Treatment of Mantle and Inverted Y Fields in Modern Era: A VMAT Approach

PURPOSE/OBJECTIVE(S)

Hodgkin's lymphoma has been treated with combined modality therapy (chemotherapy and radiation) with a very high degree of success. Total Nodal Irradiation (TNI) performed with large AP/PA mantle fields for treatment of axillary, cervical, and mediastinal lymphatics, provide adequate coverage to the mediastinum and bilateral axillae and hila, while blocking lungs. The para-aortic and pelvic lymph nodes are treated with the so called inverted-Y AP/PA fields, which often includes the spleen in cases of TNI. Multileaf Collimators (MLC) have been tried, but due to the irregular shape of the fields and necessity of island blocking in 3D treatment, they have not been successful in full elimination of Cerrobend blocks. We hypothesize that using two or three matched Volumetric Modulated Arc Therapy (VMAT) fields will not only eliminate a need for Cerrobend blocks or island blocks, but will also provide better target coverage and better organs at risk (OAR) sparing.

MATERIALS/METHODS

Under IRB study, 10 patients were retrospectively planned using two or three matched VMAT technique for mantle and inverted-Y treatments of TNI that had been previously treated using MLC and Cerrobend block combination. Pinnacle treatment planning system version 16.2.1 was used to generate plans using mantle/inverted-Y technique and corresponding VMAT plans using 2-3 arcs per isocenter (2 isocenters per plan). Optimization was performed to cover targets with the prescribed dose of 1500 cGy in 10 fractions per institutional protocol. The VMAT plans were compared with traditional 3D plans.

RESULTS

VMAT consistently provided better or similar results to traditional field arrangements. Target coverage: V15Gy - 95.45% vs 77.99% (p = 0.00017), OAR coverage: total lung V5Gy 63.7% vs 68.8% (p = 0.016), bone marrow mean dose 539.1 cGy vs 727.8 cGy (p = 0.00047), Integral Dose 464.1 mJ vs 573.9 mJ (p = 0.0026). Low isodose lines- mean volume of 5 Gy isodose line was not significantly different - 24036 cc vs 25091 cc (p = 0.271). Cord maximum dose was 40% lower for VMAT plans (p = 0.00006). Mean bladder dose was similar in VMAT plans compared to 3D plan - 821.7 cGy vs 804.9 cGy (p = 0.454). One counter-intuitive result is that the mean integral dose for 10 patients was 24% lower for VMAT plans.

CONCLUSION

VMAT based mantle fields for TNI eliminates Cerrobend blocks and improve dosimetry significantly for target volumes and all OARs; including bone marrow, which plays important role in patient's recovery after chemotherapy, radiation and often stem cell transplantation in recurrent disease. Lower integral dose for VMAT plans is explained by the large irradiated in-fields and small out-of-field volumes. The VMAT process requires minimal effort for optimization and is economical compared to the traditional planning, while improving the target coverage and decreasing dose to OARs.

Determination of bending rigidity and tilt modulus of lipid membranes from real-space fluctuation analysis of molecular dynamics simulations

We have recently developed a novel computational methodology (termed RSF for Real-Space Fluctuations) to quantify the bending rigidity and tilt modulus of lipid membranes from real-space analysis of fluctuations in the tilt and splay degrees of freedom as sampled in molecular dynamics (MD) simulations. In this article, we present a comprehensive study that combines results from the application of the RSF method to a wide range of lipid bilayer systems that encompass membranes of different fluidities and sizes, including lipids with saturated and unsaturated lipid tails, single and multi-component lipid systems, as well as non-standard lipids such as the four-tailed cardiolipin. By comparing the material properties calculated with the RSF method to those obtained from experimental data and from other computational methodologies, we rigorously demonstrate the validity of our approach and show its robustness. This should allow for future applications of even more complex lipidic assemblies, whose material properties are not tractable by other computational techniques. In addition, we discuss the relationship between different definitions of the tilt modulus appearing in current literature to address some important unresolved discrepancies in the field.