Dosimetric impact of setup accuracy for an electron breast boost technique.

PURPOSE: To determine the setup error on an electron breast boost technique using daily cone beam computed tomography (CBCT). Patient and setup attributes were studied as contributing factors to the accuracy. METHODS AND MATERIALS: Reproducibility of a modified lateral decubitus position breast boost setup was verified for 33 patients using CBCT. Three-dimensional matching was performed between the CBCT and the initial planning CT for each boost fraction by matching the tumor bed and/or surgical clips. The dosimetric impact of the daily positioning error was achieved by rerunning the initial treatment plans incorporating the recorded shifts to study the dose differences. Breast compression, decubitus angle, tumor bed location and volume, and cup size were studied for their contribution to setup error. RESULTS: The range of setup errors was: 1.5 cm anterior to 9 mm posterior, 1.3 cm superior to 2.3 cm inferior, and 3.2 cm medial to 2.4 cm lateral. Seven patients had setup errors that were ≥2-cm margin placed on the tumor bed and scar. Four of those 7 patients had unacceptable coverage as defined by the volume of the tumor bed plus scar that is covered by the 90% isodose line (V90) compared with the original plan. All other patients had no discernible difference in the coverage (V90). The use of compression, tumor bed location, or volumes >20 mL showed no effect on coverage. CONCLUSIONS: In general, this study supported that a 2-cm margin was adequate (29 of 33 patients) when patients are treated under typical conditions. Care should be taken when high electron energies are selected because the coverage at depth is more difficult to maintain in the clinical environment.

Exploring the feasibility of dose escalation positron emission tomography-positive disease with intensity-modulated radiation therapy and the effects on normal tissue structures for thoracic malignancies.

The pattern of failure is one of the major causes of mortality among thoracic patients. Studies have shown a correlation between local control and dose. Intensity-modulated radiation therapy (IMRT) has resulted in conformal dose distributions while limiting dose to normal tissue. However, thoracic malignancies treated with IMRT to highly conformal doses up to 70 Gy still have been found to fail. Thus, the need for dose escalation through simultaneous integrated boost (SIB) may prove effective in minimizing reoccurrences. For our study, 28 thoracic IMRT plans were reoptimized via dose escalation to the gross tumor volume (GTV) and planning target volume (PTV) of 79.2 Gy and 68.4 Gy, respectively. Reoccurrences in surrounding regions of microscopic disease are rare therefore, dose-escalating regional nodes (outside GTV) were not included. Hence, the need to edit GTV margins was acceptable for our retrospective study. A median dose escalation of approximately 15 Gy (64.8-79.2 Gy) via IMRT using SIB was deemed achievable with minimal percent differences received by critical structures compared with the original treatment plan. The target's mean doses were significantly increased based on p-value analysis, while the normal tissue structures were not significantly changed.