Dosimetric benefit of MR-guided online adaptive radiotherapy in different tumor entities: liver, lung, abdominal lymph nodes, pancreas and prostate.

BACKGROUND: Hybrid magnetic resonance (MR)-Linac systems have recently been introduced into clinical practice. The systems allow online adaption of the treatment plan with the aim of compensating for interfractional anatomical changes. The aim of this study was to evaluate the dose volume histogram (DVH)-based dosimetric benefits of online adaptive MR-guided radiotherapy (oMRgRT) across different tumor entities and to investigate which subgroup of plans improved the most from adaption. METHODS: Fifty patients treated with oMRgRT for five different tumor entities (liver, lung, multiple abdominal lymph nodes, pancreas, and prostate) were included in this retrospective analysis. Various target volume (gross tumor volume GTV, clinical target volume CTV, and planning target volume PTV) and organs at risk (OAR) related DVH parameters were compared between the dose distributions before and after plan adaption. RESULTS: All subgroups clearly benefited from online plan adaption in terms of improved PTV coverage. For the liver, lung and abdominal lymph nodes cases, a consistent improvement in GTV coverage was found, while many fractions of the prostate subgroup showed acceptable CTV coverage even before plan adaption. The largest median improvements in GTV near-minimum dose (D98%) were found for the liver (6.3%, p < 0.001), lung (3.9%, p < 0.001), and abdominal lymph nodes (6.8%, p < 0.001) subgroups. Regarding OAR sparing, the largest median OAR dose reduction during plan adaption was found for the pancreas subgroup (-87.0%). However, in the pancreas subgroup an optimal GTV coverage was not always achieved because sparing of OARs was prioritized. CONCLUSION: With online plan adaptation, it was possible to achieve significant improvements in target volume coverage and OAR sparing for various tumor entities and account for interfractional anatomical changes.

Novel modified patient immobilisation device with an integrated coil support system for MR-guided online adaptive radiotherapy in the management of brain and head-and-neck tumours.

Magnetic resonance imaging (MR)-guided online adaptive radiotherapy is a promising technique in the field of radiation oncology providing excellent visualisation of soft-tissues, and allowing for online plan adaptation and tumour tracking. In order to facilitate the accurate dose delivery to the target volume while sparing healthy surrounding normal tissue in the brain or head-and-neck (H&N) region, precise patient immobilisation with good image quality is pertinent. Herein, we present a customised thermoplastic mask holder with an integrated anterior MR receiver coil support system for MR-guided online adaptive radiotherapy in the brain and head-and-neck region. The approved medical product was developed by Innovative Technologie Voelp (IT-V), Innsbruck, Austria. MR image uniformity measurements demonstrated improved image uniformity at the expense of decreased signal-to-noise ratio due to a more defined and larger distance between the anterior receiver coil and the phantom or patient. This integrated coil support system represents a practical solution facilitating stable and reproducible anterior coil placement while maintaining the thermoplastic mask holder functionality, a widely established immobilisation technique.

Prefraction displacement and intrafraction drift of the prostate due to perineal ultrasound probe pressure.

OBJECTIVE: In image-guided EBRT of the prostate, transperineal ultrasound (US) probes exert pressure on the perineum both during planning and treatment. Through tissue deformation and relaxation, this causes target and risk organ displacement and drift. In this study, prefraction shift and intrafraction drift of the prostate are quantified during robotic transperineal 4DUS. METHODS: The position of the prostate was recorded for different positions of the probe before treatment in 10 patients (16 series of measurements). During treatment (15 patients, 273 fractions), intrafraction motion of the prostate was tracked (total of 27 h and 24 min) with the transperineal probe in place. RESULTS: Per 1 mm shift of the US probe in the cranial direction, a displacement of the prostate by 0.42 ± 0.09 mm in the cranial direction was detected. The relationship was found to be linear (R² = 0.97) and highly significant (p < 0.0001). After initial contact of the probe and the perineum (no pressure), a shift of the probe of about 5-10 mm was typically necessary to achieve good image quality, corresponding to a shift of the prostate of about 2-4 mm in the cranial direction. Tissue compression and prostate displacement were well visible. During treatment, the prostate drifted at an average rate of 0.075 mm/min in the cranial direction (p = 0.0014). CONCLUSION: The pressure applied by a perineal US probe has a quantitatively similar impact on prostate displacement as transabdominal pressure. Shifts are predominantly in the cranial direction (typically 2-4 mm) with some component in the anterior direction (typically <1 mm). Slight probe pressure can improve image quality, but excessive probe pressure can distort the surrounding anatomy and potentially move risk organs closer to the high-dose area.