Intensity-modulated radiation therapy as a means of reducing dose to bone marrow in gynecologic patients receiving whole pelvic radiation therapy

Abstract

Purpose: Whole pelvic radiation therapy (WPRT) is commonly used in gynecologic malignancies. However, WPRT results in irradiation of a considerable volume of the surrounding normal tissues including small bowel and pelvic bone marrow (BM). While BM suppression is not typically significant following WPRT alone, it is common in patients who receive chemotherapy. Recent data have demonstrated that intensity-modulated WPRT (IM-WPRT) significantly reduces the volume of small bowel, bladder and rectum receiving the prescription dose in gynecology patients. The purpose of this study is to evaluate IM-WPRT as a means to reduce the volume of pelvic BM irradiated. Materials and methods: Ten women with cervical or endometrial cancer previously treated using IM-WPRT were selected for this analysis. Using the treatment planning computed tomography scan, the clinical target volume (CTV) was defined to encompass the gross tumor, parametrial tissues, uterus (if present) and the regional lymph nodes. The CTV was expanded by a 1 cm margin to form the planning target volume (PTV). The bladder, rectum, small bowel were delineated in each patient. BM in the iliac crests was also contoured since studies have shown that the majority of the pelvic BM is located in these sites. Two plans were created for each patient: a standard 4-field WPRT plan with apertures shaped to the PTV in each beam’s-eye view, and an IM-WPRT plan designed to conform dose to the PTV while minimizing the dose to the normal tissues including the BM. Dose volume histograms (DVHs) for the PTV, small bowel and BM were compared for each patient. Results: For each of the ten patients, IM-WPRT treatment plans demonstrated a significant reduction of the volume of the BM receiving greater than 30% of the prescription dose (13.5 Gy). The attached table shows the average volume of the BM in the iliac crests receiving the specified dose (or greater) for the 4-field WPRT and the BM-sparing IM-WPRT plans. On average, IM-WPRT resulted in a 50% reduction in the volume of BM irradiated to doses greater than 22.5 Gy (p < 0.001), and more than 60% reduction of the BM irradiated to doses greater than 31.5 Gy (p < 0.001). Furthermore, as expected, the BM-sparing IM-WPRT plans resulted in significant sparing of all other normal tissues that was comparable to the original IM-WPRT that did not specifically consider BM as a constraint. In all ten cases, the BM-sparing treatment plan did not result in any significant differences in the PTV and small bowel DVHs. Conclusion: IM-WPRT significantly reduces the volume of pelvic BM irradiated compared to conventional WPRT. In addition, BM-sparing IM-WPRT did not compromise the improvements previously seen in IM-WPRT treatment plans that did not consider BM. Currently patients are being treated with this BM-sparing technique to determine if the dosimetric improvements seen here translate into clinical benefits. Tabled 1Dose Gy (%)WPRT (% BM Volume)BM-Sparing IM-WPRT (% BM volume)p-value9.0 (20%)95.8%98.5%< 0.00113.5 (30%)92.3%90.4%0.3522.5 (50%)87.0%43.7%< 0.00131.5 (70%)52.8%25.9%< 0.00140.5 (90%)42.5%12.1%< 0.00145.0 (100%)33.0%4.5%< 0.001 Open table in a new tab