Comparison of different contouring definitions of the rectum as organ at risk (OAR) and dose-volume parameters predicting rectal inflammation in radiotherapy of prostate cancer: which definition to use?

The British journal of radiology

PubMedID: 27936891

M N, W B, M B, D W, A K, N T, Rm H. Comparison of different contouring definitions of the rectum as organ at risk (OAR) and dose-volume parameters predicting rectal inflammation in radiotherapy of prostate cancer: which definition to use?. Br J Radiol. 2016;20160370.
OBJECTIVES
The objective of this retrospective planning study was to find a contouring definition for the rectum as an organ at risk (OAR) in curative 3D external beam radiotherapy (EBRT) for prostate cancer (PCa) with a predictive correlation between dose-volume-histograms (DVH) and rectal toxicity.

METHODS
In a pre-study the planning CT scans of n= 23 patients with PCa receiving definitive EBRT were analysed. The rectum was contoured according to 13 different definitions, and the dose distribution was correlated with the respective rectal volumes by generating DVH curves. Three definitions were identified to represent the most distinct differences in the shapes of the DVH curves: one anatomical definition recommended by RTOG, and 2 functional definitions based on the target volume. In the main study the correlation between different relative DVH parameters derived from these three contouring definitions and the occurrence of rectal toxicity during and after EBRT was studied in two consecutive collectives. The first cohort consisted of n = 97 patients receiving primary curative EBRT, the second of n = 66 patients treated for biochemical recurrence after prostatectomy. Rectal toxicity was investigated by clinical investigation and scored according to common toxicity criteria adverse events (CTC AE). Candidate parameters were volume of the rectum, mean dose, maximal dose, V60 (volume receiving at least 60 Gy), AUC(25) (area under the DVH curve up to 25 Gy) and AUC(75) in dependence of each chosen rectum definition. Multivariable logistic regression considered other clinical factors like pelvine lymphatics vs. local target volume, diabetes, prior rectal surgery, anti-coagulation, or haemorrhoids, too.

RESULTS
In cohort 1 (primary EBRT) the mean rectal volume for definition "RTOG", "PTV-based", and PTV-linked was 100cm³ (standard deviation 43cm³), 60cm³ (SD 26), and 74cm³ (SD 31), respectively (p < 0.01, ANOVA). The mean rectal dose according to these definitions was 35 Gy (SD 8 Gy), 48 Gy (SD 4), and 44 Gy (SD 5) (p < 0.01). In cohort 2 (salvage EBRT) the mean rectal volume was 114cm³ (SD 47cm³), 64cm³ (SD 26), and 81cm³ (SD 30) (p < 0.01), and the mean dose received by the rectum 36 Gy (SD 8 Gy), 49 Gy (SD 5), and 44 Gy (SD 5) (p < 0.01). Acute or subacute rectal inflammation occurred in 69 (71.9%) patients in cohort 1 and in 43 (70.5%) in cohort 2. We did not find a correlation between all investigated DVH parameters and rectal toxicity, irrespective of the investigated definition. By adding additional variables in multivariate analysis, the predictive ability was substantially improved. Still, there was essentially no difference in the probability of predicting rectal inflammation occurrence between the tested contouring definitions.

CONCLUSIONS
The RTOG anatomy-based recommendations are questionable in comparison with functional definitions, as they result in higher variances in several relative DVH parameters. Moreover, the anatomy-based definition is no-better and no-worse in predictive value concerning clinical endpoints. Advances in knowledge: Functional definitions for the rectum as OAR are easier to apply, faster to contour, have smaller variances, and do not offer less information than the anatomy-based RTOG definition.