Improved Dosimetry with Daily Online Adaptive Radiotherapy for Cervical Cancer: Waltzing the Pear.

AIMS: Standard of care radiotherapy for locally advanced cervical cancer includes large margins to ensure the uterocervix remains within the treatment fields over the course of treatment. Daily online cone-beam adaptive radiotherapy corrects for interfractional changes by adjusting the plan to match the target position during each treatment session, thus allowing for significantly reduced clinical target volume (CTV) to planning target volume (PTV) margins. We hypothesise that reduced margins from daily online adaptive radiotherapy will reduce organ at risk dose without compromising target coverage. MATERIALS AND METHODS: Ten patients with cervical cancer (stage IIB-IIIC2) were treated with definitive chemoradiation using daily online cone-beam adaptive radiotherapy in 25-27 fractions. Initial and all adapted treatment plans were generated with CTV to PTV margins versus standard of care image-guided radiotherapy (IGRT) plans as follows: cervix/uterus/gross tumour volume (0.5 versus 1.5 cm), parametria/vagina (0.5 versus 1.0 cm) and nodal chains and gross nodes (0.5 versus 0.5 cm). IGRT plans were created and copied to synthetic computed tomography scans and contours generated from each daily adapted fraction. The dosimetry of each clinically treated online adapted fraction was compared with emulated IGRT plans. Statistical significance was defined as P < 0.05. RESULTS: Daily online cone-beam adaptive radiotherapy significantly improves bowel bag dosimetry compared with IGRT, with a reduction in V40 by an average of 91.3 cm3 [V40 (-6.2%) and V45 (-6.1%)]. The daily adapted plans showed significant improvements in bladder and rectum [V40 (-25.2% and -36.0%) and V30 (-9.7% and -17.1%), respectively]. Additionally, bone marrow had a significantly reduced dose [V10 (-2.7%) and V20 (-3.3%)]. Daily online cone-beam adaptive radiotherapy improved uterocervix CTV coverage and reduced hotspots compared with IGRT [D95% (+1.6%) and Dmax (-0.9%)]. CONCLUSIONS: Reduced CTV to PTV margins achievable with daily online adaptive radiotherapy improves organ at risk dosimetry and target coverage when compared with standard of care IGRT for locally advanced cervical cancer. The clinical impact of improved dosimetry is currently undergoing investigation.

Monte Carlo dose calculations for high-dose-rate brachytherapy using GPU-accelerated processing.

PURPOSE: Current clinical brachytherapy dose calculations are typically based on the Association of American Physicists in Medicine Task Group report 43 (TG-43) guidelines, which approximate patient geometry as an infinitely large water phantom. This ignores patient and applicator geometries and heterogeneities, causing dosimetric errors. Although Monte Carlo (MC) dose calculation is commonly recognized as the most accurate method, its associated long computational time is a major bottleneck for routine clinical applications. This article presents our recent developments of a fast MC dose calculation package for high-dose-rate (HDR) brachytherapy, gBMC, built on a graphics processing unit (GPU) platform. METHODS AND MATERIALS: gBMC-simulated photon transport in voxelized geometry with physics in (192)Ir HDR brachytherapy energy range considered. A phase-space file was used as a source model. GPU-based parallel computation was used to simultaneously transport multiple photons, one on a GPU thread. We validated gBMC by comparing the dose calculation results in water with that computed TG-43. We also studied heterogeneous phantom cases and a patient case and compared gBMC results with Acuros BV results. RESULTS: Radial dose function in water calculated by gBMC showed <0.6% relative difference from that of the TG-43 data. Difference in anisotropy function was <1%. In two heterogeneous slab phantoms and one shielded cylinder applicator case, average dose discrepancy between gBMC and Acuros BV was <0.87%. For a tandem and ovoid patient case, good agreement between gBMC and Acruos BV results was observed in both isodose lines and dose-volume histograms. In terms of the efficiency, it took ∼47.5 seconds for gBMC to reach 0.15% statistical uncertainty within the 5% isodose line for the patient case. CONCLUSIONS: The accuracy and efficiency of a new GPU-based MC dose calculation package, gBMC, for HDR brachytherapy make it attractive for clinical applications.

SU-E-T-414: Clinical Implementation of Anthropomorphic Lung Phantom for Patient-Specific SAbR QA.

PURPOSE: Dose calculations for lung stereotactic ablative radiotherapy (SAbR) are challenged by the presence of extremely heterogeneous tissue and small treatment volumes. In this work, an anthropomorphic chest phantom has been constructed for the purpose of commissioning treatment planning systems (TPS) and for patient-specific SAbR QA. METHODS: A CT scan of a realistic chest phantom containing tissue equivalent materials for the spine, ribs, and lungs was imported into a Pinnacle TPS (CCC dose algorithm) where treatment plans were created for right and left-sided lung lesions. The phantom lungs are unique in that they contain embedded unit density spherical targets (2 and 4cm in diameter) that represent lung lesions. Plans directed to both tumors were designed for PTVs ranging from 2-5cm in diameter using 6, 10, and 15MV beams while passing RTOG 0813 dose and conformality criteria. Each plan was then exported to an Eclipse TPS (AAA dose algorithm) for dose calculation. Plans were delivered with a TrueBeam LINAC corrected for machine output. Point dose measurementswere verified with a 0.015 cc air ionization chamber placed in the center of each tumor. RESULTS: While the majority of plans developed in Pinnacle passed conformality criteria, the dose distribution as calculated in Eclipse failed to meet the RTOG guidelines, particularly for the small tumor at higher photon energies. All point dose measurements matched both TPS within 4%. Both TPS calculated a lower point dose than measured for large PTVs at all energies, improving as PTV size decreased. CONCLUSIONS: After comparing TPS and validating calculations with point dose measurements, the phantom was clinically implemented for patient-specific conformal SAbR QA.