Target and peripheral dose from radiation sector motions accompanying couch repositioning of patient coordinates with the Gamma Knife® Perfexion™

Abstract

BackgroundThe GammaPlan™ treatment planning system (TPS) does not fully account for shutter dose when multiple shots are required to deliver a patient’s treatment. The unaccounted exposures to the target site and its periphery are measured in this study. The collected data are compared to a similar effect from the Gamma Knife® model 4C.Materials and methods.A stereotactic head frame was attached to a Leksell® 16 cm diameter spherical phantom; using a fiducial-box, CT images of the phantom were acquired and registered in the TPS. Measurements give the relationship of measured dose to the number of repositions with the patient positioning system (PPS) and to the collimator size. An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site and all measurements were acquired with an ionization chamber.ResultsMeasured dose increases with frequency of repositioning and with collimator size. As the radiation sectors transition between the beam on and beam off states, the target receives more shutter dose than the periphery. Shutter doses of 3.53±0.04 and 1.59±0.04 cGy/reposition to the target site are observed for the 16 and 8 mm collimators, respectively. The target periphery receives additional dose that varies depending on its position relative to the target.ConclusionsThe radiation sector motions for the Gamma Knife® Perfexion™ result in an additional dose due to the shutter effect. The magnitude of this exposure is comparable to that measured for the model 4C.

Outcome of small cell lung cancer (SCLC) patients with brain metastases in a routine clinical setting

“…For extensive central nerve system involvement the treatment of choice is WBRT, however the optimal treatment strategy for a solitary brain metastasis, which is quite rare in SCLC, has still not been defined. It is known for some other cancers, such as non-small cell lung cancer and breast cancer, that surgical or stereotactic irradiation of solitary brain metastasis followed by WBRT gains better survival rates compared to WBRT alone 9,10. Despite the lack of scientific evidence based on randomized trials such an approach may be superior in selected SCLC patients as well.…”

“…20% of the patients with SCLC have detectable brain metastases, and the incidence of such metastases increases considerably during the course of disease, approaching 80% at 2 years. The standard treatment of clinically evident brain metastases is whole brain radiotherapy (WBRT), usually in combination with corticosteroids and the solitary metastasis might be additional irradiated with stereotactic radiotherapy or Gamma Knife 8–10. The response rate to brain irradiation is around 50%, but survival rates are relatively short.…”

Verification of dose profiles generated by the convolution algorithm of the gamma knife® radiosurgery planning system

“…However, when we excluded the axial planes, which were substantially affected by z‐coordinate uncertainty, the statistically meaningful differences in the GIPR between single‐shot plans and multiple‐shot plans disappeared. Possible errors related to multiple shots, such as shutter doses and intershot couch‐transport doses, which have been well‐described by Tran et al., were properly handled in treatment planning.…”

Forward treatment planning techniques to reduce the normalization effect in Gamma Knife radiosurgery

“…This would result in the need for additional shots to achieve target coverage. When more shots are used, the normal tissue may receive a higher dose because of scattering and leakage of radiation while patients are repositioned between shots 19, 20. Moreover, the beam‐on time may increase with an increasing number of shots.…”