An investigation into the range dependence of target delineation strategies for stereotactic lung radiotherapy.

BACKGROUND: The "gold standard" approach for defining an internal target volume (ITV) is using 10 gross tumor volume (GTV) phases delineated over the course of one respiratory cycle. However, different sites have adopted several alternative techniques which compress all temporal information into one CT image set to optimize work flow efficiency. The purpose of this study is to evaluate alternative target segmentation strategies with respect to the 10 phase gold standard. METHODS: A Quasar respiratory motion phantom was employed to simulate lung tumor movement. Utilizing 4DCT imaging, a gold standard ITV was created by merging 10 GTV time resolved image sets. Four alternative planed ITV's were compared using free breathing (FB), average intensity projection (AIP), maximum image projection (MIP), and an augmented FB (FB-Aug) set where the ITV included structures from FB plus max-inhale/exhale image sets. Statistical analysis was performed using the Dice similarity coefficient (DSC). Seventeen patients previously treated for lung SBRT were also included in this retroactive study. RESULTS: PTV's derived from the FB image set are the least comparable with the 10 phase benchmark (DSC = 0.740-0.408). For phantom target motion greater than 1 cm, FB and AIP ITV delineation exceeded the 10 phase benchmark by 2% or greater, whereas MIP target segmentation was found to be consistently within 2% agreement with the gold standard (DSC > 0.878). Clinically, however, the FB-Aug method proved to be most favorable for tumor movement up to 2 cm (DSC = 0.881 ± 0.056). CONCLUSION: Our results indicate the range of tumor motion dictates the accuracy of the defined PTV with respect to the gold standard. When considering delineation efficiency relative to the 10 phase benchmark, the FB-Aug technique presents a potentially proficient and viable clinical alternative. Among various techniques used for image segmentation, a judicious balance between accuracy and efficiency is inherently required to account for tumor trajectory, range and rate of mobility.

Stereotactic radiotherapy for prostate cancer: A review and future directions.

Prostate cancer affects over 200000 men annually in the United States alone. The role of conventionally fractionated external beam radiation therapy (RT) is well established as a treatment option for eligible prostate cancer patients; however, the use of stereotactic body radiotherapy (SBRT) in this setting is less well defined. Within the past decade, there have been a number of studies investigating the feasibility of SBRT as a potential treatment option for prostate cancer patients. SBRT has been well studied in other disease sites, and the shortened treatment course would allow for greater convenience for patients. There may also be implications for toxicity as well as disease control. In this review we present a number of prospective and retrospective trials of SBRT in the treatment of prostate cancer. We focus on factors such as biochemical progression-free survival, prostate specific antigen (PSA) response, and toxicity in order to compare SBRT to established treatment modalities. We also discuss future steps that the clinical community can take to further explore this new treatment approach. We conclude that initial studies examining the use of SBRT in the treatment of prostate cancer have demonstrated impressive rates of biochemical recurrence-free survival and PSA response, while maintaining a relatively favorable acute toxicity profile, though long-term follow-up is needed.