Purpose/Objective(s):
Dynamic conformal arc radiation therapy (DCA) is a three-dimensional planning technique that is commonly used in lung radiation therapy. DCA is advantageous because it limits multi-leaf collimator (MLC) modulation and the interplay effect in the presence of target motion. However, the lack of MLC and dose rate modulation limits plan conformity and organ at risk (OAR) sparing. We hypothesize that using a simple 3 split arc planning technique to adjust MLC fitting margins and arc weighting will improve dose conformity and OAR sparing compared to using a single treatment arc.
Materials/Methods:
Thirty prior lung SBRT patients were evaluated retrospectively after treatment under an IRB approved protocol. All plans were generated to deliver 54 Gy over the course of three fractions, normalized to ensure 95% of planning target volume received the prescribed dose. Three planning strategies were compared: 1) a single arc plan (SP) with a 180 degree DCA arc with default MLC margins of 0.2 mm laterally and 0.5 mm superior-inferior, 2) a plan with three arcs split into 30 degree, 120 degree and 30 degree segments using the same margins but adjusting the weights (WP), and 3) the same arc geometry as WP but allowing for variable MLC margins for each arc (MP). All plans were generated using 6FFF and either a TrueBeam or STX machine with AcurosXB V16 in Eclipse. The plans were evaluated quantitatively using the conformity index (CI) and institutional dose tolerances for the chest wall, spinal cord, esophagus, heart, lungs and skin.
Results:
The proposed technique was able to generate clinically acceptable plans for eighteen cases meeting our institutional planning dose conformity and OAR sparing goals. Cases where the technique was unable to generate acceptable plans included target volumes that invaded the chest wall, very low-density targets and proximity to OARs. Across all patients the SP, WP and MP techniques generated 3, 5 and 18 clinically acceptable plans respectively. On average, the CI was 1.35, 1.32, and 1.19 for the SP, WP and MP techniques respectively. The chest wall V30Gy was reduced from 14.1 cc using SP to 11.8 cc with WP and 9.6 cc with MP. A paired t-test revealed the MP technique yielded statistically significantly improved CI and chest wall sparing compared to the other methods. The average OAR and plan quality metrics are reported in Table 1.
Table 1: The average conformity index and OAR dose metrics across all patient studies for each planning technique.
Conclusion:
Our proposed DCA technique can improve dose conformity and OAR sparing while providing a motion robust treatment plan. On average, the MP technique yielded the highest quality plans but required more manual planning adjustments.
