Room: Exhibit Hall
Purpose: To evaluate the range uncertainty in proton treatment for a patient with bone cement (BC).
Methods: A patient with a recurrent right lower lobe lung cancer has been referred for proton therapy treatment. The planning CT showed the presence of a BC (4.3cc) (KYPHONâ„¢ HV-Râ„¢ C) at the T2 level, with an apparent physical mass density of 2.48 g/cm3 (due to the 30% w/w Barium sulfate, CT contrast agent), which is different from the true physical density. The patient treatment was planned to deliver 40 Gy, with one right lateral and one posterior beam. The true relative stopping power (RSP) for the cement example was measured directly using a multi-energy-layer-ionization chamber. The cement density was overridden and the dose was calculated on the updated CT for comparison. Close attention was paid to the dose difference seen at the organ at risk (OAR), in our case, the esophagus, which is a couple millimeters posterior to the CTV.
Results: The range difference measured revealed a calculated RSP of the BC to be 1.14, compared to the apparent RSP of 2.05). After density override, the difference between the two dose distributions showed a potential overdose from the posterior beam of 1.7Gy (5%) to 2cc, 3.35Gy (10%) to 0.89cc and 8.38Gy (25%) to 0.05cc) to the esophagus.
Conclusion: We have seen in this case study that a potential 8.38Gy or 25% overdose can occur in the OAR which can be accounted for following the method described herein. The dose difference depends on the beam angle and the proximity of the OAR to the foreign material. Accounting for the true BC RSP is more significant for serial structures where the maximum dose is key. Special care is needed for the patient with this type of bone cement near the target and OARs.