Impact of jaw position on sparing organs at risk in 3-dimensional conformal radiation therapy of pancreatic cancer

Purpose: The objective of this work is to investigate the impact of collimator jaw position on dose to organs at risk (OARs) during a 3-dimensional conformal radiotherapy (3DCRT) of pancreatic cancer and postulate a method to minimize OAR dose by proper positioning of the jaws. Methods: Clinically delivered 3DCRT treatment plans for 10 patients optimized with multiple static beams using multileaf collimator (MLC) leaves conformed to a block margin around target, and collimator jaws aligned with outer extent of the block margin were selected. Subsequent plans were generated by displacing the collimator jaws outward in lateral, superior-inferior or both directions by 1 and 2 cm without altering the MLC position. Computed dose to OARs and target with unaltered dose normalization were compared against the corresponding dose obtained from the original plans. Results: Outward displacement of the collimator jaws by 1 cm in lateral and/or superior-inferior direction resulted in a significant increase in mean dose to the studied OARs. The increase was found to be proportional to the outward displacement of the jaws. The increase in maximum dose to spinal cord was significant in a few patients while it was insignificant for all other OARs. Conclusion: Collimator jaws aligned with outer extent of a block margin minimize dose to OARs. Any gap between the block margin and the collimator jaws can lead to an inadvertent delivery of higher dose to the OARs. Hence, the use of an optimal jaw position during treatment planning becomes important to all patient plans.