SU‐E‐T‐14: Predicted Biological Effectiveness of Treatments Delivered Using Dynamic MLC IMRT versus VMAT

Abstract

Purpose: Intensity Modulated Radiation Therapy (IMRT) is an effective method of maximizing dose conformity while minimizing damage to healthy tissue. The spatial effects of IMRT are commonly used as a technique to evaluate treatment quality. However, the use of spatial information alone neglects that cell survival is directly related to the temporal dose deposition. This study uses the linear quadratic (LQ) model, with the Lea‐Catcheside dose protraction factor in order to investigate the change in biological effectiveness between IMRT modalities, including sliding window (DMLC) and volumetric modulated arc therapy (VMAT). Methods: The dose‐time relationship was measured using Lucite phantom and an ion chamber. Measurements were taken for both prostate and head/neck plans. A specific coordinate was chosen in the tumor and the dose rate was measured as a function of time. The temporal dose deposition was used to calculate the dose protraction factor and the effective total dose (ETD) with varying repair half‐times and α/β ratios. The biological effectiveness was further studied using a Poisson model of tumor control probability (TCP). Results: Switching from VMAT to DMLC, for both the prostate and head/neck treatments, revealed a drop in the biological effects. A comparison of the DMLC and VMAT techniques showed a percentage loss of ETD for both anatomical sites. Differences in the protraction factor for DMLC and VMAT also contributed to a shift in the TCP model. The tumor control results revealed an overall increase in tumor control probability for VMAT compared to DMLC Conclusions: Comparing DMLC and VMAT techniques demonstrates that temporal dose deposition is an important characteristic in determining the biological effects from different treatment modalities. Although uncertainties in biological parameters prevents clinical application of these results this study still illustrates that the shorter treatment times characteristic of VMAT plans is preferable to the longer DMLC plans.