Intra- and inter-observer variability in contouring prostate and seminal vesicles: implications for conformal treatment planning

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Abstract

Background and purpose: Accurate contouring of the clinical target volume (CTV) is a fundamental prerequisite for successful conformal radiotherapy of prostate cancer. The purpose of this study was to investigate intra- and inter-observer variability in contouring prostate (P) and seminal vesicles (SV) and its impact on conformal treatment planning in our working conditions.

Materials and methods: Inter-observer variability was investigated by asking five well-trained radiotherapists of contouring on CT images the P and the SV of six supine-positioned patients previously treated with conformal techniques. Short-term intra-observer variability was assessed by asking the radiotherapists to contour the P and SV of one patient for a second time, just after the first contouring. The differences among the inserted volumes were considered for both intra- and inter-observer variability. Regarding intra-observer variability, the differences between the two inserted contours were estimated by taking the relative differences in correspondence to the CT slices on BEV plots (antero-posterior and left-right beams). Concerning inter-observer variability, the distances between the internal and external envelopes of the inserted contours (named projected diagnostic uncertainties or PDUs) and the distances from the mean inserted contours (named mean contour distances or MCDs) were measured from BEV plots (i.e. parallel to the CT slices).

Results: Intra-observer variability was relatively small (the average percentage variation of the volume was ~5%; SD of the differences measured on BEV plots within 1.8 mm). Concerning inter-observer variability, the percentage SD of the inserted volumes ranged from 10 to 18%. Differences equal to 1 cm in the cranio-caudal extension of P+SV were found in four out of six patients. The largest inter-observer variability was found when considering the anterior margin in the left-right beam of P top (MCD=7.1 mm, 1 SD). Relatively high values for MCDs were also found for P bottom, for the posterior and lateral margins of P top (2.6 and 3.1 mm, respectively, 1 SD) and for the anterior margin of SV (2.8 mm, 1 SD). Relatively small values were found for P central (from 1.4 to 2.0 mm, 1 SD) and the posterior margin of SV (1.5 mm, 1 SD).

Conclusions: The application of larger margins taking inter-observer variability into account should be taken into consideration for the anterior and the lateral margins of SV and P top and for the lateral margin of P. The impact of short-term intra-observer variability does not seem to be relevant.

Introduction

Many sources of possible errors in planning and delivering conformal radiotherapy for prostate cancer have been investigated and their impact on treatment planning (i.e. the choice of appropriate margins in defining the planning target volume (PTV) [11]) has been widely analyzed 1, 3, 4, 11, 15, 22, 24, 29. However, even though accurate contouring of the clinical target volume (CTV) is a fundamental prerequisite for successful conformal radiotherapy, practically no literature exists about inter- and intra-observer variability in contouring prostate and seminal vesicles, which, in many cases, correspond to the CTV in the treatment of localized prostate cancer.

The reason for this lack of knowledge should depend on a generally accepted opinion that, in most cases, the prostate and the seminal vesicles should be well visible on CT images. However, as indicated by some authors, an estimate of intra- and inter-doctor variability is important 13, 29, due to the reduced margins which are applied during the beam's-eye-view (BEV)-based conformal shaping of the beams. Both intra- and inter-observer variations could also be influenced by technical aspects, such as the image resolution, the choice of grey levels, the thickness of CT slices and the distance between them and the use of contrast liquids in the bladder and/or rectum.

The aim of this current work is to investigate intra- and inter-observer variability in contouring the prostate and seminal vesicles in our routine working conditions.

Five well-trained radiotherapists were asked to contour the prostate and the seminal vesicles of six supine-positioned patients previously treated with conformal radiotherapy for prostate cancer. For one patient, all doctors were asked to reinsert the same contour after a few minutes, in order to assess (short-term) intra-observer variability.

As an example of the impact of inter-observer variability on conformal treatment planning, only one patient was considered. The PTVs corresponding to the CTVs (prostate+seminal vesicles) contoured by each doctor were generated through automatic volume expansion. A four-field conformal technique was simulated for each PTV and the relative differences in dose statistics and dose–volume histograms (DVHs) of the rectum and the bladder were reported.

Section snippets

CT simulation: procedures and patient characteristics

Five well-trained radiotherapists with at least 8 years clinical experience and 3 years experience in 3D multi-slice CT treatment planning participated in this study. The CT images of six consecutive patients with localized prostate cancer, previously irradiated with conformally-shaped fields, were considered. This sample is representative of our daily working conditions. All patients had been submitted to a CT scan in the supine position (Toshiba TCT 900S) with their bladders full and with

Volumes

The percentage variations in the inserted volumes were relatively low, ranging from 1.5 to 9% (average 5%), among the different doctors.

BEV plot analysis

When considering the BEV plot analysis, the mean differences between the first and the second contours were about 0 with SD ranging from 0.8 to 1.8 mm (see Table 1) for the various positions/directions. Intra-observer variability was higher for the lateral direction (in the AP view) with respect to the posterior and anterior directions (in the lateral view).

Discussion

A number of studies have investigated the potential impact of intra- and inter-observer variability on contouring CTVs/PTVs and organs at risk (OARs) during the treatment planning process 5, 7, 8, 10, 13, 14, 18, 19, 27, 28, especially for head and neck and lung malignancies. Jones et al. [13]roughly estimated a standard deviation in the BEV margin of ~3 mm in the prostate. However, this estimate was based on the data which referred to only three doctors contouring P of one patient.

Intra-doctor

Conclusions

To our knowledge, this is the first extensive study which tries to assess the impact of intra- and inter-observer variability on contouring P+SV in clinically suitable conditions.

The most important result is that a relatively high inter-observer variability was found for P bottom and top and for anterior/lateral margins of SV. Inter-observer variability for the anterior margin of SV was found to decrease when contrast liquid was not added in the bladder. The need to use larger margins in the

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