Lung Stereotactic Body Radiation Therapy (SBRT) dose gradient and PTV volume: a retrospective multi-center analysis

Abstract

Background The treatment of lung lesions with stereotactic body radiation therapy calls for highly conformal dose, which is evaluated by a number of metrics. Lung stereotactic body radiation therapy clinical trials constrain a plans gradient index. The purpose of this work is to describe the dependence of clinically achievable dose gradient on planning target volume. Methods Three hundred seventy-four lung stereotactic body radiation therapy treatment plans were retrospectively reviewed and selected for this study. The relationship between R50% and planning target volume size was observed and compared against the RTOG 0915 and 0813 constraints noting minor and major deviations. Then a least squares regression was used to determine the coefficients for a power functional form of the dependence of gradient measure (GM) on planning target volume size. Results Of the 317 peripheral lung SBRT plans, 142 exhibited no deviation, 135 exhibited a minor deviation, and 40 exhibited a major deviation according to the RTOG 0915 dosimetric. conformality and dose fall-off constraints. A plot of gradient measure versus planning target volume size for peripheral lesions, excluding RTOG 0915 major deviations, is fit with an power function of GM = 0.564 V 0.215 . Conclusions Using the PTV size and GM relationship we have characterized, treatment plans with PTV < 85 cm 3 can be evaluated subjectively to our previously plans, and given a percentile GM. This relationship and evaluation is useful for volumetric modulated arc therapy lung stereotactic body radiation therapy treatment planning and quality control. Electronic supplementary material The online version of this article (10.1186/s13014-019-1334-9) contains supplementary material, which is available to authorized users.

A physically meaningful relationship between R50% and PTV surface area in lung SBRT

“…The converse would be expected true for small PTVs. Of course, this is the result as reported by Hoffman et al 20 through their GM values.…”

“…This clearly establishes the relationship of R50% to both volume of the PTV and the surface area of the PTV. However, there is also a slight curvature to the distribution owing to the PTV Volume dependence of Δr = GM that was observed in the clinical data of Hoffman et al 20 This Δr dependence on PTV Volume is also visible in the R50%min GM lower bound.…”

“…Many of the steep dose falloff studies are for cranial radiosurgery targets. [14][15][16]22,23 20 Notice that the lower bound R50%min GM is always <R50% for all plans and thus is indeed a lower bound.…”

“…Figure 5 shows that as PTV D av decreases below the local minimum (D av ≈ 110%), the mean dose in the IDC50%shell increases for all PTV volumes, ie, the dose spill outside the PTV increases as the PTV dose becomes more homogeneous. 20 Notice that the lower bound R50%min GM is always less than the planned R50% and thus is indeed a lower bound. Figure 7 provides the same data as Fig.…”

On the optimal number of dose‐limiting shells in the SBRT auto‐planning design for peripheral lung cancer

“…Moreover, the conclusion of dose gradient is similar to our setting in a recently published study on lung SBRT. 16 In addition, there are few studies similar to our research, so the specific distance values of the shells cannot be referenced. The distances were 3, 5, 8, 11, 15, 20, 25, and 30 mm from PTV for the first shell to the eighth shell, which was also partly due to our years of experience.…”