Faculty Bibliography

Objectives: Published local control rates for certain stereotactic body radiotherapy (SBRT) lung cancer treatments have exceeded 97%, but several serious adverse events have been reported. No five-year normal tissue complication probability (NTCP) statistical analysis SBRT results have been published yet for skin. While we expectantly await these publications, the goal of this manuscript is to glean the published expert opinion and ongoing clinical trials for skin dose tolerance limits in one to five fractions for clinical application and research. Methods: All 42 lung tumors treated with the CyberKnife at Cooper University Hospital from July 2008 to May 2010 that had skin contours and Monte Carlo dose calculations with heterogeneity corrections were reviewed for toxicity. Twenty-one published dose tolerance limits for skin were partitioned into high-risk and low-risk categories. The DVH Evaluator software tool was developed by the first author and used to conveniently assess the dose tolerance limits for each case prior to treatment and to analyze the followup data. Results: Three of the 42 patients experienced mild Grade 1 erythema that resolved within a few months. No Grade 2 or higher adverse events of skin toxicity were encountered. A unified framework of high-risk and low-risk skin dose limits in one to five fractions at specific dose-volume levels is presented. Conclusions: When the dose tolerance limits are diligently studied and respected, SBRT has potential to be a safe and effective form of radiation therapy. Longer follow-up will still be required to statistically analyze late effects and long-term outcomes

OBJECT: Dose-volume data concerning the brainstem in stereotactic radiosurgery (SRS) for trigeminal neuralgia (TN) were analyzed in relation to associated complications. The authors present their set of data and compare it with currently cited information on brainstem dose tolerance associated with conventional fractionated radiation therapy and hypofractionated radiation treatment of other diseases. METHODS: Stereotactic radiosurgery for TN delivers a much higher radiation dose to the brainstem in a single fraction than doses delivered by any other procedures. A literature survey of articles on radiosurgery for TN revealed no incidences of severe toxicity, unlike other high-dose procedures involving the brainstem. Published data on brainstem dose tolerance were investigated and compared with dose-volume data in TN radiosurgery. The authors also performed a biological modeling study of dose-volume data involving the brainstem in cases of TN treated with the Gamma Knife, CyberKnife, and linear accelerator-based systems. RESULTS: The brainstem may receive a maximum dose as high as 45 Gy during radiosurgery for TN. The major complication after TN radiosurgery is mild to moderate facial numbness, and few other severe toxic responses to radiation are observed. The biologically effective dose of 45 Gy in a single fraction is much higher than any brainstem dose tolerance currently cited in conventional fractionation or in single or hypofractionated radiation treatments. However, in TN radiosurgery, the dose falloff is so steep and the delivery so accurate that brainstem volumes of 0.1-0.5 cm(3) or larger receive lower planned and delivered doses than those in other radiation-related procedures. Current models are suggestive, but an extensive analysis of detailed dose-volume clinical data is needed. CONCLUSIONS: Patients whose TN is treated with radiosurgery are a valuable population in which to demonstrate the dose-volume effects of an extreme hypofractionated radiation treatment on the brainstem. The result of TN radiosurgery suggests that a very small volume of the brainstem can tolerate a drastically high dose without suffering a severe clinical injury. The authors believe that the steep dose gradient in TN radiosurgery plays a key role in the low toxicity experienced by the brainstem.

PURPOSE: To investigate the dosimetric feasibility of accelerated partial breast irradiation (APBI) using CyberKnife. METHODS: Fourteen previously treated patients with early-stage breast cancer were selected for a retrospective study. Six of these patients had been treated to 38.5 Gy in 10 fractions in a phase III accelerated partial breast trial and the rest of the patients were treated to 50.4 Gy in 28 fractions. In this planning study, the guidelines in the protocol for the phase III partial breast trial were followed for organ delineation and CyberKnife planning. The achievable dosimetric parameters from all CyberKnife plans were compared to Intensity-modulated radiation therapy (IMRT) and 3D-CRT methods. The reproducibility of the dose delivery with and without respiratory motion was assessed through delivering a patient plan to a breast phantom. Different dose calculation algorithms were also compared between ray tracing and Monte Carlo. RESULTS: For all the patients in the study, the dosimetric parameters met the guidelines from the NSABP B39RTOG 0413 protocol strictly. The mean PTV volume covered by 100% of the prescription dose was 95.7 +/- 0.7% (94.7%-97.1%). The mean maximal dose was 104 +/- 2% of the prescription dose. The mean V(50%) and mean V(100%) to the ipsilateral normal breast were 23.1 +/- 11.6% and 9.0 +/- 5.8%, respectively. The conformity index of all plans was 1.14 +/- 0.04. The maximum dose to the contralateral breast varied from 1.3 cGy to 111 cGy. The mean V(5%) and mean V(30%) to the contralateral and ipsilateral lungs were 1.0 +/- 1.6% and 1.3 +/- 1.2%, respectively. In our study, the mean V(5%) to the heart was 0.2 +/- 0.5% for right-sided tumors and 9.4 +/- 10.1% for left-sided tumors. Compared with IMRT and 3D-CRT planning, the PTV coverage from CyberKnife planning was the highest, and the ratio of V(20%) to V(100%) of the breast from CyberKnife planning was the smallest. The heart and lung doses were similar in all the techniques except that the V(5%) for the lung and heart in CyberKnife planning was slightly higher. CONCLUSIONS: The dosimetric feasibility of APBI using CyberKnife was investigated in this retrospective study. All the dosimetric parameters strictly met the guidelines from the NSABP B39RTOG 0413 protocol. With advanced real-time tracking capability, CyberKnife should provide better target coverage and spare nearby critical organs for APBI treatment.