PERIPHERAL SURFACE DOSE FROM A LINEAR ACCELERATOR: RADIOCHROMIC FILM EXPERIMENTAL MEASUREMENTS OF FLATTENING FILTER FREE VERSUS FLATTENED BEAMS.

There is a growing interest in the use of flattening filter free (FFF) beams due to the shorter treatment times. The reduction of head scatter suggests a better radiation protection to radiotherapy patients, considering the expected decrease in peripheral surface dose (PSD). In this work, PSD of flattened (FF) and FFF-photon beams was compared. A radiochromic film calibration method to reduce energy dependence was used. PSD was measured at distances from 2 to 50 cm to the field border for different square field sizes, modifying relevant clinical parameters. Also, clinical breast and prostate stereotactic body radiotherapy (SBRT) plans were studied. For square beams, FFF PSD is lower compared with FF PSD (differences ranging from 3 to 64%) and 10 MV FFF yields to the lowest value, for distances greater than 5 cm. For SBRT plans, near and far away from the field border, there is a reduction of PSD for FFF-beams, but the behavior at intermediate distances should be checked depending on the case.

Evaluation of acute skin toxicity in breast radiotherapy with a new quantitative approach.

BACKGROUND AND PURPOSE: Current criteria to evaluate acute radiodermatitis are highly subjective so quantification of physiological parameters is needed. We describe a non-invasive method of assessing skin microcirculation in breast cancer patients treated with hypofractionated radiotherapy and correlate them with the CTCAE scale. METHODS: Prospective study of 63 patients where blood flow was measured with real-time laser Doppler flowmetry (LDF) at baseline, weekly, and 3-months post-radiotherapy. Skin toxicity was assessed with the microcirculation index (MCI), a novel index based on blood flow parameters obtained via LDF. RESULTS: MCI was positively correlated (R=0.647; p<0.001) with the dose. Changes in MCI from baseline to the end of radiotherapy and at 3-months post-radiotherapy were significant (p<0.001). All CTCAE groups experienced a significant increase in MCI values from baseline to end of radiotherapy (p<0.001 for CTCAE grades 0 and 1; and p=0.028 for the grade 2 group). Significant differences in MCI values were observed among CTCAE groups at the end of radiotherapy (p=0.016). CONCLUSIONS: LDF is an accurate and objective measure of changes in blood flow. The comparison with the CTCAE shows the limitations of this subjective way of classifying patients. LDF is the first step for future studies of radiodermatitis treatments and prevention.

Performance evaluation of a high resolution dedicated breast PET scanner.

PURPOSE: Early stage breast cancers may not be visible on a whole-body PET scan. To overcome whole-body PET limitations, several dedicated breast positron emission tomography (DbPET) systems have emerged nowadays aiming to improve spatial resolution. In this work the authors evaluate the performance of a high resolution dedicated breast PET scanner (Mammi-PET, Oncovision). METHODS: Global status, uniformity, sensitivity, energy, and spatial resolution were measured. Spheres of different sizes (2.5, 4, 5, and 6 mm diameter) and various 18 fluorodeoxyglucose ((18)F-FDG) activity concentrations were randomly inserted in a gelatine breast phantom developed at our institution. Several lesion-to-background ratios (LBR) were simulated, 5:1, 10:1, 20:1, 30:1, and 50:1. Images were reconstructed using different voxel sizes. The ability of experienced reporters to detect spheres was tested as a function of acquisition time, LBR, sphere size, and matrix reconstruction voxel size. For comparison, phantoms were scanned in the DbPET camera and in a whole body PET (WB-PET). Two patients who just underwent WB-PET/CT exams were imaged with the DbPET system and the images were compared. RESULTS: The measured absolute peak sensitivity was 2.0%. The energy resolution was 24.0% ± 1%. The integral and differential uniformity were 10% and 6% in the total field of view (FOV) and 9% and 5% in the central FOV, respectively. The measured spatial resolution was 2.0, 1.9, and 1.7 mm in the radial, tangential, and axial directions. The system exhibited very good detectability for spheres ≥4 mm and LBR ≥10 with a sphere detection of 100% when acquisition time was set >3 min/bed. For LBR = 5 and acquisition time of 7 min the detectability was 100% for spheres of 6 mm and 75% for spheres of 5, 4, and 2.5 mm. Lesion WB-PET detectability was only comparable to the DbPET camera for lesion sizes ≥5 mm when acquisition time was >3 min and LBR > 10. CONCLUSIONS: The DbPET has a good performance for its clinical use and shows an improved resolution and lesion detectability of small lesions compared to WB-PET.