Purpose: To maximize rectum sparing in prostate radiotherapy, both the beam angles and fluence are optimized to be deliverable on a conventional C‐arm linac equipped with robotic couch and gantry. Methods: 12 retrospective low‐risk prostate cancer patients previously treated by volumetric modulated arc therapy (VMAT) were replanned by 4π radiotherapy. Forty Gy in 5 fractions were prescribed to cover 95% of the prostate PTV. To replan, 5×5 mm2 beamlets were precomputed using convolution/superposition. A column generation method was used to optimize beam orientations and fluence in an inter‐leaved fashion. The candidate beam pool consisted of non‐coplanar beams free from collision. The price, defined as the first order inner product, of each additional beam or beamlet was evaluated and the ones with highest price were kept. A total of 30 beams were selected for each patient. The planning target volume coverage, maximum doses (D1cc) to 1 cm3 the rectum and bladder were compared. Results: Both planning methods resulted in PTV coverage according to the prescription. For the centrally located prostate target, the 4pi planning method tends to selected beams confined to non‐coplanar angles between ±30°. Nevertheless, compared with the VMAT plans using two full arcs, rectum D1cc was reduced from 30.2 Gy to 26.9 Gy (p=0.0005), the mean body dose was reduced from 2.07 Gy to 1.75 Gy (p=0.0001). The maximum bladder dose was not significantly changed. Conclusion: By adding beams with moderate non‐coplanar angles and allowing more intensity modulation at each beam orientation, superior prostate plan conformality was achieved compared against state of the planning method; the maximum rectum dose was reduced for more than 10% without compromising target coverage. While prostate SBRT was used as an example, we reasonably expect similar dosimetric improvements for conventionally fractionated prostate treatment. The study supports the development of prostate robotic radiotherapy.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging