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?

Abstract

MN and RMH contributed to the conception, design, analysis and interpretation of data and helped with the final approval of the version to be published. MN, RMH and NT helped with the drafting of the article. WB and MB contributed to the statistics of the main study. DW contributed to the acquisition of data. AK contributed to the statistics of pre-study. NT helped with the planning and implementation of radiotherapy.Objective: The objective of this retrospective planning study was to find a contouring definition for the rectum as an organ at risk (OAR) in curative three-dimensional external beam radiotherapy (EBRT) for prostate cancer (PCa) with a predictive correlation between the dosevolume histogram (DVH) and rectal toxicity. Methods: In a pre-study, the planning CT scans of 23 patients with PCa receiving definitive EBRT were analyzed. 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 the Radiation Therapy Oncology Group (RTOG) and two 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 97 patients receiving primary curative EBRT and the second cohort consisted of 66 patients treated for biochemical recurrence after prostatectomy. Rectal toxicity was investigated by clinical investigation and scored according to the Common Terminology Criteria for Adverse Events. Candidate parameters were the volume of the rectum, mean dose, maximal dose, volume receiving at least 60 Gy (V 60 ), area under the DVH curve up to 25 Gy and area under the DVH curve up to 75 Gy in dependence of each chosen rectum definition. Multivariable logistic regression considered other clinical factors such as pelvine lymphatics vs local target volume, diabetes, prior rectal surgery, anticoagulation or haemorrhoids too.

Gastrointestinal Toxicity Prediction Not Influenced by Rectal Contour or Dose-Volume Histogram Definition

“…Prior data examining truncation of rectal contours is limited. Retrospectively contouring 163 primary/salvage EBRT prostate patients, Nitsche et al 18 examined 3 different rectal definitions (chosen from 13 based on DVH heterogeneity): RTOG definition, PTV ± 1 cm, PTV ± 0 cm. No relationship was seen between various DVH parameters and worst late rectal inflammation G1+ (Common Terminology Criteria for Adverse Events).…”

“…The choice of truncations to take forward to toxicity analysis (0 and 2 cm) was made pragmatically based on prior work in the field. 18 …”

“…Third, variation in the superior/inferior extent of the contoured rectum might be reduced through procedural truncation relative to the PTV. Such truncation has previously been investigated, without a clear link to toxicity, 18 although in a comparably small cohort of 23 patients. A larger study might therefore be warranted to see if the greater rectal consistency from PTV-based truncation might lead to improved toxicity prediction performance.…”

Autosegmentation based on different-sized training datasets of consistently-curated volumes and impact on rectal contours in prostate cancer radiation therapy

“…In RT, differences in OAR volumes do not always translate into large dose differences [9] , [21] although compliance to trial protocols have been reported to be compromised due to incorrect delineations [2] . We found in our data that autosegmented rectal volumes, irrespective of algorithm version, were overall closer to the manually-delineated uncurated clinical rectal volumes (Δ≈0.5–1 cm 3 ) than to the manually-delineated curated rectal volumes (Δ≈1-3 cm 3 ), with the larger differences typically generated by the smaller-sized training datasets.…”

“…In general, overestimated dose for OARs brings the dose distributions to the safer side, however, underdosage of tumours may be a consequence if the OAR is directly adjacent to the planning target volume and unintendedly included in the high-dose region on the basis of an incorrectly-defined volume [18] . Furthermore, variations in rectum volumes and the impact on rectal toxicity modelling has been investigated in a retrospective study where the same rectum definition as used for our curated volumes was included as one of 13 investigated OAR definitions [21] . This study found that the more distinct differences in DVH shapes between the OAR definitions related to volumes of 60–110 cm 3 , mean doses of 35–50 Gy, and maximum doses of 70–73 Gy (n = 163; 3D-CRT primary and salvage prostate cancer treatments; prescribed dose = 67–76 Gy).…”