Peripheral dose distributions for a linear accelerator equipped with a secondary multileaf collimator and universal wedge

Abstract

The American Association of Physicists in Medicine Task Group 36 (AAPM TG‐36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have been shown to significantly affect PD distributions and TG‐36 reported data. Measurements were performed to evaluate PD distributions for a linear accelerator equipped with a secondary MLC, backup diaphragms, and UW. This data can be used to compliment the TG‐36 report for estimation of doses to critical structures outside primary radiation fields. For the evaluated linear accelerator, an MLC is incorporated in the upper secondary collimator jaws. Backup shielding diaphragms are located underneath the MLC. At the nominal collimator position, the MLC and the backup diaphragm provide collimation primarily in the transverse direction. Conventional, solid tungsten‐alloy jaws, located underneath the backup diaphragms, provide secondary collimation in the longitudinal direction. The universal wedge provides dose modulation in the direction of the conventional jaws. Measurements were made with an ionization chamber inserted into a 20×40×120cm3 water‐equivalent plastic phantom with the secondary collimator and MLC settings of 5×5,10×10,15×15, and 25×25cm2 with and without UW. Data was acquired along the machine's longitudinal axis for 6, 10, and 18 MV photons. Peripheral dose distributions were measured with the collimator rotated to 0° and 270° for open field measurements and to 0°, 180°, and 270° for wedged fields (IEC 1217). This allowed evaluation of peripheral dose distributions as a function of collimator rotation. Wedged fields were normalized to deliver the same dose at the depth of maximum dose on the central axis as open fields. The measured PD distributions were generally comparable to data reported by TG‐36. At distances close to the field edge (less than 30 or 40 cm), the measured PD distributions were lower when the measurement point was shielded by solid jaws than with MLC and backup diaphragm. At longer distances, this trend reversed for all energies and evaluated field sizes. However, the difference in PD distribution with collimator rotation was not large enough to warrant strategic positioning of the collimator to reduce dose to critical structures outside the primary radiation field. Because internal scatter dominates close to the field edge, wedged PD distributions were comparable to open field doses at distances closer than 30 cm. However, at distances larger than 30 cm from the field edge, wedged PD distributions were significantly grater than those for open fields due to increased contribution of leakage radiation. Increased leakage radiation is due to the increase in wedged field monitor units, which is related to a small wedge factor (0.27 to 0.29).PACS number(s): 87.53.–j, 87.66.–a

AAPM TG158: Measurement and calculation of doses outside the treated volume from external‐beam radiation therapy

“…Universal wedges result in virtually no increase in out‐of‐field dose within 30 cm of the field edge compared with an open field. The dose beyond 30 cm increased by a factor of 4, corresponding to the increased head leakage necessitated by the wedge factor of around 0.28 . Similarly, for high‐energy beams, neutron production is increased by a factor of 6, arising from the increased beam time as well as the increased neutron production in the wedge .…”

Dosimetric review of cardiac implantable electronic device patients receiving radiotherapy

“…Typically, CIEDs are located at the periphery of the treatment field(s) where clinical dose calculation algorithms are less accurate (13‐17) . Peripheral dose (PD) is made up of contributions from leakage radiation, collimator head scatter, and internal scatter (15,16,18) . The contributions of each of these components vary based on the distance from the field edge (15) and the treatment technique.…”

“…Additionally, the dosimetric accuracy of TPSs are known to decrease with distance from the treatment field aperture(s) (19,20) . Since TPSs are not commissioned for PD calculations and their accuracy is known to decrease with increasing distance from the field edge, (19) published data are often used to estimate PDs (14‐18,21) . Regardless of the technique utilized to assess dose to CIEDs, the method's accuracy should be high enough for the determination of the potential risk the treatment may pose (7) …”

Radiation treatment planning and delivery strategies for a pregnant brain tumor patient

“…The TrueBeam treatment plans used collimator rotations of 90° to place the distal x jaws in the patient superior–inferior direction and avoided the use of physical wedges; EDW fields may increase peripheral dose by 10%–20% in very close proximity to the treatment field, compared to 200%–400% increases for physical wedges 4, 5, 7, 8. Tertiary MLC collimation was used for all TrueBeam treatment plans.…”

“…It may be possible to achieve a clinically acceptable plan while substantially reducing peripheral dose by modifying these standard treatment planning strategies. Numerous reports have detailed planning strategies to reduce peripheral dose 4, 5, 6, 7, 8, 9, 10. While IMRT is a common choice for intracranial treatments, IMRT often results in higher peripheral dose than 2D‐ or 3D‐conformal treatment techniques 5, 10…”