Dosimetry Modeling for Focal LDR Prostate Brachytherapy

S150

International Journal of Radiation Oncology  Biology  Physics

Radiological Technology, Kanazawa University Hospital, Kanazawa City, Japan

Digital Poster Abstract 1007; Table

Purpose/Objective(s): Intensity modulated radiation therapy (IMRT) is not widely used for the treatment of tumors that can move with respiration. The goal of this study was to establish a new irradiation method to accurately deliver radiation therapy to those types of tumors using breath holding volumetric modulated radiation therapy (VMAT) based on fiducial marks. Materials/Methods: Patients with lung or upper abdominal cancers were eligible for this treatment. The fiducial markers used in this treatment were gold markers for lung cancer, visicoils for hepatic cancer, metallic stents for cholangiocarcinoma, and embolization coils for pancreatic cancer. A VMAT plan using Monte Carlo dose calculation algorithm was established based on data from computerized tomography (CT) imaging during breath holding. Patients were treated using image guided radiation therapy (IGRT) functions as follows: (1) set-up patients with bone match using cone beam CT, (2) fluoroscopic observation of fiducial markers under free breathing using the fluoroscopic IGRT function, (3) breath hold when the fiducial markers matched the location of those on the reference digitally reconstructed radiograph (DRR) image established from the planning CT, and (4) beam on during the possible breath holding time with repeat of this procedure until the end of the fraction. The feasibility and utility of this irradiation method was assessed. Results: From May 2012, 13 patients (lung cancer, n Z 2; hepatocellular carcinomas, n Z 2; cholangiocarcinoma, n Z 1; liver metastases, n Z 3; pancreatic cancers, n Z 3; hilar cholangiocarcinoma, n Z 1; pancreatic head lymph node metastasis, n Z 1) were treated with this irradiation method. Fiducial markers described above suitable for each lesion were safely and prospectively implanted in all patients, except for in two patients in whom embolization coils had already been safely inserted. The fiducial markers were clearly visible, and the delivery of VMAT under several breath holds for each fraction with exact matching of the fiducial markers with the location of those on the DRR image using the fluoroscopic IGRT function was successful in all patients. The irradiation doses were 50-61.8 Gy/25-30 conventional fractions to 10 abdominal lesions and 50-66 Gy/5-11 stereotactic fractions to two lung cancers and one liver metastasis. No severe complications were seen throughout the radiation procedures in any patient; this was also true for the pancreatic cancer patients despite the fact that somewhat larger irradiation dose than usual were delivered via VMAT. Conclusions: This new VMAT irradiation method enables accurate and rapid delivery of radiation for tumors that move with respiration, including lung cancers and upper abdominal cancers. Author Disclosure: T. Takanaka: None. T. Kumano: None. T. Minami: None. W. Koda: None. O. Matsui: None. K. Noto: None. S. Ueda: None. Y. Kurata: None.

Static 0 Static 90 Step and Shoot at 0 , 45 , and 90 Convention arc 0 -90 Conical arc 45

1007 Dosimetric Verification of the Small Animal Radiation Research Platform (SARRP) Treatment Planning System (TPS) in Mice H. Korideck,1 W. Ngwa,1,2 R. Kumar,3 M. Makrigiorgos,1,2 and R. Berbeco1,2; 1Dana-Farber Cancer Institute/Harvard Medical School, Boston, MA, 2Brigham and Women’s Hospital/Harvard Medical School, Boston, MA, 3Northeastern University, Boston, MA Purpose/Objective(s): The Small Animal Radiation Research Platform (SARRP) enables state-of-the-art image guided radiation therapy (RT) research to be performed in animal models. In this work, treatment planning accuracy was verified in mice for various RT treatment delivery modes using the SARRP treatment planning system (TPS). Materials/Methods: Calibrated micro-MOSFET dosimeters were surgically implanted post-euthanasia, by a trained surgeon, into mouse brain, kidney and Lung with minimal organ disruption. After implanting the MOSFET, the mouse was CT-imaged with the SARRP. The CT images were exported to the TPS, and the micro-MOSFET was selected as the

Delivery mode

Comparison with expected values (%) 0.4 1.8 9.7 4.5 1.0

isocenter (target) to receive 100% of the dose. The TPS was then used to generate a treatment plan which provided the ‘beam on’ time (s) for treatment delivery. The target was treated as planned; dose measurements by the MOSFETs were compared to planned (calculated) doses by the TPS for the following treatment delivery modes: static delivery at gantry (0 and 90 ), Step and Shoot delivery (0 , 45 , and 90 ), conventional arc delivery, and conical arc delivery. Results: For micro-MOSFETS implanted in the lung, measured and calculated doses agreed to within  2% for the static and conical arc delivery modes, within 4% for the conventional arc, and within 10% for the step and shoot delivery modes. Higher discrepancies for the step and shoot mode could partly be attributed to treatment time rounding errors when the treatment time is split amongst the different beam delivery angles. The impact of these errors is minimized for treatment at higher doses. Agreement for sample measurements at other anatomical locations: brain and kidney was also within 10%. Conclusions: The SARRP TPS is designed to enable researchers to replicate their clinical techniques, allow for image fusion with other imaging modalities, and provide dose computation and graphical visualization of treatment plans consisting of multiple x-ray beams and conformable arcs. The preliminary results obtained suggest that the TPS calculates dose to 10% or better accuracy in conditions simulating treatment planning for typical tumor models. Author Disclosure: H. Korideck: None. W. Ngwa: None. R. Kumar: None. M. Makrigiorgos: None. R. Berbeco: None.

1008 Dosimetry Modeling for Focal LDR Prostate Brachytherapy B. Al-Qaisieh,1 J. Mason,1 A. Henry,1 P. Bownes,1 L. Dickinson,2 A. Hashim,2 M. Emberton,3 and S. Langley4; 1Leeds Teaching Hospitals NHS, Leeds, United Kingdom, 2University College London Hospital, London, United Kingdom, 3University College London Hospital, London, United Kingdom, 4St Luke’s Cancer Centre, Guildford, United Kingdom Purpose/Objective(s): Following the report of a consensus meeting on focal prostate brachytherapy, it was agreed to retrospectively model treatment planning techniques. Focal LDR treatment plans targeting discrete lesion(s) identified on prostate multi-parametric MRI and transperineal template prostate mapping biopsy (TPM) was compared with standard whole prostate LDR planning. The robustness of treatment plans against a range of random and systematic seed movement was evaluated and compared with standard prostate LDR planning. Inter seed attenuation (ISA) was measured and its effect on the quality of implant was assessed. Materials/Methods: Nine were selected to be suitable for this study according to their PSA, Gleason score, and concordance of TPM and mpMRI for the location of the cancer lesion. MRI T2 and diffusionweighted images were manually manipulated to match patient’s position as in treatment and treatment planning setup. Imaging template biopsy results were used to contour target volumes and organs at risk on the treatment planning system. Three treatment plans were produced for each case: standard whole prostate, hemi and ultra focal as recommended by the consensus report. Six thousand seven hundred eleven seed type was selected with source strength of 0.5 U and a dose prescription of 145 Gy. GEC-ESTRO recommendations were followed to meet dose constraints.

Volume 87  Number 2S  Supplement 2013 DVHs were generated and compared. Monte Carlo calculations were performed to simulate ISA and to verify plan robustness. Results: On average the total number of seeds compared to the standard plan is 32% and 70% less for hemi and ultra focal plans respectively, whereas the average needles were 38% and 58% less for hemi and ultra focal respectively. Target volume coverage was adequately achieved for all plans with V100 above 97.8%. D90 was increased by 19% and 69% for hemi and ultra focal plans respectively. Meanwhile, D10 of urethral dose was reduced by 7% and 55% for hemi and ultra focal plans respectively. D2cc of rectum was reduced by 28% and 60% for hemi and ultra focal respectively. Plan robustness shows that a random shift in seed position of 4 mm would result in V100 and D90 dropping by 7.2% and 14%, respectively, for standard plans, with a corresponding reduction of 7.3% and 20% for hemi plans and 5% and 32% for ultra focal plans. Despite the fact that seed density is higher for the hemi and ultra focal approaches, ISA has similar effect on DHV parameters. For example, V100 and D90 are reduced by less than 1% and 3%, respectively, for all types of plans. Conclusions: Treatment planning for hemi and ultra focal options is feasible. Dose constraints are easily met with a notable reduction to organs at risk such as the bladder, urethra and rectum. Treating smaller targets, such as for ultra focal, makes seed positioning more critical. This should be taken into consideration during treatment planning. ISA has the same effect on DVH calculations for standard, hemi and ultra focal plans. Author Disclosure: B. Al-Qaisieh: G. Consultant; GE Oncura Ltd. J. Mason: None. A. Henry: None. P. Bownes: None. L. Dickinson: None. A. Hashim: None. M. Emberton: None. S. Langley: None.

1009 Toward Customizable Radiation Therapy Enhancement (CuRE) With Gold Nanoparticles Released, In Situ, From Gold-Loaded Brachytherapy Spacers W. Ngwa,1 H. Korideck,2 R. Kumar,2,3 S. Sridhar,4,3 K. David,1 N. Paul,1 R. Berbeco,1 R. Cormack,1 and G. Makrigiorgos1; 1Dana-Farber Cancer Institute, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, 2Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA, 3Northeastern University, Boston, MA, 4Harvard Medical School, Boston, MA Purpose/Objective(s): Loading routinely used brachytherapy spacers with non-toxic/biocompatible gold nanoparticles (AuNP), which can be released in situ to serve as radiosensitizers is a novel potential strategy for prostate tumor sub-volume boosting or dose-painting. This study investigates the release and diffusion of the AuNP from the brachytherapy spacer in vivo using CT imaging. Materials/Methods: Gold-loaded brachytherapy spacers (GBS) (ca. 0.8 mm x 5 mm) were produced using AuNP and biodegradable PLGA copolymer. The GBS was implanted in mice using a brachytherapy needle and CBCT-imaged over time using the small animal radiation research platform (SARRP). The CT images were exported to imageJ and the intensity of the implanted spacer evaluated at different time points. Also, to simulate a burst release of the AuNP from an implanted spacer, 20 mg Au/ mL concentration of 15 nm polymer coated AuNP suspended in 20 mL phosphate-buffered saline at pH 7.4, was injected intratumorally, and the AuNP diffusion monitored via serial CT imaging over 5 time points in hours: 0.5, 48, 120, 144, and 160. The CT images were then exported to imageJ and the pixel intensities for 5 sample tumor regions of interest (at: left, right, center, anterior, posterior) extracted to assess AuNP diffusion. Results: Results for the implanted GBS showed that the CT intensity of the spacer decreased over time, indicating the release of the AuNP as the PLGA degrades in vivo. Meanwhile, the CBCT images from the AuNP burst release study showed clear evidence of AuNP diffusion. For example, at the site of AuNP release, the CT image intensity decreased by over 160% during the investigated time range (0.5 h e 160 h). During the same time, the intensity in the tumor subvolume or ROI on the opposite side of the release site increased by over 140%. The diffusion results also showed

Digital Poster Discussion Abstracts S151 that for the 15 nm AuNP size, potent AuNP concentrations can reside within the tumor sub-volume for a sustained period of time. Conclusions: The results provide the first in vivo evidence of AuNP release from gold loaded brachytherapy spacers. The sustained residence of potent AuNP concentrations in the tumor sub-volume could allow for significant dose boosting during brachytherapy. Such boosting could be further customized (e.g., by varying AuNP size, functionalization, spacer degradation time, etc.) since the intra-tumor biodistribution depends on such parameters. Overall, the results provide a useful basis for future R&D towards the development of Customizable Radiation therapy Enhancement (CuRE) with AuNP for prostate cancer. Potential clinical applications for such a new approach are anticipated in salvage brachytherapy, and subvolume radiation boosting during initial treatment to help prevent prostate cancer recurrence. Author Disclosure: W. Ngwa: None. H. Korideck: None. R. Kumar: None. S. Sridhar: None. K. David: None. N. Paul: None. R. Berbeco: None. R. Cormack: None. G. Makrigiorgos: None.

1010 Clinical Implementation of a Comprehensive EPID-Based 3D/4D Patient Dose Reconstruction Framework for Complex Treatment Validations M. Lin, L. Jinsheng, and C.M. Charlie Ma; Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective(s): A program was developed and clinically implemented to reconstruct 3D/4D patient dose distributions utilizing on-line measured EPID transmission images to enable DVH-based dose validation for complex delivery techniques including VMAT and IMRT. Materials/Methods: The program reconstructs the entrance intensity map using the MLC apertures continuously measured by the EPID during the treatment and the corresponding monitor unit (MU) fractions recorded in the DynaLog file. The patient dose is calculated based on the reconstructed entrance intensity map and patient’s CT images using a Monte Carlo dose calculation engine and realistic linac beam data. For patients with fiducial markers, the program is capable of extracting the translational motion information and incorporating it in the dose reconstruction process. Isodose-based and DVH-based dose comparisons can then be performed after the DICOM dose and structure files are imported from the treatment planning system. This method holds under the assumption that MLC apertures can be reconstructed accurately using the measured EPID transit images. The feasibility of utilizing transit images for the MLC aperture detection and intensity map reconstruction was evaluated with and without an anthropomorphic phantom in the beam for various MLC patterns covering an effective field size of 1 x 1 cm2 to the largest detectable field size of the EPID. The accuracy of 3D and 4D dose reconstruction was then validated with a cylindrical diode array for ten IMRT and ten VMAT treatment plans covering PTV sizes from 9.2 to 1453 cm3. Furthermore, two lung patients were used to demonstrate the 4D dose reconstruction with the proposed method. Results: The geometrical accuracy of the MLC aperture detection using the EPID transit images is within 1.4 mm. Better than 98.5% gamma passing rate (2%/2 mm) is achieved when the entrance intensity map was compared with the EPID in-air measured fluence map. These results demonstrated that the photon scattering from the patient body has no significant effects on the fluence and dose reconstructions. The reconstructed 3D dose shows at least a 97.6% gamma passing rate (3%/3 mm) for the peripheral dose. The reconstructed dose distributions for moving targets are also consistent with the measured ones. For the patient dose reconstruction, differences in the target dose coverage between measured and planned dose distributions were observed for cases in which a significant tumor movement occurred during the treatment. Conclusions: On-line measured EPID transit cine-images together with the delivered MUs can be used for the entrance fluence reconstruction and subsequent fractional dose calculation for both small and large lesions