, AP, and LR directions. Conclusions 3D–CBCT is an adequate alternative to 4D–CBCT when lipoid is used for localizing HCC during free–breathing SBRT. Similar margins are anticipated with 3D– and 4D–CBCT.

SU‐E‐T‐202: Comparison of 4D‐Measurement‐Guided Dose Reconstructions (MGDR) with COMPASS and OCTAVIUS 4D System

Purpose: To cross-validate the MGDR of COMPASS (IBA dosimetry, GmbH, Germany) and OCTAVIUS 4D system (PTW, Freiburg, Germany). Methods: Volumetric-modulated arc plans (5 head-and-neck and 3 prostate) collapsed to 40° gantry on the OCTAVIUS 4D phantom in QA mode on Monaco v5.0 (Elekta, CMS, Maryland Heights, MO) were delivered on a Elekta Agility linac. This study was divided into two parts: (1) error-free measurements by gantry-mounted EvolutionXX 2D array were reconstructed in COMPASS (IBA dosimetry, GmbH, Germany), and by OCTAVIUS 1500 array in Versoft v6.1 (PTW, Freiburg, Germany) to obtain the 3D doses (COM4D and OCTA4D). COM4D and OCTA4D were compared to the raw measurement (OCTA3D) at the same detector plane for which OCTAVIUS 1500 was perpendicular to 0° gantry axis while the plans were delivered at gantry 40°; (2) beam steering errors of energy (Hump=-2%) and symmetry (2T=+2%) were introduced during the delivery of 5 plans to compare the MGDR doses COM4D_Hump (COM4D_2T), OCTA4D_Hump (OCTA4D_2T), with raw doses OCTA3D_Hump (OCTA3D_2T) and with OCTA3D to assess the error reconstruction and detection ability of MGDR tools. All comparisons used Υ-criteria of 2%(local dose)/2mm and 3%/3mm. Results: Averaged Υ passing rates were 85% and 96% for COM4D,and 94% and 99% for OCTA4D at 2%/2mm and 3%/3mm criteria respectively. For error reconstruction, COM4D_Hump (COM4D_2T) showed 81% (93%) at 2%/2mm and 94% (98%) at 3%/3mm, while OCTA4D_Hump (OCTA4D_2T) showed 96% (96%) at 2%/2mm and 99% (99%) at 3%/3mm. For error detection, OCTA3D doses were compared to COM4D_Hump (COM4D_2T) showing Υ passing rates of 93% (93%) at 2%/2mm and 98% (98%), and to OCTA4D_Hump (OCTA4D _2T) showing 94% (99%) at 2%/2mm and 81% (96%) at 3%/3mm, respectively. Conclusion: OCTAVIUS MGDR showed better agreement to raw measurements in both error- and error-free comparisons. COMPASS MGDR deviated from the raw measurements possibly owing to beam modeling uncertainty.

SU‐F‐T‐297: Quality Assurance of Multiple Brain Metastases with Single Isocenter Using Measurement Guided Dose Reconstruction

PURPOSE To evaluate the use of measurement guided dose reconstruction (MGDR) in quality assurance (QA) of stereotactic radiosurgery (SRS) of multiple brain metastases (MBM) planned with single isocenter (SI). METHODS Seven clinically approved multi-isocenter MBM (MI-MBM) plans were re-optimized using SI (SI-MBM) by coplanar volumetric arc therapy (VMAT) in Monaco v5.0 (Elekta CMS, Maryland Heights, MO, USA). These plans were delivered on an Elekta Agiltiy Linac and measured by OCTAVIUS 4D system with OCTAVIUS 1500 2D array (PTW, Freiburg, Germany). Measurements were repeated with a shift of the phantom by 5mm to double the detector resolution. 3D γ analysis in Verisoft 6.1 with 1.5mm/1.5%, 1.5mm/2% and 2mm/2% at local-dose passing criteria and 20% dose suppression were made. RESULTS 3D γ passing rates are 94.3±2.7%, 85.2±5.4% and 84±5.7% at 1.5mm/1.5%, 1.5mm/2% and 2mm/2% criteria. CONCLUSION MGDR of OCTAVIUS 4D system provides adequate and efficient VMAT QA solution for SI-MBM SRS. However, the results showed significant spatial dependence due to rapid dose fall-off in radiosurgery. Further improvement in detector spatial resolution is desired. PTV margins should be carefully adopted for those MBM treated with single isocenter.

SU-G-TeP2-10: Feasibility of Newly Designed Applicator for High Dose Rate Brachytherapy Treatment of Patients with Vaginal Vault Recurrence

PURPOSE To compare the dose of an in-house 3D-printed gynecology applicator (TMHGA) for vaginal vault recurrence of corpus cancer patients after operation for high dose rate brachytherapy treatment with commercially available applicators. METHODS A newly designed applicator is made from 3D-printing methods using ABSM30i. The isodose of the applicator is compared with Elekta multi-channel (MC) applicator and titanium Rotterdam applicator with coupling central tube and vaginal cylinder (RC). Three plans are created using three applicators in a CT set of water phantom. The applicators are anchored using the applicator library and implant library in the Elekta Oncentra treatment planning system (ver.4.5). The rectum is mimicked by creating a 2cm diameter cylinder, with a distance 1mm posteriorly away from the high risk CTV (HR-CTV). Similarly, the bladder is replicated by a 6cm diameter cylinder with distance 1mm anteriorly from the HR-CTV. Three plans are all normalized 1.5cm superior, 0.5cm anterior and 0.5cm posterior of the applicator surface. By fixing D90 of HR-CTV to 6Gy, the D2cc of rectum and bladder of three plans are compared. RESULTS The D2cc of the bladder for using TMHGA is lower than MC and RC by 14.0% and 11.9% respectively. While the D2cc of the rectum for using TMHGA is lower than MC and RC by 18.9% and 12.4% respectively. The total treatment time of TMHGA plan is shorter than MC and RC by 11.2% and 12.9%. CONCLUSION The applicator created via 3D printing delivers a lower dose to the bladder and the rectum while keeping the same coverage to HR-CTV as other commercially available applicators. Additionally, the new applicator resulted in a reduction of treatment time, which is always welcome.

SU-F-T-16: Experimental Determination of Ionization Chamber Correction Factors for In-Phantom Measurements of Reference Air Kerma Rate and Absorbed Water Dose Rate of Brachytherapy 192Ir Source

PURPOSE Following the method of in-phantom measurements of reference air kerma rate (Ka) at 100cm and absorbed water dose rate (Dw1) at 1cm of high-dose-rate 192Ir brachytherapy source using 60Co absorbed-dose-to-water calibrated (ND,w,60Co) ionization chamber (IC), we experimentally determined the in-phantom correction factors (kglob) of the PTW30013 (PTW, Freiburg, Germany) IC by comparing the Monte Carlo (MC)-calculated kglob of the other PTW30016 IC. METHODS The Dw1 formalism of in-phantom measurement is: M*ND,w,60Co*(kglob)Dw1, where M is the collected charges, and (kglob)Dw1 the in-phantom Dw1 correction factor. Similarly, Ka is determined by M*ND,w,60Co*(kglob)ka, where (kglob)ka the in-phantom Ka correction factor. Two thimble ICs PTW30013 and another PTW30016 having a ND,w,60Co from the German primary standard laboratory (PTB) were simultaneously exposed to the microselectron 192Ir v2 source at 8cm in a PMMA phantom. A reference well chamber (PTW33004) with a PTB transfer Ka calibration Nka was used for comparing the in-phantom measurements to derive the experimental (kglob)ka factors. We determined the experimental (kglob)Dw1 of the PTW30013 by comparing the PTW30016 measurements with MC-calculated (kglob)Dw1. RESULTS Ka results of the PTW30016 based on ND,w,60Co and MC-calculated (kglob)ka differ from the well chamber results based on Nka by 1.6% and from the manufacturer by 1.0%. Experimental (kglob)ka factors for the PTW30016 and two other PTW30013 are 0.00683, 0.00681 and 0.00679, and vary <0.5% with 1mm source positioning uncertainty. Experimental (kglob)Dw1 of the PTW30013 ICs are 75.3 and 75.6, and differ by 1.6% from the conversion by dose rate constant from the AAPM report 229. CONCLUSION The 1.7% difference between MC and experimental (kglob)ka for the PTW30016 IC is within the PTB 2.5% expanded uncertainty in Ka calibration standard. Using a single IC with ND,w,60Co to calibrate the brachytherapy source and dose output in external radiotherapy is feasible. MC validation of the PTW30013(kglob)Dw1 is warranted.

SU-F-T-377: Monte Carlo Re-Evaluation of Volumetric-Modulated Arc Plans of Advanced Stage Nasopharygeal Cancers Optimized with Convolution-Superposition Algorithm

BACKGROUND Commercial treatment planning system Pinnacle3 (Philips, Fitchburg, WI, USA) employs a convolution-superposition algorithm for volumetric-modulated arc radiotherapy (VMAT) optimization and dose calculation. Study of Monte Carlo (MC) dose recalculation of VMAT plans for advanced-stage nasopharyngeal cancers (NPC) is currently limited. METHODS Twenty-nine VMAT prescribed 70Gy, 60Gy, and 54Gy to the planning target volumes (PTVs) were included. These clinical plans achieved with a CS dose engine on Pinnacle3 v9.0 were recalculated by the Monaco TPS v5.0 (Elekta, Maryland Heights, MO, USA) with a XVMC-based MC dose engine. The MC virtual source model was built using the same measurement beam dataset as for the Pinnacle beam model. All MC recalculation were based on absorbed dose to medium in medium (Dm,m). Differences in dose constraint parameters per our institution protocol (Supplementary Table 1) were analyzed. RESULTS Only differences in maximum dose to left brachial plexus, left temporal lobe and PTV54Gy were found to be statistically insignificant (p> 0.05). Dosimetric differences of other tumor targets and normal organs are found in supplementary Table 1. Generally, doses outside the PTV in the normal organs are lower with MC than with CS. This is also true in the PTV54-70Gy doses but higher dose in the nasal cavity near the bone interfaces is consistently predicted by MC, possibly due to the increased backscattering of short-range scattered photons and the secondary electrons that is not properly modeled by the CS. The straight shoulders of the PTV dose volume histograms (DVH) initially resulted from the CS optimization are merely preserved after MC recalculation. CONCLUSION Significant dosimetric differences in VMAT NPC plans were observed between CS and MC calculations. Adjustments of the planning dose constraints to incorporate the physics differences from conventional CS algorithm should be made when VMAT optimization is carried out directly with MC dose engine.

SU-E-T-790: Validation of 4D Measurement-Guided Dose Reconstruction (MGDR) with OCTAVIUS 4D System

Purpose: To validate the MGDR of OCTAVIUS 4D system (PTW, Freiburg, Germany) for quality assurance (QA) of volumetric-modulated arc radiotherapy (VMAT). Methods: 4D-MGDR measurements were divided into two parts: 1) square fields from 2×2 to 25×25 cm2 at 0°, 10° and 45° gantry, and 2) 8 VMAT plans (5 nasopharyngeal and 3 prostate) collapsed to gantry 40° in QA mode in Monaco v5.0 (Elekta, CMS, Maryland Heights, MO) were delivered on the OCTAVIUS 4D phantom with the OCTAVIUS 1500 detector plane perpendicular to either the incident beam to obtain the reconstructed dose (OCTA4D) or the 0° gantry axis to obtain the raw doses (OCTA3D) in Verisoft 6.1 (PTW, Freiburg, Germany). Raw measurements of OCTA3D were limited to 0.5% variation of detector angular response with respect to 0° gantry as determined previously. Reconstructed OCTA4D and raw OCTA3D doses for all plans were compared at the same detector plane using γ criteria of 2% (local dose)/2mm and 3%/3mm criteria. Results: At gantry 0° and 10°, the γ results for all OCTA4D on detector plane coinciding with OCTA3D were over 90% at 2%/2mm except for the largest field (25×25 cm2 ) showing >88%. For square field at 45° gantry, γ passing rate is > 90% for fields smaller than 15x 15cm2 but < 80% for field size of 20 x20 cm2 upward. For VMAT, γ results showed 94% and 99% passing rate at 2%/2mm and 3%/3mm, respectively. Conclusion: OCTAVIUS 4D system has compromised accuracy in reconstructing dose away from the central beam axis, possibly due to the off-axis softening correction and errors of the percent depth dose data necessary as input for MGDR. Good results in VMAT delivery suggested that the system is relatively reliable for VMAT with small segments.

SU‐E‐T‐05: 4D Measurement‐Guided Dose Reconstruction (4D‐MGDR) in End‐End Quality Assurance (E2E QA) for Assessing Safety Margin in Radiosurgery (SRS) From Clinical Perspectives

Purpose: To assess the plan robustness and safety margin in SRS from 4DMGDR in E2E QA based on clinical objectives. Methods: OCTAVIUS SRS 1000 detector array and 4D phantom (PTW, Freiburg, Germany) were used to measure 5 coplanar SRS plans with 1 and 2 mm planning target volume (PTV). 3 targets were clinical, and 2 were virtual simulated to be 1mm from the brainstem (BS), and between chiasm (CS) and optic nerve (ON). Planning was done on Monaco v5.0 (Elekta, Maryland Heights, MO) to achieve 95–99% PTV and 100% gross tumor volume (GTV) prescription dose coverage. CBCT setup of the 4D phantom by 6D robotic couch was performed as for real patient. 4D-MGDR in patient CT and dosimetric analysis were performed in PTW Verisoft v6.1. The safety margin that achieved 100% GTV coverage was determined, and doses to 2% (D2%) of BS, ON and CS were assessed from E2E QA. Results: 100% GTV coverage was achieved with 1mm margin for 2 plans and 2mm margin for all plans. 98.3% and 99.4% GTV coverage were found in E2E QA for 1mm PTVs that either had sharp changing contour, or was nearby CS and ON or BS, and had either low planned minimum GTV dose (∼101% of the prescribed dose vs.∼106%) or compromised PTV coverage (95% vs. 99%). D2% to CS obtained with 4D-MGDR for one virtual target were 18.8Gy for 1mm PTV and 19.2Gy for 2mm PTV, exceeding the planned tolerance of 18Gy/3 fractions for prescription dose of 24Gy. Conclusion: 1mm margin is generally sufficient for dose planning and machine delivery errors. Irregular GTV with just enough dose coverage to spare critical organs may need 2mm margin at the costs of possible higher organ doses. 4D MGDR in an E2E QA approach can put the treatment plan evaluation in clinical perspectives.