Part 2: Dynamic mixed beam radiotherapy (DYMBER): Photon dynamic trajectories combined with modulated electron beams

PURPOSE

The purpose of this study was to develop a treatment technique for dynamic mixed beam radiotherapy (DYMBER) utilizing increased degrees of freedom (DoF) of a conventional treatment unit including different particle types (photons and electrons), intensity and energy modulation and dynamic gantry, table, and collimator rotations.

METHODS

A treatment planning process has been developed to create DYMBER plans combining photon dynamic trajectories (DTs) and step and shoot electron apertures collimated with the photon multileaf collimator (pMLC). A gantry-table path is determined for the photon DTs with minimized overlap of the organs at risk (OARs) with the target. In addition, an associated dynamic collimator rotation is established with minimized area between the pMLC leaves and the target contour. pMLC sequences of photon DTs and electron pMLC apertures are then simultaneously optimized using direct aperture optimization (DAO). Subsequently, the final dose distribution of the electron pMLC apertures is calculated using the Swiss Monte Carlo Plan (SMCP). The pMLC sequences of the photon DTs are then re-optimized with a finer control point resolution and with the final electron dose distribution taken into account. Afterwards, the final photon dose distribution is calculated also using the SMCP and summed together with the one of the electrons. This process is applied for a brain and two head and neck cases. The resulting DYMBER dose distributions are compared to those of dynamic trajectory radiotherapy (DTRT) plans consisting only of photon DTs and clinically applied VMAT plans. Furthermore, the deliverability of the DYMBER plans is verified in terms of dosimetric accuracy, delivery time and collision avoidance. For this purpose, The DYMBER plans are delivered to Gafchromic EBT3 films placed in an anthropomorphic head phantom on a Varian TrueBeam linear accelerator.

RESULTS

For each case, the dose homogeneity in the target is similar or better for DYMBER compared to DTRT and VMAT. Averaged over all three cases, the mean dose to the parallel OARs is 16% and 28% lower, D2% to the serial OARs is 17% and 37% lower and V10% to normal tissue is 12% and 4% lower for the DYMBER plans compared to the DTRT and VMAT plans, respectively. The DYMBER plans are delivered without collision and with a 4-5 min longer delivery time than the VMAT plans. The absolute dose measurements are compared to calculation by gamma analysis using 2% (global)/2 mm criteria with passing rates of at least 99%.

CONCLUSIONS

A treatment technique for DYMBER has been successfully developed and verified for its deliverability. The dosimetric superiority of DYMBER over DTRT and VMAT indicates utilizing increased DoF to be the key to improve brain and head and neck radiation treatments in future.

Anatomy driven optimization strategy for total marrow irradiation with a volumetric modulated arc therapy technique

The purpose of this study was to evaluate the possibility of dose distribution optimization for total marrow irradiation (TMI) employing volumetric‐modulated arc therapy (VMAT) with RapidArc (RA) technology setting isocenter's positions and jaw's apertures according to patient's anatomical features. Plans for five patients were generated with the RA engine (PROIII): eight arcs were distributed along four isocenters and simultaneously optimized with collimator set to 90°. Two models were investigated for geometrical settings of arcs: (1) in the “symmetric” model, isocenters were equispaced and field apertures were set the same for all arcs to uniformly cover the entire target length; (2) in the “anatomy driven” model, both field sizes and isocenter positions were optimized in order to minimize the target volume near the field edges (i.e., to maximize the freedom of motion of MLC leaves inside the field aperture (for example, avoiding arcs with ribs and iliac wings in the same BEV)). All body bones from the cranium to mid of the femurs were defined as PTV; the maximum length achieved in this study was 130 cm. Twelve (12) Gy in 2 Gy/fractions were prescribed in order to obtain the covering of 85% of the PTV by 100% of the prescribed dose. For all organs at risk (including brain, optical structures, oral and neck structures, lungs, heart, liver, kidneys, spleen, bowels, bladder, rectum, genitals), planning strategy aimed to maximize sparing according to ALARA principles, looking to reach a mean dose lower than 6 Gy (i.e., 50% of the prescribed dose). Mean MU/fraction resulted 3184±354 and 2939±264 for the two strategies, corresponding to a reduction of 7% (range −2% to 13%) for (1) and (2). Target homogeneity, defined as D2%−D98% was 18% better for (2). Mean dose to the healthy tissue, defined as body minus PTV, had 10% better reduction with (2). The isocenter's position and the jaw's apertures are significant parameters in the optimization of the TMI with RA technique, giving the medical physicist a crucial role in driving the optimization and thus obtaining the best plan. A clinical protocol started in our department in October 2010.

PACS numbers: 87.55.de, 87.55.dk, 87.56.nk, 87.57.uq