Purpose: To evaluate intensity-modulated whole pelvis radiotherapy (IM-WPRT) (with bone marrow [BM] as a planning constraint) as a means to reduce the volume of pelvic BM irradiated.
Methods and materials: Ten women with cervical or endometrial cancer previously treated using IM-WPRT were selected for this analysis. Using the treatment planning CT scan, the clinical target volume was defined to encompass the gross tumor, parametrial tissues, uterus (if present), and regional lymph nodes. The clinical target volume was expanded by a 1-cm margin to form the planning target volume (PTV). The bladder, rectum, small bowel, and pelvic BM were delineated in each patient. Three plans were created for each patient: a standard four-field WPRT plan, an IM-WPRT treatment plan designed to conform the dose to the PTV while minimizing dose to the normal tissues (excluding BM), and a BM-sparing (BMS) IM-WPRT plan that included the BM as an additional treatment planning constraint. Dose-volume histograms for the PTV, small bowel, and BM were compared for each patient.
Results: For each of the 10 patients, BMS IM-WPRT treatment plans demonstrated a significant reduction of the volume of BM receiving >40% (18 Gy) of the prescription dose (45 Gy) compared with both IM-WPRT and four-field treatment. On average, BMS IM-WPRT resulted in only 60% of the BM volume irradiated to >50% of the dose compared with 87.4% (p <0.001) of the BM volume in a four-field plan and 75.7% (p < 0.003) of the volume in an IM-WPRT plan. Furthermore, the BMS IM-WPRT plans resulted in significant sparing of all other normal tissues that was comparable to the original IM-WPRT. In all 10 cases, the BMS IM-WPRT treatment plan did not result in any significant differences in the PTV and small bowel dose-volume histograms compared with the IM-WPRT treatment plans.
Conclusion: BMS IM-WPRT significantly reduces the volume of pelvic BM irradiated compared with conventional WPRT. In addition, BMS IM-WPRT did not compromise the improvements previously seen in IM-WPRT treatment plans that did not consider BM. Clinical studies are necessary to assess the significance of BMS IM-WPRT in reducing hematologic toxicity.