On the beam hardening correction of the Transit-Guided Radiation Therapy attenuation model.

PURPOSE: The Transit-Guided Radiation Therapy (TGRT) technique is a novel technique aimed to quantify the position error of a patient by using the transit portal images (TPI) of the treatment fields. Despite of the promising preliminary results, about 4% of the cases would have led to position overcorrections. In this work, the TGRT formalism is refined to improve its accuracy and, especially, to decrease the risk of overcorrections. METHODS: A second free parameter accounting for beam hardening has been added to the attenuation model of the TGRT formalism. Five treatment plans combining different delivery techniques and tumour sites have been delivered to an anthropomorphic phantom. TPIs have been obtained under a set of random couch shifts for each field. For each TPI, both the original and the refined TGRT formalism have been used to estimate the underlying true shift. RESULTS: With respect the original formalism, the refined formalism: (i) decreased both the number (from 5% to 1%) and the magnitude of the overcorrections; (ii) lowered the detection threshold (from approximately 1 mm to <0.3 mm); (iii) largely improved the accuracy in tumour sites with large mass thickness variations; and (iv) largely improved the accuracy for true shifts below 5 mm. For true shifts above 5 mm, the accuracy was slightly impaired. CONCLUSIONS: The refined TGRT formalism performed globally better than the original TGRT formalism and it largely reduced the risk of overcorrections. Further refinements of the TGRT formalism should focus on true shifts above 5 mm.

Transit-guided radiation therapy: proof of concept of an on-line technique for correcting position errors using transit portal images.

Objective.Transitin vivodosimetry methods monitor that the dose distribution is delivered as planned. However, they have a limited ability to identify and to quantify the cause of a given disagreement, especially those caused by position errors. This paper describes a proof of concept of a simplein vivotechnique to infer a position error from a transit portal image (TPI).Approach.For a given treatment field, the impact of a position error is modeled as a perturbation of the corresponding reference (unperturbed) TPI. The perturbation model determines the patient translation, described by a shift vector, by comparing a givenin vivoTPI to the corresponding reference TPI. Patient rotations can also be determined by applying this formalism to independent regions of interest over the patient. Eight treatment plans have been delivered to an anthropomorphic phantom under a large set of couch shifts (<15 mm) and rotations (<10°) to experimentally validate this technique, which we have named Transit-Guided Radiation Therapy (TGRT).Main results.The root mean squared error (RMSE) between the determined and the true shift magnitudes was 1.0/2.4/4.9 mm for true shifts ranging between 0-5/5-10/10-15 mm, respectively. The angular accuracy of the determined shift directions was 12° ± 14°. The RMSE between the determined and the true rotations was 0.5°. The TGRT technique decoupled translations and rotations satisfactorily. In 96% of the cases, the TGRT technique decreased the existing position error. The detection threshold of the TGRT technique was around 1 mm and it was nearly independent of the tumor site, delivery technique, beam energy or patient thickness.Significance.TGRT is a promising technique that not only provides reliable determinations of the position errors without increasing the required equipment, acquisition time or patient dose, but it also adds on-line correction capabilities to existing methods currently using TPIs.

Pseudo skin flash on VMAT in breast radiotherapy: Optimization of virtual bolus thickness and HU values.

PURPOSE: Optimisation strategies for volumetric modulated arc therapy (VMAT) in most treatment planning systems for breast cancer do not account for patient positioning, breathing, or anatomical changes. To overcome this limitation, a pseudo-skin flash strategy using a virtual bolus has been proposed. Using this strategy, we determined optimal thickness and value of Hounsfield units (HU) assigned to the virtual bolus to ensure adequate CTV irradiation. MATERIALS AND METHODS: We modified the original computed tomography data (CT0) by adding combinations of thicknesses and densities of a virtual bolus on PTVs (CT') of seven bilateral breast cancer patients. Using a single optimization objective template, we obtained a VMAT plan on CT' and recalculated this on the CT0. Optimal CT' parameters were defined as those that minimized dose differences between CT' and CT0 plans regarding PTV and OAR dose-volume parameters. We studied bolus parameters regarding robustness by shifting the isocenter 5 and 10 mm in the breathing direction for each CT0 plan. RESULTS: The minimal dosimetric impact was between -400 and -600 HU depending on bolus thickness. OARs doses were not significantly affected. Best robustness was found for -500 HU and 15 mm bolus thickness against shifts of up to 10 mm in the breathing direction. CONCLUSION: Our results support a bolus thickness equal to the CTV-PTV margin plus 5 mm and a virtual bolus HU value around -500 and -400 depending on the bolus thickness chosen. These findings could play a useful role in maximisingrobustness and minimising the need for plan renormalization.