We compared two intensity-modulated radiotherapy techniques for left-sided breast treatment, involving lymph node irradiation including the internal mammary chain. Inverse planned arc-therapy (VMAT) was compared with a forward-planned multi-segment technique with a mono-isocenter (MONOISO). Ten files were planned per technique, delivering a 50-Gy dose to the breast and 46.95 Gy to nodes, within 25 fractions. Comparative endpoints were planning target volume (PTV) coverage, dose to surrounding structures, and treatment delivery time. PTV coverage, homogeneity and conformality were better for two arc VMAT plans; V95%PTV-T was 96% for VMAT vs 89.2% for MONOISO. Homogeneity index (HI)PTV-T was 0.1 and HIPTV-N was 0.1 for VMAT vs 0.6 and 0.5 for MONOISO. Treatment delivery time was reduced by a factor of two using VMAT relative to MONOISO (84 s vs 180 s). High doses to organs at risk were reduced (V30left lung = 14% using VMAT vs 24.4% with MONOISO; dose to 2% of the volume (D2%)heart = 26.1 Gy vs 32 Gy), especially to the left coronary artery (LCA) (D2%LCA = 34.4 Gy vs 40.3 Gy). However, VMAT delivered low doses to a larger volume, including contralateral organs (mean dose [Dmean]right lung = 4 Gy and Dmeanright breast = 3.2 Gy). These were better protected using MONOISO plans (Dmeanright lung = 0.8 Gy and Dmeanright breast = 0.4 Gy). VMAT improved PTV coverage and dose homogeneity, but clinical benefits remain unclear. Decreased dose exposure to the LCA may be clinically relevant. VMAT could be used for complex treatments that are difficult with conventional techniques. Patient age should be considered because of uncertainties concerning secondary malignancies.
PurposeThis study evaluates the benefit of a virtual bolus method for volumetric modulated arc therapy (VMAT) plan optimization to compensate breast modifications that may occur during breast treatment.MethodsTen files were replanned with VMAT giving 50 Gy to the breast and 47 Gy to the nodes within 25 fractions. The planning process used a virtual bolus for the first optimization, then the monitors units were reoptimized without bolus, after fixing the segments shapes. Structures and treatment planning were exported on a second scanner (CT) performed during treatment as a consequence to modifications in patient's anatomy. The comparative end‐point was clinical target volume's coverage. The first analysis compared the VMAT plans made using the virtual bolus method (VB‐VMAT) to the plans without using it (NoVB‐VMAT) on the first simulation CT. Then, the same analysis was performed on the second CT. Finally, the level of degradation of target volume coverage between the two CT using VB‐VMAT was compared to results using a standard technique of forward‐planned multisegment technique (Tan‐IMRT).ResultsUsing a virtual bolus for VMAT does not degrade dosimetric results on the first CT. No significant result in favor of the NoVB‐VMAT plans was noted. The VB‐VMAT method led to significant better dose distribution on a second CT with modified anatomies compared to NoVB‐VMAT. The clinical target volume's coverage by 95% (V95%) of the prescribed dose was 98.9% [96.1–99.6] on the second CT for VB‐VMAT compared to 92.6% [85.2–97.7] for NoVB‐VMAT (P = 0.0002). The degradation of the target volume coverage for VB‐VMAT is not worse than for Tan‐IMRT: the median differential of V95% between the two CT was 0.9% for VMAT and 0.7% for Tan‐IMRT (P = 1).ConclusionThis study confirms the safety and benefit of using a virtual bolus during the VMAT planning process to compensate potential breast shape modifications.
Recent conventional medical linear particle accelerators (linacs) come with the capacity to deliver flattening filter free (FFF) beams, allowing higher dose rates. Intensity modulated radiation therapy (IMRT) and small-fields stereotactic techniques no longer seem to require flattened beams. To implement such beams, it is important to correctly measure and model the dose distribution they deliver. The objective of this work was to model the recent VERSA-HD (Elekta®) FFF beams using RayStation® TPS, as done externally on Monaco TPS. There exists no publication on the modeling of this machine in FFF mode. Also, no published work relates to the use of RayStation® to model unflattened beams. Flattened X6 and unflattened 6FFF and 10FFF beams have been modeled. Lateral and depth dose profiles as well as output factors were measured using ionization chambers and diodes detectors. Models were created in RayStation® and Monaco® treatment planning systems (TPS), offering analytically-and Monte Carlo-algorithms, respectively. Surface dose, out-of-field dose and collimator transmission were also measured to improve models. All steps and parameters used during the modeling process are presented, along with measured and calculated dose profiles. Agreement between measurements and models was assessed by one-dimensional gamma analysis. Stereotactic and VMAT (volumetric modulated arc therapy) treatment plans were controlled by measuring isocenter absolute dose and dose distribution with the ArcCheck device. The impacts of the model parameters on each region of the depth and lateral dose profiles were explained. FFF beams models exhibited different energy spectra, smaller source sizes and lower electron contamination compared to flattened beams, in both TPS. Monaco® and RayStation® exhibit good agreement with depth and lateral dose profile measurements, 1% and 1 mm criteria being respected in the high dose regions. Stereotactic and VMAT treatment plans measured led to good agreement with calculation.
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