Novel approach for the evaluation of dose conformity in radiotherapy

Abstract

Purpose We describe a new approach to evaluate conformity of dose distributions in radiotherapy. Methods The suggested conformity factor λ is defined by using existing conformity indices and expansion of the planning target volume (PTV). If the average distance (d¯$\bar d$) between the PTV and reference isodose surface and an arbitrarily selected PTV expansion margin (dexp${d_{exp}}$) are both much smaller than the size of the PTV, then λ approximately equals the ratio trued¯dexp$\frac{{\bar d}}{{{d_{exp}}}}$. We use λ to analyze several cases of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). Results In the case of SRS with a single target or multiple targets, treatment plans produced with the help of volumetric modulated arc therapy (VMAT) have smaller λ than plans produced by using dynamic conformal arcs (DCA). Likewise, it is demonstrated that in the case of SBRT, λ is reduced by employing VMAT instead of DCA. It is also shown that if the distance between the reference isodose surface and surface of the PTV is fixed, λ varies less with variations in PTV volume compared to frequently used conformity indices. Conclusions The described conformity factor λ can be applied clinically to compare and rank treatment plans for lesions of different sizes. It is suggested that conditions λ<1$\lambda &lt; 1$ and λ > 1 can be employed as “pass” and “fail” criteria, respectively, for dose conformity assessment with appropriate choice of dexp${d_{exp}}$.

Toward an improved assessment of dose conformity in radiotherapy

“…In addition, $$C{I_{{d_{exp}}}}$$ can be used to distinguish between overdosing normal tissue and underdosing treatment target. The latter feature of $$C{I_{{d_{exp}}}}$$ differentiates it from the conformity factor recently described in 18 . Note also that in the presented framework (see Section 2), the “pass” and “fail” criteria for dose conformity assessment can be defined by the conditions $$C{I_{{d_{exp}}}} &lt; 1$$ and $$C{I_{{d_{exp}}}} \ge 1$$, respectively, for appropriately chosen $${d_{exp}}$$.…”

“…Let $${\bar d_{\bot} }$$ denote average distance between the surfaces of the PTV and $${A_{{D_{ref}}}}$$ (see Appendix ). If the distance between the surfaces is sufficiently small (i.e., $${\bar d_{\bot} }$$ is much smaller than the sizes of the PTV and $${A_{{D_{ref}}}}$$), then we have 18 $$$$\begin{equation}{V_{uni}} - {V_{int}} \approx {\bar d_{\bot} }{S_{PTV}}.\end{equation}$$$$Likewise, in the case when $${d_{exp}}$$ is much smaller than the size of the PTV, we also have $$$$\begin{equation}{V_{PT{V_{exp}} - }}{V_{PTV}} \approx {d_{exp}}{S_{PTV}}\end{equation}$$$$From Equations (), (), and (), it follows that $$$$\begin{equation}C{I_{{d_{exp}}}} \approx \frac{{{{\bar d}_{\bot} }}}{{{d_{exp}}}}.\end{equation}$$$$…”

“…1 To quantitatively assess dose conformity, a number of indices (referred to as conformity indices) were introduced previously (e.g., see refs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] ). Computation of these indices generally requires several input parameters: .…”

“…The latter feature of CI d exp differentiates it from the conformity factor recently described in. 18 Note also that in the presented framework (see Section 2), the "pass" and "fail" criteria for dose conformity assessment can be defined by the conditions CI d exp < 1 and CI d exp ≥ 1, respectively, for appropriately chosen d exp .…”

“…It has been long recognized that the quality of radiotherapy plans can be assessed by examining how prescribed dose conforms to the shape of the PTV 1 . To quantitatively assess dose conformity, a number of indices (referred to as conformity indices ) were introduced previously (e.g., see refs 1–18 …”