Nuclear Medicine & Radiation Therapy

Target Volume Heterogeneity Index, a Potentially Valuable Metric in IMRT Prostate Cancer Treatment Planning

Abstract

Michael M. Dominello, Isaac Kaufman, Erin McSpadden, Michael Snyder, Mark Zaki, Jordan Maier, Peter Paximadis and Steven Miller

Abstract
Purpose/Objectives: Heterogeneity index (HI) has been described in the literature as a tool for evaluating dose gradients within a planning target volume (PTV). HI may be expressed as D1/D95 where D1 and D95 equal the dose encompassing 1% and 95% of the target volume. The purpose of this study is to evaluate the effect of target volume dose heterogeneity on dose received by local organs at risk in the treatment of low and intermediate risk prostate cancer.
Materials/Methods: Treatment plans were reviewed for 157 patients with low or intermediate risk prostate cancer treated with dose-escalated radiation therapy between 6/2007 and 2/2012. Patients treated in the post-operative setting or receiving pelvic nodal irradiation were excluded. Patients were treated with either standard intensity modulation (IMRT) using 7 or 8 fields or 2-arc volumetric modulated arc therapy (VMAT). All patients had daily image-guidance. PTV HI (D1/D95) and dose-volume histogram (DVH) data at 8 dose levels for rectum and bladder were recorded. Patients were categorized into two groups (low HI or high HI) with respect to median index score. A two-tailed t-test was used to test for differences in dose received by rectum and bladder for the two groups.

Results: For the 157 plans evaluated, mean PTV volume was 164cc and mean prescription dose was 7833cGy. Median HI was 1.04 (range 1.0-1.08). Low HI (≤1.04) was found to correlate with significantly lower rectal V50 (p=0.02), V55 (p=0.01), V60 (p=0.01), V65 (p=0.01), and V70 (p=0.01). There was no significant correlation with dose received by bladder at any dose level. HI was similar for patients treated with standard IMRT and VMAT (p=0.85).
Conclusions: Target volume HI ≤1.04 is associated with more favorable rectal doses at clinically relevant dose-levels. We believe HI may serve as a valuable metric in prostate cancer treatment planning. Further work is needed to correlate these dosimetric findings with clinical outcomes.