Direct visualization and correlation of liver stereotactic body radiation therapy treatment delivery accuracy with interfractional motion.

This study used the visualization of hypo-intense regions on liver-specific MRI to directly quantify stereotactic body radiation therapy (SBRT) spatial delivery accuracy. Additionally, the interfractional motion of the liver region was determined and compared with the MRI-based evaluation of liver SBRT spatial treatment delivery accuracy. Primovist® -enhanced MRI scans were acquired from 17 patients, 8-12 weeks following the completion of liver SBRT treatment. Direct visualization of radiation-induced focal liver reaction in the form of hypo-intensity was determined. The auto-delineation approach was used to localize these regions, and center-of-mass (COM) discrepancy was quantified between the MRI hypo-intensity and the CT-based treatment plan. To assess the interfractional motion of the liver region, a planning CT was registered to a Cone Beam CT obtained before each treatment fraction. The interfractional motion assessed from this approach was then compared against the localized hypo-intense MRI regions. The mean ± SD COM discrepancy was 1.4 ± 1.3 mm in the left-right direction, 2.6 ± 1.8 mm in an anteroposterior direction, and 1.9 ± 2.6 mm in the craniocaudal direction. A high correlation was observed between interfractional motion of visualized hypo-intensity and interfractional motion of planning treatment volume (PTV); the quantified Pearson correlation coefficient was 0.96. The lack of correlation was observed between Primovist® MRI-based spatial accuracy and interfractional motion of the liver, where Pearson correlation coefficients ranged from -0.01 to -0.26. The highest random and systematic errors quantified from interfractional motion were in the craniocaudal direction. This work demonstrates a novel framework for the direct evaluation of liver SBRT spatial delivery accuracy.

Towards an objective evaluation of tolerances for beam modeling in a treatment planning system.

The performance of a convolution/superposition based treatment planning system depends on the ability of the dose calculation algorithm to accurately account for physical interactions taking place in the tissue, key components of the linac head and on the accuracy of the photon beam model. Generally the user has little or no control over the performance of the dose calculation algorithm but is responsible for the accuracy of the beam model within the constraints imposed by the system. This study explores the dosimetric impact of limitations in photon beam modeling accuracy on complex 3D clinical treatment plans. A total of 70 photon beam models was created in the Pinnacle treatment planning system. Two of the models served as references for 6 MV and 15 MV beams, while the rest were created by perturbing the reference models in order to produce specific deviations in specific regions of the calculated dose profiles (central axis and transverse). The beam models were then used to generate 3D plans on seven CT data sets each for four different treatment sites (breast and conformal prostate, lung and brain). The equivalent uniform doses (EUD) of the targets and the principal organs at risk (OARs) of all plans ( approximately 1000) were calculated and compared to the EUDs delivered by the reference beam models. In general, accurate dosimetry of the target is most greatly compromised by poor modeling of the central axis depth dose and the horns, while the EUDs of the OARs exhibited the greatest sensitivity to beam width accuracy. Based on the results of this analysis we suggest a set of tolerances to be met during commissioning of the beam models in a treatment planning system that are consistent in terms of clinical outcomes as predicted by the EUD.

Intensity modulated and three-dimensional conformal radiation therapy plans for oropharyngeal cancer: a comparison of their sensitivity to set-up errors and uncertainties.

We compared the effect of set-up error and uncertainty on two radiation therapy treatment plans for head and neck cancer: one using intensity modulated radiation therapy (IMRT) and one using conventional three-dimensional conformal radiation therapy (3D-CRT). We used a Pinnacle3 (Philips Medical Systems, Markham, Ontario) system to create the two treatment plans (7-beam IMRT and 5-beam 3D-CRT) for the same volumetric data set, based on the objectives and constraints defined in the Radiation Therapy Oncology Group H-0022 protocol. In both plans, the dose-volume constraints for the targets and the organs at risk (oars) were met as closely as the beam geometries would allow. Monte Carlo-based simulations of set-up error and uncertainty were performed in three orthogonal directions for 840 simulated "courses of treatment" for each plan. A systematic error (chosen from distributions characterized by standard deviations ranging from 0 mm to 6 mm) and random uncertainties (2 mm standard deviation) were incorporated. We used a probability approach to compare the sensitivities of the IMRT and the 3D-CRT plans to set-up error and uncertainty in terms of equivalent uniform dose (EUD) to targets and oars.Based on the EUD analysis, the targets and oars showed considerably greater sensitivity to set-up error with the IMRT plan than with the 3D-CRT plan. For the IMRT plan, target EUDS were reduced by 4%, 7.5%, and 10% for 2-mm, 4-mm, and 6-mm set-up errors respectively. However, even with set-up error, the mandible, spinal cord, and parotid EUDS always remained lower with the IMRT plan than with the 3D-CRT plan.We conclude that, when quantified by EUD, IMRT-plan doses to oars and targets are more sensitive to set-up error than are 3D-CRT-plan doses. However, as judged by the differences between target and OAR doses, IMRT retains its superiority over 3D-CRT, even in the presence of set-up error.

Use of novel fibre-coupled radioluminescence and RADPOS dosimetry systems for total scatter factor measurements in small fields.

A dosimetry system based on Al2O3:C radioluminescence (RL), and RADPOS, a novel 4D dosimetry system using microMOSFETs, were used to measure total scatter factors, (S(c,p))(f(clin))(det), for the CyberKnife robotic radiosugery system. New Monte Carlo calculated correction factors are presented and applied for the RL detector response for the 5, 7.5 and 10 mm collimators in order to correct for the detector geometry and increased photoelectric cross section of Al2O3:C relative to water. For comparison, measurements were also carried out using a micro MOSFET, PTW60012 diode and GAFCHROMIC(®) film (EBT and EBT2). Uncorrected (S(c,p))(f(clin))(det) were obtained by taking the ratio of the detector response for each collimator to that for the 60 mm diameter reference field. Published Monte Carlo calculated correction factors were applied to the RADPOS, microMOSFET and diode detector measurements to yield corrected field factors, Ω(f(clin),f(msr))(Q(clin),Q(msr)), following the terminology of a recent formalism introduced for small and composite field relative dosimetry. With corrections, the RL measured Ω(f(clin),f(msr))(Q(clin),Q(msr)) were 0.656 ± 0.002, 0.815 ± 0.002 and 0.865 ± 0.003 for the 5, 7.5 and 10 mm collimators, respectively. This was in good agreement with RADPOS corrected field factors of 0.650 ± 0.010, 0.816 ± 0.024 and 0.867 ± 0.010 for the 5, 7.5 and 10 mm collimators, respectively. Both RL and RADPOS total scatter factors agreed within approximately two standard deviations of the GAFCHROMIC film values (average of EBT and EBT2) of 0.640 ± 0.006, 0.806 ± 0.007 and 0.859 ± 0.09. Corrected total scatter factors for all dosimetry systems agreed within one standard deviation for collimator sizes 10-60 mm. Our study suggests that the microMOSFET/RADPOS and optical fibre-coupled RL dosimetry system are well suited for total scatter factor measurements over the entire range of field sizes, provided that appropriate correction factors are applied for the collimator diameters smaller than 10 mm.

Sci-Thur AM: Planning - 10: Improved dosimetric accuracy for patient specific quality assurance using a dual-detector measurement method for cyberknife output factors.

The measurement of output factors for small fields is challenging and can lead to large dose errors in patient treatments if corrections for detector size and scatter from high-Z material are not applied. Due to its high spatial resolution and near tissue equivalence, GAFCHROMIC® film potentially provides a correction free measure of output factors but it can be challenging to obtain high quality dosimetric results using this film. We propose minimizing errors in the clinical determination of small field output factors by employing diode measurements with Monte-Carlo generated corrections for small fields ≤10 mm diameter and using small volume ion chambers for apertures >10 mm diameter with independent validation using radiochromic film. We performed patient specific quality assurance (QA) measurements for 9 patients using GAFCHROMIC® film and an A16 small volume ion chamber in a head-shaped phantom, employing this hybrid dual detector method for relative output factor measurements within the Multiplan treatment planning system. Our results suggest that consistent output factors can be determined using this method with experimental verification using GAFCHROMIC® film dosimetry. For the patient specific QA using film, we achieve good dosimetric agreement (<2σ) of the measured and calculated average dose for pixels within the 80% isodose line. For patient specific QA using the micro-ion chamber, we get good agreement (<3%) for cone sizes greater than 5 mm. The differences observed for the 5 mm cone plans are consistent with a 1 mm radial setup uncertainty for patient positioning using the Cyberknife system.

Sci-Fri PM: Delivery - 07: Cyberknife relative output factor measurements using fiber-coupled luminescence, MOSFETS and RADPOS dosimetry system.

Novel dosimetry systems based on Al2 O3 :C radioluminescence (RL) and a 4D dosimetry system (RADPOS) from Best Medical Canada were used to measure the relative output factor (ROF) on Cyberknife. Measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm for cones from 5 to 60 mm. ROFs were also measured using a mobileMOSFET system (Best Medical Canada) and EBT1 and EBT2 GAFCHROMIC® (ISP, Ashland) radiochromic films. For cone sizes 12.5-60 mm all detector results were in agreement within the measurement uncertainty. The microMOSFET/RADPOS measurements (published corrections applied) yielded ROFs of 0.650 ± 1.9%, 0.811 ± 0.9% and 0.843 ± 1.7% for the 5, 7.5 and 10 mm cones, respectively, and were in excellent agreement with radiochromic film values (averaged for EBT1 and EBT2) of 0.645 ± 1.4%, 0.806 ± 1.1% and 0.859 ± 1.1%. Monte-Carlo calculated correction factors were applied to the RL readings to correct for excessive scatter due to the relatively high effective atomic number of Al2 O3 (Z=10.2) compared to water for the 5, 7.5 and 10 mm cones. When these corrections are applied to our RL detector measurements, we obtain ROFs of 0.656 ± 0.3% and 0.815 ± 0.3% and 0.865 ± 0.3% for 5, 7.5 and 10 mm cones. Our study shows that the microMOSFET/RADPOS and optical fiber-coupled RL dosimetry system are well suited for Cyberknife cone output factors measurements over the entire range of field sizes, provided that appropriate correction factors are applied for the smallest cone sizes (5, 7.5 and 10 mm).