Characterization of the ZAP-X® Peripheral Dose Fall-Off

Abstract

Various small-field radiation dose detectors were systematically compared and their impact on measured beam performance of the ZAP-X® dedicated stereotactic radiosurgery system (ZAP Surgical Systems, Inc., San Carlos, CA, USA) was determined. Three Physikalische Technische Werkstaetten (PTW) diodes, i.e., the microSilicon, the microDiamond, and the Stereotactic Radiosurgery (SRS) diode detectors of (PTW-Freiburg, Freiburg, Germany), as well as Gafchromic™ External Beam Therapy 3 (EBT) film (Ashland, Inc., Wilmington, DE, USA), were used and compared to arrive at a recommended standard for this critical component of small-field beam measurements. Beam profiles, including the dose fall-off region near the edge of the beam, were measured with the PTW diodes and EBT3 film and subsequently contrasted. The impact of detector physical and dosimetric characteristics on the results of the measurements was investigated and compared with film measurements. The beam penumbra was used to quantify the dose falloff. The measurement acquired with the diodes and film showed the most significant differences in the falloff region near the field edge. The film-based measurements clearly showed the steepest dose gradient verified by the penumbra value of 1.21 mm, followed by the SRS diode with 1.60 mm, the microSilicon diode with 1.67 mm, and the microDiamond diode with 1.83 mm. A clear correlation of each detector's sensitive area with the penumbra was found, with the microDiamond detector at 2.2 mm diameter sensitive area having the largest penumbra, followed by the microSilicon and SRS diodes. Beam measurements for the purposes of system characterization or treatment planning system beam data acquisition depend, to a large extent, on detector characteristics. This is especially true for small-field dosimetry performed during stereotactic radiosurgery beam measurements. Careful consideration should be practiced which allows for the measurements to represent true beam characteristics and minimize the impact of the detector on the measurements. We conclude that film should be considered the reference method for such measurements with the ZAP-X due to its smallest physical measurement resolution of 23.1 µm. Potential drawbacks to this methodology are the need to calibrate the film relative to the dose and possible problems with saturation and non-linear film response for very high and very low optical densities.

Treatment planning with a 2.5 MV photon beam for radiation therapy

“…Achieving a rapid dose fall-off beyond the target (PTV), and consequently, reducing the dose of adjacent OARs using low MV has been traditionally used in radiotherapy in general and especially in LINACbased stereotactic radiosurgery (SRS). 39,40 By designing hybrid plans with higher energies, the steeper fall-off of PDD with 2.5 MV photons can be used in treatment of cases where normal tissues with low-dose tolerances, such as the heart in the liver case, are in the path of the beam exit. For example, in the liver plan, the higher energy fields can be arranged in the anterior and posterior directions that do not pass through the heart, but the lateral fields, which exit through the heart could be planned with 2.5 MV fields.…”

“…Achieving a rapid dose fall‐off beyond the target (PTV), and consequently, reducing the dose of adjacent OARs using low MV has been traditionally used in radiotherapy in general and especially in LINAC‐based stereotactic radiosurgery (SRS) 39,40 . By designing hybrid plans with higher energies, the steeper fall‐off of PDD with 2.5 MV photons can be used in treatment of cases where normal tissues with low‐dose tolerances, such as the heart in the liver case, are in the path of the beam exit.…”

Technical note: Absolute dose measurements of a vault‐free radiosurgery system

“…For AD calibration with the modified TG‐21 protocol, the calculated N gas value using Table 1 parameters was 2.677 × 10 8 Gy/C. After measurements with the modified TG‐21 protocol, as described in the methods section, the “working primary factor” was set to 1.128 to achieve a reading of 1.00 cGy/MU.…”

“…As the monoenergetic electrons incident on the X-ray target have an energy of 3.0 MeV, the Nominal Accelerating Potential (NAP) is designated to be 3.0 MV for product specification purposes. 8 The system has no bending magnet or flattening filter. The nominal maximum dose rate is 1500 monitor units (MUs) per minute.…”

The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

“…The 60023‐type detector has also been tested in a 6 MV circular field of diameter 5 mm by Francescon et al., 14 who found that the on‐axis dose measured at 1.5 cm depth required a 2% correction compared to 5–6% for other stereotactic diodes. Wiedlich et al 15 . tested the detector in 3 MV circular fields down to a diameter of 4 mm and reported that the penumbra width measured using this detector was greater than that measured using film or a PTW 60018‐type diode, but narrower than the width measured using a microDiamond detector, reflecting the relative diameters of the detectors’ sensitive volumes.…”