Technical Note: Characteristics of a microSilicon X shielded diode detector for photon beam dosimetry.

PURPOSE: This study investigated the characteristics of a new shielded diode detector, microSilicon X (model 60022: MSX), for small-field and large-field dosimetry. METHODS: The percent depth dose (PDD), beam profiles, detector output factor (OFdet ), temperature dependence, dose rate dependence, dose-per-pulse (DPP) dependence, and dose-response linearity of MSX were evaluated in Cyberknife and TrueBeam photon beams and compared with various detectors including microDiamond (PTW model 60019: MD), Sun Nuclear EDGE detector, Photon diode (PTW model 60016: PD), and semiflex ionization chamber (PTW model 31010: IC). RESULTS: For field sizes ranging from 50 × 50 mm2 to 400 × 400 mm2 , MSX-measured OFdet values were within 1% of the IC-measured values. For the CyberKnife small fields, the maximum difference between the MSX-measured OFdet and the MD-measured field output factor (Ω) was 4.0%, while the maximum differences were 8.8% and 10.9% for PD and EDGE, respectively. MSX showed a stable response within 0.7% for water temperatures of 5°C to 34°C, while PD and EDGE showed a linear correlation between the water temperature and the response. MSX showed small variations within 0.2% for the dose rate, and PD and EDGE showed logarithmic increases in the response with the dose rate. MSX and MD had smaller DPP dependences than PD and EDGE. CONCLUSIONS: The characteristics of MSX for measurements of small- and large-field photon beams are favorable. Compared to PD, MSX exhibited significant improvement in the over-response for small fields. The OFdet values measured by MSX were approximately in-between those measured by MD and PD. MSX showed stable responses against water temperature, dose rate, and DPP variations and provided suitable data for a wide range of field sizes. However, careful attention is required for measurements of OFdet for field sizes of <10 × 10 mm2 and PDD for field sizes of ≥200 × 200 mm2 .

Repositioning accuracy of a novel thermoplastic mask for head and neck cancer radiotherapy.

PURPOSE: The aim of this study was to assess the reproducibility of patient shoulder position immobilized with a novel and innovative prototype mask (E-Frame, Engineering System). METHODS: The E-frame mask fixes both shoulders and bisaxillary regions compared with that of a commercial mask (Type-S, CIVCO). Thirteen and twelve patients were immobilized with the Type-S and E-Frame mask systems, respectively. For each treatment fraction, cone-beam CT (CBCT) images of the patient were acquired and retrospectively analyzed. The CBCT images were registered to the planning CT based on the cervical spine, and then the displacements of the acromial extremity of the clavicle were measured. RESULTS: The systematic and random errors between the two mask systems were evaluated. The differences of the systematic errors between the two mask systems were not statistically significant. The mean random errors in the three directions (AP, SI and LR) were 2.7 mm, 3.1 mm and 1.5 mm, respectively for the Type-S mask, and 2.8 mm 2.5 mm and 1.4 mm, respectively for the E-Frame mask. The random error of the E-Frame masks in the SI direction was significantly smaller than that of the Type-S. The number of cases showing displacements exceeding 10 mm in the SI direction for at least one fraction was eight (61% of 13 cases) and three (25% of 12 cases) for Type-S and E-Frame masks, respectively. CONCLUSIONS: The E-Frame masks reduced the random displacements of patient's shoulders in the SI direction, effectively preventing large shoulder shifts that occurred frequently with Type-S masks.

Characterization of a microSilicon diode detector for small-field photon beam dosimetry.

This study characterized a new unshielded diode detector, the microSilicon (model 60023), for small-field photon beam dosimetry by evaluating the photon beams generated by a TrueBeam STx and a CyberKnife. Temperature dependence was evaluated by irradiating photons and increasing the water temperature from 11.5 to 31.3°C. For Diode E, microSilicon, microDiamond and EDGE detectors, dose linearity, dose rate dependence, energy dependence, percent-depth-dose (PDD), beam profiles and detector output factor (OFdet) were evaluated. The OFdet of the microSilicon detector was compared to the field output factors of the other detectors. The microSilicon exhibited small temperature dependence within 0.4%, although the Diode E showed a linear variation with a ratio of 0.26%/°C. The Diode E and EDGE detectors showed positive correlations between the detector reading and dose rate, whereas the microSilicon showed a stable response within 0.11%. The Diode E and microSilicon demonstrated negative correlations with the beam energy. The OFdet of microSilicon was the smallest among all the detectors. The maximum differences between the OFdet of microSilicon and the field output factors of microDiamond were 2.3 and 1.6% for 5 × 5 mm2 TrueBeam and 5 mm φ CyberKnife beams, respectively. The PDD data exhibited small variations in the dose fall-off region. The microSilicon and microDiamond detectors yielded similar penumbra widths, whereas the other detectors showed steeper penumbra profiles. The microSilicon demonstrated favorable characteristics including small temperature and dose rate dependence as well as the small spatial resolution and output factors suitable for small field dosimetry.

Involvement of histamine released from mast cells in acute radiation dermatitis in mice.

A possible involvement of histamine in acute radiation dermatitis in mice was investigated. The dose of 40 Gy of gamma irradiation induced erythema and edema in C57BL/6 mice treated with vehicle. However, in C57BL/6 mice treated with chlorpheniramine and WBB6F1-W/Wv mice, erythema and edema were not observed. In all of these mice, epilation and dry desquamation were induced, but bepotastine significantly reduced the extent of these areas. These results suggest that gamma irradiation-induced erythema and edema were caused by histamine released from mast cells via histamine H1 receptor, and epilation was induced by other inflammatory mediators.