International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationPreoperative Helical Tomotherapy and Megavoltage Computed Tomography for Rectal Cancer: Impact on the Irradiated Volume of Small Bowel
Introduction
Short-term preoperative radiotherapy was reported to decrease the risk of local recurrence in patients with rectal cancer who underwent total mesorectal excision (TME) from 10.9% to 5.6% after a follow-up of 5 years (1). In addition, the Swedish Rectal Cancer Trial demonstrated an overall survival benefit of 8% after a median follow-up time of 13 years for patients who underwent preoperative irradiation (2). The addition of concomitant chemotherapy has become a standard of care in many countries, as it seems to be improving resectability and sphincter preservation rates 3, 4. However, the toxicity of chemoradiation is considerable 5, 6, 7, 8. The German Rectal Cancer Study Group reported acute and late Grade 3 or 4 toxicity in 27% and 14% of the patients respectively (8). Recently, a long-term analysis of the patients participating in the Swedish Rectal Cancer Trial showed a more then twofold excessive risk of late small bowel obstruction and abdominal pain in patients irradiated preoperatively compared with surgery alone (5).
A growing body of evidence indicates a strong dose–volume relationship for the development of small-bowel toxicity 9, 10, 11, 12. Baglan et al. observed no Grade 3+ toxicity in patients receiving 15 Gy to less than 150 cc of small bowel, whereas 50% of the patients receiving 15 Gy to more than 150 cc developed Grade 3+ toxicity (9). The irradiated volume of small bowel may be reduced by creating a conformal dose distribution that tightly match the horseshoe-shape PTV of rectal cancer, using intensity-modulated radiotherapy (IMRT) delivery techniques such as intensity-modulated arc therapy (IMAT) and helical tomotherapy 13, 14. To further reduce this volume, it is mandatory to reduce the CTV-PTV margin and hence the volume of small bowel that is intentionally irradiated 15, 16. We recently reported that with the use of daily megavoltage (MV) CT imaging the CTV-PTV margins could be reduced from 15, 15, 15, and 10 mm to 8, 11, 7, and 10 mm for the X, Yant, Ypost and Z direction, respectively, compared with results achieved with laser-skin marks (17).
The aim of this study was to explore to what extent helical tomotherapy and daily MV-CT imaging may reduce the irradiated volume of small bowel and its normal tissue complication probability (NTCP). To do so, we performed a planning comparison in 11 patients with Stage II/III rectal cancer between 3D-conformal radiotherapy (3D-CRT) with classical CTV-PTV margins, helical tomotherapy (IMRT), and helical tomotherapy with reduced margins (IMRT/IGRT). The different plans were compared by analyzing target and intersection volumes, dose volume histograms (DVH) and NTCP values for predicting acute Grade 2+ diarrhea.
Section snippets
Volume delineation
The study involved 11 patients (10 male, 1 female) with Stage II/III rectal cancer that were consecutively treated in our department. All patients underwent planning CT in prone position using a conventional helical CT scanner (Siemens Emotion 16, Erlangen, Germany) with a 3-mm slice thickness. Patients were asked to urinate and drink 250 ml of water 1 hour before the planning CT and each treatment session.
Delineation of the CTV and organ at risks (OARs) was performed as described elsewhere (14)
Margins and volume reduction
The CTV-PTV margin reduction obtained by daily MV-CT imaging compared with laser-skin marks, decreased the mean PTV from 1857.4 ± 256.6 cc to 1462.0 ± 222.3 cc (p < 0.01), corresponding to a relative volume reduction of 21.4% (Table 1).
The mean volume of small bowel in the pelvis was 225.2 ± 142.5 cc (range, 11.9–429 cc). The mean intersection volume of PTV and the small bowel (PTV ∩ SB) decreased from 31.4 ± 28.0 cc (laser-skin marks) to 19.5 ± 19.7 cc (daily MV-CT, p < 0.01), corresponding to
Discussion
The introduction of preoperative (chemo)radiotherapy in rectal cancer has led to reduced local recurrence rates and improved survival 1, 2. Unfortunately, acute and late gastrointestinal toxicity remains a clinical concern and dose limiting factor 5, 8. The irradiated volume of small bowel is the most predictive determinant for the development of gastrointestinal toxicity 9, 10, 11, and the role of IMRT in reducing the irradiated volume of small bowel was recently emphasized in preoperative
Conclusion
In conclusion, the combination of helical tomotherapy and daily MV-CT imaging in the preoperative radiotherapy of rectal cancer decreases the irradiated volume of small bowel and its NTCP. This may extend the indications of preoperative radiotherapy and allow exploration of more effective treatment schedules.
Acknowledgments
This research was funded by grants the Foundation against Cancer, foundation of public interest (SCIE2005-29), the “Wetenschappelijk Fonds W. Gepts UZ Brussel” and the “Fonds voor Wetenschappelijk Onderzoek – Vlaanderen” (G.0386.07 – G.0486.06).
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Conflict of interest: The authors have a research agreement with TomoTherapy Inc., Madison, WI.