Modelling of the multi-leaf collimator (MLC) in treatment planning systems (TPS) is crucial for the dose calculation accuracy of intensity-modulated radiation therapy plans. However, no standardised methodology for their configuration exists to date. In this study we present a method that separates the effect of each dosimetric characteristic of the MLC, offering comprehensive equations for the determination of the configuration parameters used in the TPS model. The main advantage of the method is that it only requires prior knowledge of the nominal leaf width and is based on doses measured with a Farmer chamber, which is a very well established and robust methodology. Another significant advantage is the required time, since measuring the tests takes only about 30 minutes per energy. Firstly, we provide a theoretical general formalism in terms of the primary fluence constructed from the transmission map obtained from an MLC model for synchronous and asynchronous sweeping beams. Secondly, we apply the formalism to the RayStation TPS as a proof of concept and we derive analytical expressions that allow the determination of the configuration parameters (leaf tip width, tongue-and-groove width, x-position offset and MLC transmission) and describe how they intertwine. Finally, we apply the method to Varian’s Millennium120 and HD120 MLCs in a TrueBeam linear accelerator for different energies and determine the optimal configuration parameters. The proposed procedure is much faster and streamlined than the typical trial-and-error methods and increases the accuracy of dose calculation in clinical plans. Additionally, the procedure can be useful for standardising the MLC configuration process and it exposes the limitations of the implemented MLC model, providing guidance for further improvement of these models in TPSs.

We demonstrate the application of mixture density networks (MDNs) in the context of automated radiation therapy treatment planning. It is shown that an MDN can produce good predictions of dose distributions as well as reflect uncertain decision making associated with inherently conflicting clinical tradeoffs, in contrast to deterministic methods previously investigated in the literature. A twocomponent Gaussian MDN is trained on a set of treatment plans for postoperative prostate patients with varying extents to which rectum dose sparing was prioritized over target coverage. Examination on a test set of patients shows that the predicted modes follow their respective ground truths well, both spatially and in terms of their dose-volume histograms. A special dose mimicking method based on the MDN output is used to produce deliverable plans and thereby showcase the usability of voxel-wise predictive densities. Thus, this type of MDN may serve to support clinicians in managing clinical tradeoffs and has the potential to improve the quality of plans produced by an automated treatment planning pipeline.

Background and purpose: In 2017 the ACROP guideline on SBRT for peripherally located early stage NSCLC was published. Later that year ICRU-91 about prescribing, recording and reporting was published. The purpose of this study is to quantify the current variation in prescription practice in the institutions that contributed to the ACROP guideline and to establish the link between the ACROP and ICRU-91 recommendations. Material and methods: From each of the eight participating centres, 15 SBRT plans for stage I NSCLC were analyzed. Plans were generated following the institutional protocol, centres prescribed 3 Â 13.5 Gy, 3 Â 15 Gy, 3 Â 17 Gy or 3 Â 18 Gy. Dose parameters of the target volumes were reported as recommended by ICRU-91 and also converted to BED 10Gy . Results: The intra-institutional variance in D98%, Dmean and D2% of the PTV and GTV/ITV is substantially smaller than the inter-institutional spread, indicating well protocollised planning procedures are followed. The median values per centre ranged from 56.1 Gy to 73.1 Gy (D2%), 50.4 Gy to 63.3 Gy (Dmean) and 40.5 Gy to 53.6 Gy (D98%) for the PTV and from 57.1 Gy to 73.6 Gy (D2%), 53.7 Gy to 68.7 Gy (Dmean) and 48.5 Gy to 62.3 Gy (D98%) for the GTV/ITV.Comparing the variance in PTV D98% with the variance in GTV Dmean per centre, using an F-test, shows that four centres have a larger variance in GTV Dmean, while one centre has a larger variance in PTV D98% (p values <0.01). This shows some centres focus on achieving a constant PTV coverage while others aim at a constant GTV coverage. Conclusion: More detailed recommendations for dose planning and reporting of lung SBRT in line with ICRU-91 were formulated, including a minimum PTV D98% of 100 Gy BED 10Gy and minimum GTV/ITV mean dose of 150 Gy BED 10Gy and a D2% in the range of 60-70 Gy.

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A decade after its first introduction into the clinic, little is known about the clinical impact of helical tomotherapy (HT) on head and neck cancer (HNC) treatment. Therefore, we analyzed the basics of this technique and reviewed the literature regarding HT's potential benefit in HNC. The past two decades have been characterized by a huge technological evolution in photon beam radiotherapy (RT). In HNC, static beam intensitymodulated radiotherapy (IMRT) has shown superiority over three-dimensional conformal RT in terms of xerostomia and is considered the standard of care. However, the next-generation IMRT, the rotational IMRT, has been introduced into the clinic without any evidence of superiority over static beam IMRT other than being substantially faster. Of these rotational techniques, HT is the first system especially developed for IMRT in combination with image-guided RT. HT is particularly promising for the treatment of HNC because its sharp dose gradients maximally spare the many radiosensitive organs at risk nearby. In addition, HT's integrated computed tomography scan assures a very precise dose administration and allows for some adaptive RT. Because HT is specifically developed for IMRT in combination with (integrated) imageguidance, it allows for precise dose distribution ("dose painting"), patient setup, and dose delivery. As such, it is an excellent tool for difficult HNC irradiation. The literature on the clinical results of HT in HNC all show excellent short-term (Յ2 years) results with acceptable toxicity profiles. However, properly designed trials are still warranted to further substantiate these results. The Oncologist 2013;18:697-706 Implications for Practice: This article highlights the advantages of helical tomotherapy in the treatment of head and neck cancer as it combines three recent evolutions and challenges in radiotherapy in an integrated system. (1) A rotational IMRT technique that is very efficient in generating homogeneous dose distributions to the target volume while sparing the organs at risk more precisely than conventional IMRT approaches. (2) Three-dimensional image guidance permits a more precise administration of the radiation than the classical two-dimensional imaging. This allows for reduction of the safety margins, which in turn, reduces toxicity. (3) Adaptive radiotherapy is still very labor intensive and no software is able to provide daily online adaptation. Tomotherapy offers a platform that allows for pioneering attempts at adaptive radiotherapy with the possibility of recalculating the administered treatment based on the daily CT scan. Properly designed trials are still warranted to further substantiate the results of this promising treatment modality. INTRODUCTIONDespite major progress in locoregional control, the treatment outcomes of locoregionally advanced head and neck cancer (HNC) remain poor, with a 5-year survival rate of approximately 35% due to the development of distant metastases, locoregional failures, second primaries, and/or comorbidities [1][...