Intensity-modulated radiotherapy in patients with locally advanced rectal cancer reduces volume of bowel treated to high dose levels

doi:10.1016/j.ijrobp.2005.12.056

International Journal of Radiation OncologyBiologyPhysics

Volume 65, Issue 3, 1 July 2006, Pages 907-916

https://doi.org/10.1016/j.ijrobp.2005.12.056 Get rights and content

Purpose: To investigate the potential for intensity-modulated radiotherapy (IMRT) to spare the bowel in rectal tumors.

Methods and Materials: The targets (pelvic nodal and rectal volumes), bowel, and bladder were outlined in 5 patients. All had conventional, three-dimensional conformal RT and forward-planned multisegment three-field IMRT plans compared with inverse-planned simultaneous integrated boost nine-field equally spaced IMRT plans. Equally spaced seven-field and five-field and five-field, customized, segmented IMRT plans were also evaluated.

Results: Ninety-five percent of the prescribed dose covered at least 95% of both planning target volumes using all but the conventional plan (mean primary and pelvic planning target volume receiving 95% of the prescribed dose was 32.8 ± 13.7 Gy and 23.7 ± 4.87 Gy, respectively), reflecting a significant lack of coverage. The three-field forward planned IMRT plans reduced the volume of bowel irradiated to 45 Gy and 50 Gy by 26% ± 16% and 42% ± 27% compared with three-dimensional conformal RT. Additional reductions to 69 ± 51 cm³ to 45 Gy and 20 ± 21 cm³ to 50 Gy were obtained with the nine-field equally spaced IMRT plans—64% ± 11% and 64% ± 20% reductions compared with three-dimensional conformal RT. Reducing the number of beams and customizing the angles for the five-field equally spaced IMRT plan did not significantly reduce bowel sparing.

Conclusion: The bowel volume irradiated to 45 Gy and 50 Gy was significantly reduced with IMRT, which could potentially lead to less bowel toxicity. Reducing the number of beams did not reduce bowel sparing and the five-field customized segmented IMRT plan is a reasonable technique to be tested in clinical trials.

Introduction

Preoperative chemoradiotherapy has become the standard of care for locally advanced rectal cancer, because it has been shown to improve resectability (1, 2, 3) and sphincter preservation rates (4). The bowel is a radiosensitive organ, and acute radiation enteritis occurs in most patients undergoing radiotherapy (RT) for rectal cancer, with severe acute toxicity reported in 16–39% patients treated with preoperative RT (5, 6, 7) and 21–23% when concomitant chemotherapy was used (7, 8). In addition, late bowel toxicity (diarrhea, bowel stricture, perforation, or hemorrhage) is frequently irreversible, most often presenting within the first year after RT (9), but with longer latency periods reported. The bowel tolerant dose, defined as the dose that gives a 5% risk of late toxicity at 5 years, has been suggested to be 45–50 Gy (10). Clinical studies have suggested that increasing the dose and volume of bowel (BV) irradiated are related to the development of late toxicity. Overall, Grade 3-4 toxicity occurs in about 5% of patients with pelvic doses of 45–50 Gy (11, 12, 13). Increasing the dose to >50 Gy was associated with an increased rate of Grade 3 or worse toxicity of 37% (9) and of 40% if the treated BV was >328 cm³ (14). Gallagher et al. (12) have suggested that the absolute BVs irradiated to >50 Gy (17 cm³) and 45 Gy (78 cm³) were associated with increased late toxicity.

Intensity-modulated RT (IMRT) produces concave dose distributions that can potentially reduce the BV irradiated to, or greater than, the tolerance levels. The ability of IMRT to reduce bowel irradiation has been demonstrated in prostate, cervical, and endometrial cancers. Nutting et al. (15) showed that the amount of bowel treated to 45 Gy could be reduced to 5% of the delineated bowel in pelvic RT for prostate tumors compared with 18% using three-dimensional conformal RT (3D-CRT). In gynecologic malignancies, reductions to 13.5–17.4% were demonstrated (16, 17). In addition, lower acute toxicity was shown by Mundt et al. (18). A recent study by Duthoy et al. (19) showed increased bowel sparing with intensity-modulated arc therapy in rectal cancer patients. To date, however, the only report of the dosimetric advantages of IMRT comes from Robertson et al. (20), who found a reduction in the BV irradiated to ≥70% of the prescribed dose using IMRT.

This retrospective planning study was designed to evaluate the potential dosimetric advantages of inverse planned IMRT (IMRT_inv) using a simultaneous integrated boost (SIB) (21, 22) compared with conventional 3D-CRT and three-field forward planned IMRT (IMRT_3Ffwd) with regard to target coverage and bowel sparing. The effect of reducing the number of intensity-modulated beams and segmentation for step-and-shoot delivery was also investigated. Previous studies have suggested that a larger number of beams provides the optimal IMRT dose distribution (23), although successful attempts have been made at reducing the number of beams (15, 24), with the added advantage of shorter delivery times (25).

Section snippets

Target volume definition

Five patients with histologically proven locally advanced adenocarcinoma of the rectum treated with preoperative concomitant chemoradiotherapy were studied. Each had undergone RT planning, noncontrast-enhanced, prone CT of the pelvis with a full bladder at 5-mm intervals from the L2–L3 junction to the perineum. The CT data sets were transferred to the planning module of PINNACLE³ (Philips Radiation Oncology Systems, Milpitas, CA). For the purpose of this study, the target volumes outlined were

Results

The typical dose distributions produced by 3D-CRT and IMRT_9Feq at the level of the S1–S2 junction are shown in Fig. 1. For the IMRT_9Feq plan, the 95% and 90% isodose curves produced a concavity that reduced irradiation of the bowel. DVHs comparing the 3D-CRT and IMRT_9Feq plans for the same patient are shown in Fig. 2. Table 2 lists the dose statistics for the conventional, 3D-CRT, IMRT_3Ffw, and IMRT_9Feq plans.

Discussion

The results of this planning study have shown that the use of SIB IMRT_inv techniques in rectal cancer patients is associated with an ∼64% reduction in the percentage of BV irradiated to 45 and 50 Gy compared with 3D-CRT. Several authors have suggested that the incidence of late effects is related to the BV irradiated (7, 9, 12, 31). Gallagher et al. (12) reported no late toxicity (i.e., Radiation Therapy Oncology Group Grade 0) provided the BV irradiated to 45 Gy and 50 Gy was <78 cm³ and <17 cm

Conclusion

Clear evidence from published reports has shown that irradiation of larger BVs is associated with increased acute and late toxicity (11, 12, 13, 14). We have shown that IMRT techniques can reduce the BV treated to higher dose levels while maintaining target coverage. In addition, a reduction in the number of fields is possible without significant loss of sparing or coverage with the five-field IMRT_inv plan with gantry angles optimized for delivery and subsequently segmented for step-and-shoot

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Physics contributionIntensity-modulated radiotherapy in patients with locally advanced rectal cancer reduces volume of bowel treated to high dose levels

Introduction

Section snippets

Target volume definition

Results

Discussion

Conclusion

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Gynecol Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Physics contribution
Intensity-modulated radiotherapy in patients with locally advanced rectal cancer reduces volume of bowel treated to high dose levels