Robust dose planning objectives for mesorectal radiotherapy of early stage rectal cancer - A multicentre dose planning study

Abstract

Background and purpose: Organ preservation strategies are increasingly being explored for early rectal cancer. This requires revision of target volumes according to disease stage, as well as new guidelines for treatment planning. We conducted an international, multicentre dose planning study to develop robust planning objectives for modern radiotherapy of a novel mesorectal-only target volume, as implemented in the STAR-TReC trial (NCT02945566).

Materials and methods: The published literature was used to establish relevant dose levels for organ at risk (OAR) plan optimisation. Ten representative patients with early rectal cancer were identified. Treatment scans had mesorectal target volumes as well as bowel cavity, bladder and femoral heads outlined, and were circulated amongst the three participating institutions. Each institution produced plans for short course (SCRT, 5 × 5 Gy) and long course (LCRT, 25 × 2 Gy) treatment, using volumetric modulated arc therapy on different dose planning systems. Optimisation objectives for OARs were established by determining dose metric objectives achievable for ≥90% of plans.

Results: Sixty plans, all fulfilling target coverage criteria, were produced. The planning results and literature review suggested optimisation objectives for SCRT: V 10Gy < 180 cm3, V 18Gy < 110 cm3, V 23Gy < 85 cm3 for bowel cavity; V 21Gy < 15% and V 25Gy < 5% for bladder; and V 12.5Gy < 11% for femoral heads. Corresponding objectives for LCRT: V 20Gy < 180 cm3, V 30Gy < 130 cm3, V 45Gy < 90 cm3 for bowel cavity; V 35Gy < 22% and V 50Gy < 7% for bladder; and V 25Gy < 15% for femoral heads. Constraints were validated across all three institutions.

Conclusion: We utilized a multicentre planning study approach to develop robust planning objectives for mesorectal radiotherapy for early rectal cancer.

Keywords: Intensity-modulated; Organ preservation; Radiotherapy; Rectal neoplasms.

Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

© 2019 The Authors.

Figures

Fig. 1

Between-centre variation in dose planning, for patients with limited and larger variation in organ at risk (OAR) dose metrics. Short-course radiotherapy (SCRT, 25 Gy/5 fractions) and long-course radiotherapy (LCRT, 50 Gy/25 fractions) chosen for illustration, not to indicate that either of the treatment schedules demonstrated more planning heterogeneity. Treatment planning details for each centre are described in the main text. Red outline: Gross tumour volume, GTV. Dark blue outline: Clinical target volume, CTV. Green outline: Planning target volume, PTV. Orange outline: Bowel cavity. Yellow outline: Bladder. Light blue outline: Femoral heads. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Boxplots of dose metrics for bowel cavity (a and b) and small bowel loops (c and d) for long- and short-course mesorectal-only radiotherapy. Dotted lines indicate optimisation objectives as suggested in Table 2. Circles represent outliers outside 1.5 times the interquartile range (IQR).

Fig. 3

Boxplots of dose metrics for bladder (a and b) and femoral heads (c and d) for long- and short-course mesorectal-only radiotherapy. Dotted lines indicate optimisation objectives as suggested in Table 2. Note that centre 2 did not spare the dose to the femoral heads as much as possible, the suggested objective for femoral heads was only added if necessary.

Fig. 4

Boxplots of conformity indices (CI1 and CI2, see main text) for high dose to the PTV (a and b) and spill-over of median dose levels into surrounding normal tissue (c and d) for long- and short-course mesorectal-only radiotherapy. PTV: Planning target volume.