Plan Quality and Dosimetric Association of Acute and Late Rectal and Urinary Toxicities for Prostate Stereotactic Body Radiotherapy

Abstract

To quantify the magnitude of quality of life (QOL) changes (up to 3 years) with dosimetric parameters for prostate cancer patients treated with stereotactic body radiotherapy (SBRT). A total of 73 patients treated and followed between 2010 through 2013 were analyzed. All patients received SBRT with a prescription dose of 40 Gy in 5 fractions delivered through volumetric modulated arc therapy. Geometric/dosimetric parameters were extracted automatically through an institution-specific data parser, including the following quantitates: the volumes of the target, rectum and bladder, the overlap volume between PTV and organ at risk (OARs), PTV100 (PTV volume receiving 100% prescription dose), conformity index (CI, the ratio between the volume getting 50% prescription dose to PTV volume), V50, V80, V90, V100, maximum and mean dose (for rectum), V50, V100, maximum and mean dose (for bladder), etc. The patient-reported QOL data were measured by the Expanded Prostate Cancer Index Composite (EPIC) at the completion of the treatment (baseline) and every 3 months up to 3 years. The rectal/urinary QOL reduction at 6 months (acute toxicity) and 12 months (late toxicity) were assessed and correlated with the dosimetric parameters using linear regression models. For the entire cohort, PTV volume coverage at 40 Gy was 95.0 ± 1.0 (%), with a CI = of 3.2 ± 0.2. The achieved V50 and V100 were 24.1 ± 7.3 (%) and 1.5 ± 1.1 (%) for the rectums, and 21.1 ± 12.6 (%) and 3.2 ± 2.4 (%) for the bladders. The overlap volumes of the PTV and OARs showed good predictive power for the dosimetric endpoints. The correlation between bowel QOL reduction and the maximum rectal doses increased dramatically from 6 month to 12 month (slopes changed 60 fold from 0.001 to 0.06). The bowel QOL reduction at 12 months was linearly correlated to the increasing maximum rectal doses (p = 0.01) and volume of the rectum (p = 0.03), respectively. We did not observe a significant correlation between urinary QOL score and the aforementioned dosimetric endpoints at either 6 or 12 months. The current study is a novel study demonstrating the correlation between dosimetric plan parameters and patient-reported quality of life outcomes. Specifically, maximum rectum dose was the parameter most clearly showing linear correlation with late rectal toxicity. The correlations between dosimetric parameters and urinary toxicities were not as clear. Longer follow-up for larger patient cohorts are expected to further clarify potential correlations over longer time periods.