Acuros® dose verification of ultrasmall lung lesions with EBT-XD film in a homogeneous and heterogeneous anthropomorphic phantom setup.

PURPOSE: Modern type 'c' dose calculation algorithms like Acuros® can predict dose for lung tumors larger than approximately 4 cm3 with a relative uncertainty up to 5%. However, increasingly better tumor diagnostics are leading to the detection of very small early-stage lung tumors that can be treated with stereotactic body radiotherapy (SBRT) for inoperable patients. This raises the question whether dose algorithms like Acuros® can still accurately predict dose within 5% for challenging conditions involving small treatment fields. Current recommendations for Quality Assurance (QA) and dose verification in SBRT treatments are to use phantoms that are as realistic as possible to the clinical situation, although water-equivalent phantoms are still largely used for dose verification. In this work we aim to demonstrate that existing dose verification methods are inadequate for accurate dose verification in very small lung tumors treated with SBRT. METHOD: The homogeneous PTW Octavius4D phantom with the Octavius 1000 SRS detector ("Octavius4D phantom") and the heterogeneous CIRS Dynamic Thorax phantom ('CIRS phantom') were used for dose measurements. The CIRS phantom contained different lung-equivalent film-holding cylindrical phantom inserts ("film inserts") with water-equivalent spherical targets with diameters 0.5, 0.75, 1, 2, and 3 cm. Plans were calculated for 6 and 10 MV for each spherical target in the CIRS phantom, resulting in 14 treatment plans. The plans were delivered to both Octavius4D and CIRS phantom to compare measured dose in a commonly used homogeneous and more realistic heterogeneous phantom setup. In addition, treatment plans of seven clinical lung cancer patients with lung tumors below approximately 1.0 cm3 were irradiated in the heterogeneous CIRS phantom. The actual tumor size within the clinical treatment plans determined the choice of the spherical target size, such that both measurement geometry and clinical target volumes match as closely as possible. The Acuros® dose algorithm (version 15.5.11) was used for all dose calculations reporting dose-to-medium using a 0.1-cm-grid size. RESULTS: The measurement discrepancies in the homogeneous Octavius4D phantom for the fourteen treatment plans were within 1.5%. Dose discrepancies between measurement and treatment planning systems (TPS) for the heterogeneous CIRS phantom increased for both 6 and 10 MV with decreasing target diameters up to 23.7 ± 1.0% for 6 MV and 8.8 ± 1.1% for 10 MV for the smallest target of 0.5 cm in diameter with a 2-mm-CTV-PTV margin. For the seven clinical plans this trend of increasing dose difference with decreasing tumor size is less pronounced although the smallest tumors show the largest differences between measurement and TPS up to 16.6 ± 0.9%. CONCLUSION: Current verification methods using homogenous phantoms are not adequate for lung tumors with diameters below approximately 0.75 cm. The current Acuros® dose calculation algorithm underestimates dose in very small lung tumors. Dose verification of small lung tumors should be performed in an anthropomorphic lung phantom incorporating a water-equivalent target that matches clinical tumor size as closely as possible.

The potential of an optical surface tracking system in non-coplanar single isocenter treatments of multiple brain metastases.

To evaluate the accuracy of a commercial optical surface tracking (OST) system and to demonstrate how it can be implemented to monitor patient positioning during non-coplanar single isocenter stereotactic treatments of brain metastases. A 3-camera OST system was used (Catalyst HD™, C-RAD) on a TruebeamSTx with a 6DoF couch. The setup accuracy and agreement between the OST system, and CBCT and kV-MV imaging at couch angles 0° and 270°, respectively, were examined. Film measurements at 3 depths in the Rando-Alderson phantom were performed using a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Setup of the phantom was performed with CBCT at couch 0° and subsequently monitored by OST at other couch angles. Setup data for 7 volunteers were collected to evaluate the accuracy and reproducibility of the OST system at couch angles 0°, 45°, 90°, 315°, and 270°. These results were also correlated to the couch rotation offsets obtained by a Winston-Lutz (WL) test. The Rando-Alderson phantom, as well as volunteers, were fixated using open face masks (Orfit). For repeated tests with the Rando-Alderson phantom, deviations between rotational and translational isocenter corrections for CBCT and OST systems are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the film and TPS doses in the center of the 4 lesions were -1.2%, -0.1%, -0.0%, and -1.9%. Local gamma evaluation criteria of 2%/2 mm and 3%/1 mm yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for the 4 lesions. Regarding the volunteers, the mean translational and rotational isocenter shift values were (0.24 ± 0.09) mm and (0.15 ± 0.07) degrees. Largest isocenter shifts were found for couch angles 45˚ and 90˚, confirmed by WL couch rotation offsets. Patient monitoring during non-coplanar VMAT treatments of brain metastases is feasible with submillimeter accuracy.

Should dose from small fields be limited for dose verification procedures?: uncertainty versus small field dose in VMAT treatments.

Over the years, radiotherapy treatments have become more complex and conformal, leading to an increased use of small field segments in volumetric modulated arc therapy (VMAT) arcs. The impact of small field dose inaccuracy on dose verification methods has not been studied yet. The aim of this work is therefore to quantify the relationship between the uncertainty of a 2D pre-treatment dose prediction model and the proportion of dose coming from small fields in VMAT arcs for a range of clinical plans. The model evaluated in this work predicts 2D portal dose images (PDIs) without a patient or phantom in the beam. The uncertainty of the model was calculated through simulation of model parameter deviations. The proportion of dose from small fields in a VMAT arc was determined by comparing a PDI with only dose from small fields with the original PDI. The uncertainty and proportion of dose from small fields were calculated for 109 VMAT arcs (41 head and neck, 33 lung, 35 prostate). The correlation was assessed with a linear regression. There is a statistically significant positive correlation between the uncertainty of the model and the proportion of dose from small fields in a VMAT arc, for each treatment site individually, as well as for all tumor sites together. The strongest relationship is found for the prostate cases. As there is a positive relationship between the uncertainty of the 2D pre-treatment dose prediction model, it may be wise to limit the dose from small fields in VMAT arcs, to avoid additional uncertainty in the dose verification process.

Influence of the jaw tracking technique on the dose calculation accuracy of small field VMAT plans.

PURPOSE: The aim of this study was to evaluate experimentally the accuracy of the dose calculation algorithm AcurosXB in small field highly modulated Volumetric Modulated Arc Therapy (VMAT). METHOD: The 1000SRS detector array inserted in the rotational Octavius 4D phantom (PTW) was used for 3D dose verification of VMAT treatments characterized by small to very small targets. Clinical treatment plans (n = 28) were recalculated on the phantom CT data set in the Eclipse TPS. All measurements were done on a Varian TrueBeamSTx, which can provide the jaw tracking technique (JTT). The effect of disabling the JTT, thereby fixing the jaws at static field size of 3 × 3 cm2 and applying the MLC to shape the smallest apertures, was investigated for static fields between 0.5 × 0.5-3 × 3 cm2 and for seven VMAT patients with small brain metastases. The dose calculation accuracy has been evaluated by comparing the measured and calculated dose outputs and dose distributions. The dosimetric agreement has been presented by a local gamma evaluation criterion of 2%/2 mm. RESULTS: Regarding the clinical plans, the mean ± SD of the volumetric gamma evaluation scores considering the dose levels for evaluation of 10%, 50%, 80% and 95% are (96.0 ± 6.9)%, (95.2 ± 6.8)%, (86.7 ± 14.8)% and (56.3 ± 42.3)% respectively. For the smallest field VMAT treatments, discrepancies between calculated and measured doses up to 16% are obtained. The difference between the 1000SRS central chamber measurements compared to the calculated dose outputs for static fields 3 × 3, 2 × 2, 1 × 1 and 0.5 × 0.5 cm2 collimated with MLC whereby jaws are fixed at 3 × 3 cm2 and for static fields shaped with the collimator jaws only (MLC retracted), is on average respectively, 0.2%, 0.8%, 6.8%, 5.7% (6 MV) and 0.1%, 1.3%, 11.7%, 21.6% (10 MV). For the seven brain mets patients was found that the smaller the target volumes, the higher the improvement in agreement between measured and calculated doses after disabling the JTT. CONCLUSION: Fixing the jaws at 3 × 3 cm2 and using the MLC with high positional accuracy to shape the smallest apertures in contrast to the JTT is currently found to be the most accurate treatment technique.