ABSTRACT
Hippocampi plays a fundamental role in immediate or long-term memory and the spatial learning. This structure is rarely involved by metastasis and their irradiation is at the origin of some impairment of the neurocognitive function. Sparing hippocampi during whole brain radiation therapy becomes possible with volumetric modulated arc therapy (VMAT) or with helical tomotherapy. The delineation of the structures should be performed after coregistration of gadolinium-enhanced T1-weighted MR-images with the planning. The D40 to both hippocampi should not be greater than 7.3 Gy. Patients who are more likely to benefit from a hippocampal-sparing strategy must have a 6 months or longer life expectancy and a Karnosky index above 70. Hence, patients who are more likely to be deemed fit for this strategy are frequently patients with NSCLC, breast cancer, gastointestinal cancers or patients. Patients with small cell lung carcinoma who are selected for prophylactic cerebral irradiation should be also considered, as they are unfit for ablative treatments such as stereotactic radiotherapy or brain surgery. Moreover, brain metastasis located in the area surrounding the hippocampi are unlikely. To date, no randomized study is available to confirm these assumptions. Two on-going prospective trials (RTOG 0933 and a French phase II trial) are currently investigating whether breast cancer patients with a single resected metastasis could benefit from the hippocampal-sparing strategy during whole brain radiotherapy.
Subject(s)
Brain Neoplasms/radiotherapy , Cognition Disorders/prevention & control , Hippocampus/physiology , Organ Sparing Treatments , Brain Neoplasms/secondary , Cranial Irradiation , Hippocampus/radiation effects , Humans , Life Expectancy , Patient Selection , Radiotherapy DosageABSTRACT
For treatments with dynamic intensity modulated radiotherapy (IMRT), the adjustment of multileaf collimator (MLC) parameters affecting both the optimization algorithm and dose distributions is crucial. The main parameters characterizing the MLC are the transmission (T) and the dosimetric leaf separation (DLS). The aim of this study is twofold: a methodology based on the 'sliding slit' test is proposed to determine (T, DLS) combinations inducing the best conformity between calculations and measurements. Secondly, the effects of the MLC adjustment on measured dose and on optimization are presented for different configurations as the chair test and for the patient dosimetric quality control (DQC). Tests were performed with a Varian 23EX linac operated at 20 MV and equipped with a 120 leaf Millenium dynamic collimator. The treatment planning system was CadPlan/Helios (version 6.3.6). Results demonstrated that the sliding width (SW) strongly depends on the (T, DLS) combinations, and the measured dose is a linear function of the SW. Different (T, DLS) combinations induced a good agreement between calculations and measurements. The influence of the MLC calibration was found to be particularly important on the 'sliding slit' test (11.8% for a gap change of 0.8 mm) but not so much on the chair test and on the DQC. To detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients, a daily check called IMRT MU check is proposed.
