Feasibility of Using Stereotactic Body Radiation as an Alternative to HDR for Treatment of Cervical Cancers
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Purpose: Brachytherapy is frequently used to boost volumes at risk in the treatment of cervical cancers. Not all centers have HDR, or LDR capabilities, however, all have at least a linear accelerator. The possibility of using external beam radiation therapy utilizing a Stereotactic Body Radiation Therapy (SBRT) or Intensity Modulated SBRT (IM-SBRT) approach was evaluated in this project. Method and Materials: Volumes covered by the HDR prescription were used to define a Clinical Target Volume (CTV), with a prescription of 3250 cGy to the CTV over 5 fractions. Planning started with 33 equally spaced non-coplanar beams. Beam weight optimization was used to choose the most effective beam orientations. An SBRT plan was generated using unmodulated beams and then, by allowing beam modulation, IM-SBRT plans were generated. Minimum doses to 0.5, 1, 2, and 5 cc of rectum and bladder in the highest dose region were compared among HDR, SBRT, and IM-SBRT plans. Total Dose Volume Histograms (DVHs) were also compared. Results: 98% of the CTV was covered by the prescription dose for SBRT and IM-SBRT plans. Comparing plans, the highest doses to small volumes, showed that the dose to the high dose region of the rectum was increased by 8% for both SBRT and IM-SBRT plans, and the dose to the bladder was 8% higher using SBRT and 3% lower using IM-SBRT planning. The rectal DVHs for two patients were very similar for all three plans while IM SBRT showed an advantage for three patients. The bladder DVHs were similar for all plans for all patients. In general the rectum and bladder received a higher dose using SBRT, while the femoral heads received less dose using HDR. As expected, a larger volume of normal tissue was exposed to radiation using external beam irradiation. Conclusion: SBRT and IM-SBRT methods provided similar tumor coverage to HDR though normal structures received less dose using IM-SBRT approach than SBRT approach. It is feasible to replace HDR with external beam. This substitution makes the standard of care available to females worldwide and has the potential to treat large and irregularly shaped tumors.