Discrepancies in Out-Of-Field Dose Prediction Between Treatment Planning Systems
T Tyson*, M Mistro, A Kubli, A Harrison, K Mooney, Thomas Jefferson University Hospital, Philadelphia, PA
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PO-GePV-T-289 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Physicians routinely rely on dosimetric parameters for organs outside of the treatment field to make clinical decisions regarding plan quality. The purpose of this study is to quantify variance in predicted out-of-field dose by the TPS for relevant OAR constraints in a simple treatment plan.
Methods: This investigation compared dosimetric data from a simple breast tangent plan prescribed to 4005 cGy in 15 fractions with open jaw-based rectangular fields calculated using different beam models and calculation algorithms within several software systems. The plan parameters such as energy, field size, gantry angle, collimator angle, MU, and isocenter were kept consistent throughout all iterations and the same CT and structure set was utilized. The plan was evaluated on a Varian Truebeam, Varian Edge, Varian Trilogy and Elekta Synergy with an Agility head (Elekta-SA) modeled in various treatment planning systems: Eclipse using AAA, Xio using CCC, and Monaco using Monte Carlo.
Results: The mean heart dose was recorded for each iteration of this plan as well as the Lung V20. The mean heart doses ranged from 44.7 cGy (Trilogy, Eclipse AAA) to 98.3 cGy (Elekta-SA, Eclipse AAA), the standard deviation was 19.58 cGy. This research also found a 29% difference in the calculated mean heart dose for the same plan on the same machine that had been modeled in two different TPS. Total Lung V20 did not change significantly with distinct beam model, ranging from 12.2% to 12.5%.
Conclusion: The research reinforces that out-of-field dose cannot be quantified with consistent accuracy across treatment planning systems. The relationship between out-of-field dose predicted by a particular beam model and absolute dose being delivered to that area is not well characterized and necessitates further investigation. This calls into question the strength with which physicians should weigh dosimetric values outside of the treatment field.
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