Reducing PTV margins for prostate SBRT with motion compensation and gating techniques

Abstract

The purpose of this study is to investigate the dosimetric accuracy of prostate SBRT when motion is considered. To account for target movement, motion compensation and gating techniques were investigated with PTV margins reduced to 2 mm. To allow for dosimetric measurements a Delta4 phantom, Gafchromic film, and Hexamotion motion platform were utilized. Four motion files were utilized that represent a range of motions. Analysis of measured prostate motions for fifteen patients was performed to ensure detected motions were similar to those previously reported and motion files utilized were suitable. Five patient plans were utilized to allow for the effects of MLC and target motion interplay to be investigated. For both motion compensation and gating techniques, plans were delivered to the stationary phantom and for each of four motion types with/without compensation/gating enabled. Using a 3%, 2 mm and 80% threshold gamma criteria, film measurements had an average pass rate of 80.5% for uncorrected deliveries versus 96.0% for motion compensated deliveries. For gated techniques average pass rates increased from 89.9% for uncorrected to 94.8% with gating enabled. Measurements with the Delta4 arrays were analyzed with a 3%, 2 mm and 10% threshold dose. An average pass rate of 83.8% was measured for uncorrected motions versus 94.8% with motion compensation. For the gated technique an average pass rate of 87.2% was found for uncorrected motions versus 96.9% with gating enabled. These results show that very high gamma pass rates are achievable when motion compensation or gating techniques are applied. When target motion is not accounted for shifts up to 5 mm in planned versus delivered isodose distributions were found. However, when motion compensation, or gated techniques were applied, much smaller differences between planned and delivered isodose distributions were found. With these techniques dose delivery accuracy is greatly improved, allowing for PTV margins to be reduced.

Dosimetric benefit of online treatment plan adaptation in stereotactic ultrahypofractionated MR-guided radiotherapy for localized prostate cancer

“…Real-time tracking of the prostate via surrogate fiducial markers may permit a similar reduction in PTV margins ( 30 ). However, fiducial marker implantation is an invasive procedure accompanied by a slightly increased risk for infection or even fiducial migration in rare cases which might lead to impaired tracking ( 31 ).…”

Stereotactic Body Radiation Therapy for Prostate Cancer Using Tomotherapy With Synchrony Fiducial Tracking

“…With the introduction of the Synchrony system, tomotherapy can be used to track SBRT [ 9 , 10 ]. By using the system, the 3D fiducial position is calculated based on successive 2D kV radiographs, and the translational motion can be compensated by jaw sweeping in the longitudinal direction and MLC shifting in the lateral and vertical directions.…”

Optimal planning target margin for prostate radiotherapy based on interfractional and intrafractional variability assessment during 1.5T MRI-guided radiotherapy

“…Minor deviations may lead to undercoverage of target volumes and higher toxicity rates ( 10 ). Several studies have attempted to assess the optimal target margin in prostate SBRT using image-guided RT (IGRT); however, they obtained limited information on prostate movement owing to the lack of full-time continuous monitoring or the use of X-ray-based image guidance ( 11 , 12 ).…”