SU-F-P-26: Study of Radiation Dose Evaluation for Organs at Risk Using MRI in Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

Purpose: This article compares the dosimetric differences between jaw tracking and no jaw tracking technique in static intensity-modulated radiation therapy plans of large and small tumors. Methods: Eight plans with large tumor (nasopharyngeal carcinoma, volume range: 510.9 to 768.0 cm3) and 8 plans with small tumor (single brain metastasis, volume range: 5.3 to 9.9 cm3) treated with jaw tracking on Varian EDGE LINAC were chosen and recalculated with no jaw tracking to study the dosimetric differences. We compared the differences of organ-at-risk doses (Dmax, Dmean), monitor units, and γ passing rate of plan verification (3mm/3%, threshold 10%; 2mm/2%, threshold 10%) between the 2 techniques. Results: The organ-at-risk doses of nasopharyngeal carcinoma cases having jaw tracking are all less than those with no jaw tracking. The Dmax and Dmean of organ-at-risks reduced 0.61% to 17.65% and 2.17% to 19.32%, P < .05, respectively. In cases with single brain metastasis, the organ-at-risk doses with jaw tracking were also lower than no jaw tracking. The Dmax and Dmean of organ-at-risk doses reduced 0.84% to 1.52% and 0.90% to 1.86%, P < .05, respectively. The monitor units for the large tumor and small tumor were increased by 2.41% and 1.1%, respectively. The γ passing rates (3mm/3%, th10%; 2mm/2%, th10%) of nasopharyngeal carcinoma plans are 99.89% ± 0.06% (jaw tracking) versus 99.56% ± 0.19% (no jaw tracking; P = .127); 97.15% ± 0.98% (jaw tracking) versus 91.90% ± 1.40% (no jaw tracking; P = .000), and the γ passing rates (3mm/3%, th10%; 2mm/2%, th10%) of brain metastasis plans are 99.97% ± 0.05% (jaw tracking) versus 99.44% ± 1.24% (no jaw tracking; P = .251), 98.65% ± 1.27% (jaw tracking) versus 93.35% ± 2.72% (no jaw tracking; P = .000). Conclusion: Jaw tracking can reduce the dose of organ-at-risks compared to no jaw tracking, and the effect is more significant for plans with large tumor. The γ passing rate of plans with jaw tracking is also higher than the plans with no jaw tracking. Although the monitor units in plans of jaw tracking will increase slightly, it is recommended to use jaw tracking in static intensity-modulated radiation therapy both in large and in small tumors.

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Intensity-modulated radiation therapy (IMRT): Differences in target volumes and improvement in clinically relevant doses to small bowel in rectal carcinoma.

Journal of Clinical Oncology ◽

10.1200/jco.2011.29.4_suppl.586 ◽

2011 ◽

Vol 29 (4_suppl) ◽

pp. 586-586

Author(s):

H. Mok ◽

C. H. Crane ◽

T. Briere ◽

S. Beddar ◽

M. E. Delclos ◽

...

Keyword(s):

At Risk ◽

Rectal Cancer ◽

Radiation Therapy ◽

Small Bowel ◽

Rectal Carcinoma ◽

Organs At Risk ◽

Intensity Modulated Radiation Therapy ◽

Preoperative Treatment ◽

Conformal Radiation Therapy ◽

Intensity Modulated

586 Background: In the treatment of rectal cancer, 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. Highly conformal treatment approaches, such as intensity-modulated radiation therapy (IMRT), may reduce dose to adjacent organs-at-risk (OAR). We performed a dosimetric evaluation of IMRT compared to 3-dimensional conformal radiation therapy (3DCRT) in standard, preoperative treatment for rectal cancer. Methods: Using RTOG consensus contouring atlas, treatment volumes were generated for ten patients treated preoperatively, with IMRT plans compared to 3DCRT plans derived from classic anatomic landmarks, as well as modified 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. Bowel displacement was achieved using a carbon-fiber bellyboard apparatus with prone positioning. Results: IMRT 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 modified 3DCRT plans, IMRT achieved a concomitant reduction in doses to the bowel, bladder, pelvic bones, and femoral heads, with an improvement in absolute volumes of small bowel receiving dose levels known to induce clinically-relevant acute toxicity. In the six patients with the highest volume of small bowel (range: 209-537-cc), the volume of bowel receiving 15-Gy was reduced from a median of 224-cc in the modified 3DCRT plans to 185-cc with IMRT. Also, the IMRT volumes were typically larger than that covered by classic 3DCRT fields, without incurring penalty with respect to adjacent OAR. Conclusions: For rectal carcinoma, IMRT, compared to 3DCRT, yielded plans with superior target coverage, homogeneity, and conformality, while lowering dose to adjacent OAR. This is 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. No significant financial relationships to disclose.

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