Part II of the application of robotic stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) using the CyberKnife^® platform is presented in this issue of Technology in Cancer Research and Treatment (TCRT).

The CyberKnife^® is a linear accelerator (linac) based platform that utilizes a robotic delivery system that is coupled with real-time imaging to track and compensate for patient or target motion. Evolution of this technology has compensated for respiratory movements with Xsight^™ lung and has the ability to utilize skeletal tracking. These advances have allowed for the expansion from intra­cranial SRS to multisystem SBRT.

Bucholz and colleagues (1) have completed an extensive review of CyberKnife^® stereotactic radiosurgery for several intracranial tumors including metastatic and glial tumors. The authors discuss the importance of dose conformality and precision targeting to minimize the risk of radiation injury to adjacent normal brain. A literature review comparing the CyberKnife^® frameless systems to other frame-based system in various pathologies suggest equivalent results and the unique ability to hypofractionate with frameless systems provides advantages with larger benign and malignant tumors. The authors suggest that the use of SRS for malignant gliomas is still however, controversial.

This issue includes three articles detailing the use of SBRT for prostate and lung cancer as well as two articles investigating technical aspects of SBRT delivery methods. First, Torshabi et al. (2) presents a comparative study of three different approaches to assess the accuracy of external surrogates in evaluating tumor motion. They suggest that their implemented models lead to an effective improvement over the CyberKnife Synchrony^®, which is currently used for real-time tumor tracking. Second, a multi-institutional study reported by Ma and colleagues (3) investigating apparatus-dependent dose distribution differences in spinal SBRT. They compared CyberKnife^® radiosurgery, intensity-modulated proton therapy (IMP) and multi-leaf collimator (MLC) fixed-field IMRT in the treatment of 1-3 vertebral bodies. This dosimetric study demonstrated the feasibility of each of the modalities to deliver spinal radiosurgery with the customary dose constraints to the organs at risk (OARs). There were some differences with the CyberKnife^® plans having more dose inhomogeneity and perhaps marginally diminished target coverage in meeting the constraints for the OARs. Not surprisingly, smaller MLC based plans fared favorably in target coverage and dose to nearby critical structures. IMP appeared to offer similary coverage of the planning target volume as the other two modalities reported but its clinically utility has yet to be reported.

In the first of two articles on prostate cancer, Katz (4) reported the data on 73 patients with locally-advanced prostate cancer treated with external beam radiation therapy and an SBRT boost with the hopes of reducing treatment-related toxicities They suggest that at 33 months follow-up the late toxicity and biochemical control rates are promising but long-term durability data has not been determined at this point. Suy and colleagues (5) describe the first histopathologic analysis of prostate tissue following SRBT in a 66 year old man treated with 37.5 Gy in five fractions. Histopathological analysis showed changes consistent with radiation including inflammation, gland atrophy and hyperplasia but no evidence of residual cancer, fibrosis or necrosis.

In the second review article, Gibbs and Loo (6) discussed the role of stereotactic ablative radiotherapy for lung tumors. This technique was shown to have high rates of local tumor control especially in smaller peripheral tumors with acceptable toxicity utilizing a short treatment course. The CyberKnife^® utilizing respiratory gating was thought to be well-suited for the group of patients with early stage lung cancers who are medically or physiologically inoperable.

This second edition of TCRT focusing on CyberKnife^® robotic radiosurgery has addressed some of the technical aspects and provides two comprehensive reviews on brain and lung tumor treatment. We hope that this edition has served to further inform the readers on the utilization and technical nuances of this novel and rapidly changing technology.

References

1. Bucholz, R. D., Laycock, K. A., Cuff, R. N. CyberKnife Stereotactic Radiosurgery for Intracranial Neoplasms, with a Focus on Malignant Tumors. Technol Cancer Res Treat 9, 541-550 (2010).
2. Torshabi, A. E., Pella, A., Riboldi, M., Baroni, G. Targeting Accuracy in Real-Time Tumor Tracking via External Surrogates: a Comparative Study. Technol Cancer Res Treat 9, 551-561 (2010).
3. Ma, L., Sahgal, A., Cozzi, L., Chang, E., Shiu, A., Létourneau, D., Yin. F-F., Fogliata, A., Kaissl, W., Hyde, D., Laperriere, N. J., Shrieve, D. C., Larson, D. A. Apparatus-Dependent Dosimetric Differences in Spine Stereotactic Body Radiosurgery. Technol Cancer Res Treat 9, 563-574 (2010).
4. Katz, A. L., Santoro, M., Ashley, R., Diblasio, F., Witten, M. Stereo­tactic Body Radiotherapy as Boost for Organ-Confined Prostate Cancer. Technol Cancer Res Treat 9, 575-582 (2010).
5. Suy, S., Oermann, E., Hanscom, H., Lei, S., Vahdat, S., Yu, X., Park, H. U., Chen, V., Collins, B. T., McGeagh, K., Dawson, N., Jha, R., Azumi, N., Dritschilo, A., Lynch, J., Collins, S. P. Histopathologic Effects of Hypofractionated Robotic Radiation Therapy on Malignant and Benign Prostate Tissue. Technol Cancer Res Treat 9, 583-587 (2010).
6. Gibbs, I. C., Loo, Jr, B. W. CyberKnife Stereotactic Ablative Radiotherapy for Lung Tumors. Technol Cancer Res Treat 9, 589-596 (2010).