Abstract
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.
Highlights
Pancreatic cancer has been projected to become second leading cause of cancer related death in the United States by 2030.1 Surgery alone is not an obvious option for pancreatic cancer treatment because of its aggressive biology, late diagnosis, encasement of large blood vessels and the presence of metastasis.[2,3] Despite the high chances of distant metastases of pancreatic cancer, radiotherapy may provide a survival advantage.[4]
Changes in mean dose to the organs at risk (OARs) were larger than the changes in maximum dose
These effects on most of the patients were smaller for larger structures such as liver and bowels, and other structures that extend beyond 2 cm from the target
Summary
Pancreatic cancer has been projected to become second leading cause of cancer related death in the United States by 2030.1 Surgery alone is not an obvious option for pancreatic cancer treatment because of its aggressive biology, late diagnosis, encasement of large blood vessels and the presence of metastasis.[2,3] Despite the high chances of distant metastases of pancreatic cancer, radiotherapy may provide a survival advantage.[4] Current radiation prescription dose (~ 54 Gy) is not adequate for the tumor control.[5,6] One approach for better tumor control is to increase the prescription dose but that option comes at the cost of higher toxicity to the OARs.[7] reducing dose to OARs becomes extremely important. Intensity modulated radiation therapy (IMRT) is a technique that can spare critical structures well. Some studies suggest that IMRT did not present a significant advantage over three dimensional conformal radiation therapy (3DCRT) in terms of OARs sparing.[8]
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