MR-guidance in clinical reality: current treatment challenges and future perspectives.

Magnetic Resonance-guided radiotherapy (MRgRT) marks the beginning of a new era. MR is a versatile and suitable imaging modality for radiotherapy, as it enables direct visualization of the tumor and the surrounding organs at risk. Moreover, MRgRT provides real-time imaging to characterize and eventually track anatomical motion. Nevertheless, the successful translation of new technologies into clinical practice remains challenging. To date, the initial availability of next-generation hybrid MR-linac (MRL) systems is still limited and therefore, the focus of the present preview was on the initial applicability in current clinical practice and on future perspectives of this new technology for different treatment sites.MRgRT can be considered a groundbreaking new technology that is capable of creating new perspectives towards an individualized, patient-oriented planning and treatment approach, especially due to the ability to use daily online adaptation strategies. Furthermore, MRL systems overcome the limitations of conventional image-guided radiotherapy, especially in soft tissue, where target and organs at risk need accurate definition. Nevertheless, some concerns remain regarding the additional time needed to re-optimize dose distributions online, the reliability of the gating and tracking procedures and the interpretation of functional MR imaging markers and their potential changes during the course of treatment. Due to its continuous technological improvement and rapid clinical large-scale application in several anatomical settings, further studies may confirm the potential disruptive role of MRgRT in the evolving oncological environment.

Comparison of DCE-CT models for quantitative evaluation of K(trans) in larynx tumors.

Dynamic contrast enhanced CT (DCE-CT) can be used to estimate blood perfusion and vessel permeability in tumors. Tumor induced angiogenesis is generally associated with disorganized microvasculature with increased permeability or leakage. Estimated vascular leakage (K(trans)) values and their reliability greatly depend on the perfusion model used. To identify the preferred model for larynx tumor analysis, several perfusion models frequently used for estimating permeability were compared in this study. DCE-CT scans were acquired for 16 larynx cancer patients. Larynx tumors were delineated based on whole-mount histopathology after laryngectomy. DCE-CT data within these delineated volumes were analyzed using the Patlak and Logan plots, the Extended Tofts Model (ETM), the Adiabatic Approximation to the Tissue Homogeneity model (AATH) and a variant of AATH with fixed transit time (AATHFT). Akaike's Information Criterion (AIC) was used to identify the best fitting model. K(trans) values from all models were compared with this best fitting model. Correlation strength was tested with two-tailed Spearman's rank correlation and further examined using Bland-Altman plots. AATHFT was found to be the best fitting model. The overall median of individual patient medians K(trans) estimates were 14.3, 15.1, 16.1, 2.6 and 22.5 mL/100 g min( - 1) for AATH, AATHFT, ETM, Patlak and Logan, respectively. K(trans) estimates for all models except Patlak were strongly correlated (P < 0.001). Bland-Altman plots show large biases but no significant deviating trend for any model other than Patlak. AATHFT was found to be the preferred model among those tested for estimation of K(trans) in larynx tumors.

[Saliva and intensity modulated radiotherapy]. / Speeksel en intensiteitsgemoduleerde radiotherapie.

A dry mouth (xerostomia) is a serious side effect for head and neck cancer patients treated with radiotherapy. The degree of xerostomia is dependent on the dosage in the parotid glands. New, advanced radiation techniques, such as intensity-modulated radiotherapy, can reduce the dosage in the parotid glands, resulting in a significant improvement in the functioning of these glands by comparison with conventional radiation techniques.

Dose optimization for the MRI-accelerator: IMRT in the presence of a magnetic field.

A combined system of a 6 MV linear accelerator and a 1.5 T MRI scanner is currently being developed. In this system, the patient will be irradiated in the presence of a 1.5 T magnetic field. This causes a strong dose increase at tissue-air interfaces. Around air cavities in the patient, these effects may become problematic. Homogeneous dose distributions can be obtained around regularly shaped symmetrical cavities using opposing beams. However, for more irregularly shaped cavities this approach may not be sufficient. This study will investigate whether IMRT can be used to cope with magnetic field dose effects, in particular for target volumes adjacent to irregularly shaped air cavities. Therefore, an inverse treatment planning approach has been designed based on pre-calculated beamlet dose distribution kernels. Using this approach, optimized dose distributions were calculated for B = 1.5 T and for B = 0 T. Investigated target sites include a prostate cancer, a laryngeal cancer and an oropharyngeal cancer. Differences in the dose distribution between B = 0 and 1.5 T were minimal; only the skin dose increased for B = 1.5 T. Homogeneous dose distributions were obtained for target structures adjacent to air cavities without the use of opposing beams. These results show that a 1.5 T magnetic field does not compromise the ability to achieve desired dose distributions with IMRT.

Influence of the linac design on intensity-modulated radiotherapy of head-and-neck plans.

In this study, we quantify the impact of linac/MLC design parameters on IMRT treatment plans. The investigated parameters were leaf width in the MLC, leaf transmission, related to the thickness of the leaves, and penumbra related primarily to the source size. Seven head-and-neck patients with stage T1-T3N0-N2cM0 oropharyngeal cancer were studied. For each patient nine plans were made with a different set of linac/MLC parameters. The plans were optimized in Pinnacle(3) v7.6c and PLATO RTS v2.6.4, ITP v1.1.8. A hypothetical ideal linac/MLC was introduced to investigate the influence of one parameter at a time without interaction of other parameters. When any of the three parameters was increased from the ideal set-up values (leaf width 2.5 mm, transmission 0%, penumbra 3 mm), the mean dose to the parotid glands increased, given the same tumour coverage. The largest increase was found for increasing leaf transmission. The investigation showed that by changing more than one parameter of the ideal linac/MLC set-up, the increase in the mean dose was smaller than the sum of dose increments for each parameter separately. As a reference to clinical practice, we also optimized the plans of the seven patients with the clinically used Elekta SLi 15, equipped with a standard MLC with a leaf width of 10 mm. As compared to the ideal linac, this resulted in an increase of the average dose to the parotid glands of 5.8 Gy.

Assessment of induction of secondary tumours due to various radiotherapy modalities.

One of the objectives of the European Sixth Framework integrated project MAESTRO is to perform an assessment of risk due to various radiotherapy modalities, regarding secondary tumour induction. Initially, the study will focus on cancer of the prostate and the present work represents the first step towards that goal. One of the intended tools, to be used in the assessment, is the Monte Carlo radiation transport code ORANGE. A validation of the ORANGE code's capability to tally dose on a grid superimposed on an existing MCNP geometry is given. Preliminary results on the dose distribution due to conventional radiotherapy treatment of prostate cancer are discussed. Two mathematical models of the patient are proposed and the clinical relevance of the ADAM phantom is investigated. A problem in comparing average doses provided by commercial treatment planning systems and those calculated with Monte Carlo is noticed. The two proposed models are shown to receive a lower dose and average energy deposition than a 'real' patient.