Abstract
BackgroundA strong dose-volume relationship exists between the amount of small bowel receiving low- to intermediate-doses of radiation and the rates of acute, severe gastrointestinal toxicity, principally diarrhea. There is considerable interest in the application of highly conformal treatment approaches, such as intensity-modulated radiation therapy (IMRT), to reduce dose to adjacent organs-at-risk in the treatment of carcinoma of the rectum. Therefore, we performed a comprehensive dosimetric evaluation of IMRT compared to 3-dimensional conformal radiation therapy (3DCRT) in standard, preoperative treatment for rectal cancer.MethodsUsing RTOG consensus anorectal contouring guidelines, treatment volumes were generated for ten patients treated preoperatively at our institution for rectal carcinoma, with IMRT plans compared to plans derived from classic anatomic landmarks, as well as 3DCRT plans treating the RTOG consensus volume. The patients were all T3, were node-negative (N = 1) or node-positive (N = 9), and were planned to a total dose of 45-Gy. Pairwise comparisons were made between IMRT and 3DCRT plans with respect to dose-volume histogram parameters.ResultsIMRT plans had superior PTV coverage, dose homogeneity, and conformality in treatment of the gross disease and at-risk nodal volume, in comparison to 3DCRT. Additionally, in comparison to the 3DCRT plans, IMRT achieved a concomitant reduction in doses to the bowel (small bowel mean dose: 18.6-Gy IMRT versus 25.2-Gy 3DCRT; p = 0.005), bladder (V40Gy: 56.8% IMRT versus 75.4% 3DCRT; p = 0.005), pelvic bones (V40Gy: 47.0% IMRT versus 56.9% 3DCRT; p = 0.005), and femoral heads (V40Gy: 3.4% IMRT versus 9.1% 3DCRT; p = 0.005), with an improvement in absolute volumes of small bowel receiving dose levels known to induce clinically-relevant acute toxicity (small bowel V15Gy: 138-cc IMRT versus 157-cc 3DCRT; p = 0.005). We found that the IMRT treatment volumes were typically larger than that covered by classic bony landmark-derived fields, without incurring penalty with respect to adjacent organs-at-risk.ConclusionsFor rectal carcinoma, IMRT, compared to 3DCRT, yielded plans superior with respect to target coverage, homogeneity, and conformality, while lowering dose to adjacent organs-at-risk. This is achieved despite treating larger volumes, raising the possibility of a clinically-relevant improvement in the therapeutic ratio through the use of IMRT with a belly-board apparatus.
Highlights
A strong dose-volume relationship exists between the amount of small bowel receiving low- to intermediate-doses of radiation and the rates of acute, severe gastrointestinal toxicity, principally diarrhea
Dose to target volumes When comparing the 3-field belly board (3FBB) treatment volumes to the contoured volumes based on RTOG consensus guidelines, it was evident that the contoured planning target volume (PTV) encompassed a typically larger volume than that treated in the 3FBB plans
Dosimetric comparisons between 3FBB and intensity-modulated radiation therapy (IMRT) plans, as shown in Table 1, revealed that the percentage of the PTV receiving the prescription dose was significantly lower for the 3FBB plans than with IMRT (V45Gy: median 3FBB 87.2% versus IMRT 99.5%; p = 0.005)
Summary
A strong dose-volume relationship exists between the amount of small bowel receiving low- to intermediate-doses of radiation and the rates of acute, severe gastrointestinal toxicity, principally diarrhea. Sauer and colleagues, demonstrated that preoperative chemoradiation was superior with respect to the rates of local recurrence and sphincter preservation compared to postoperative therapy [4]. The safe, effective, and tolerable administration of preoperative chemoradiation in rectal cancer is not without challenge, owing in part to the irradiation of a large volume at risk for microscopic disease spread, with potential toxicity to nearby bowel, bladder, and bones. A strong dosevolume relationship between the amount small bowel receiving intermediate- and low-doses of radiation and the rates of severe diarrhea has been demonstrated, at the 15-Gy dose level [7,8,9,10]. Higher rates of acute severe toxicity may potentially lead to breaks in treatment or mitigate compliance, which may confer untoward consequences with respect to local control or survival [11]
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