Optimal planning parameters for simultaneous boost IMRT treatment of prostate cancer using a Beam Modulator™

Abstract

Objective

To determine the optimum energy and beam arrangement for prostate intensity-modulated radiation therapy (IMRT) delivery using an Elekta Beam Modulator™ linear accelerator, in order to inform decisions when commissioning IMRT for prostate cancer.

Methods

CMS XiO was used to create IMRT plans for a prostate patient. Arrangements with 3, 5, 7, 9 and 11 equally spaced fields, containing both a direct anterior and a direct posterior beam were used, with both 6[[ce:hsp sp="0.25"/]]MV and 10[[ce:hsp sp="0.25"/]]MV photons. The effects of varying the maximum number of iterations, leaf increment, number of intensity levels and minimum segment size were investigated. Treatment plans were compared using isodose distributions, conformity indices for targets and critical structures, target dose homogeneity, body dose and plan complexity.

Results

Target dose conformity and homogeneity and sparing of critical structures improved with an increasing number of beams, although any improvements were small for plans containing more than five fields. Set-ups containing a direct posterior field provided superior conformality around the rectum to anterior beam arrangements. Mean non-target dose and total number of monitor units were higher with 6[[ce:hsp sp="0.25"/]]MV for all beam arrangements. The dose distribution resulting from seven 6[[ce:hsp sp="0.25"/]]MV beams was considered clinically equivalent to that with five 10[[ce:hsp sp="0.25"/]]MV beams.

Conclusion

Methods have been developed to plan IMRT treatments using XiO for delivery with a Beam Modulator™ that fulfil demanding dose criteria, using many different set-ups. This study suggests that 6[[ce:hsp sp="0.25"/]]MV photons can produce prostate IMRT plans that are comparable to those using 10[[ce:hsp sp="0.25"/]]MV. Work is ongoing to develop a complete class solution.