M. R. Bussiere1,2, and N. Depauw2,3; 1Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, 2Harvard Medical School, Boston, MA, 3Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA
Purpose/Objective(s): At our center, proton PBS field-specific QA is performed using detector array consisting of 1020 ion chambers with individual diameters of 4.5 mm, spacing resolution of 7.6 mm and an overall measurement area of 244 x 244 mm2. The detector’s dimensional characteristics make it difficult to measure small fields and we aim to restrict patient fields such that FWHM = 40 mm. QA for smaller fields, such as those used for proton PBS radiosurgery use a diamond detector for absolute dosimetry and film for lateral profile analysis. Our film-based QA workflow is more time consuming and error prone than using a detector array which can be used for both absolute and relative dosimetry. A recently released high resolution detector array designed for proton therapy consists of 1521 ion chambers with individual diameters of 2.5 mm, spacing resolution of 2.5 mm for the central area of 65 x 65 mm2, and 5.0 mm for the remaining area to 150 x 150 mm2. Our goal is to assess this high-resolution array to reduce the minimum PBS clinical field diameter. Materials/
Methods: We fabricated a mounting adapter to enable the same table indexing QA setup for both the current and new high-resolution detector arrays. A programming environment script used for g-analysis, was modified to read the data format of both devices fabricated by different manufacturers and enable the same QA workflow. Plans were generated using a pencil beam algorithm TPS. Field delivery was performed on a cyclotron using brass apertures for collimation and a synchrotron without apertures. Clinical fields were measured at various depths using both devices to compare treatment plans. Specialized small fields were also created with complex dose gradients and field FWHM ranging from 14-58 mm. Plan agreement was assessed using various PASS criteria of 85-95% and 1%/1mm to 3%/3mm and 10% threshold. Results: Our TPS dose output has some limitations modeling the halo effect, resulting in the need to scale overall monitor units. g-analysis pass rates of dose-uncorrected deliveries are similar for both devices, however, as expected, the higher resolution detector proved more sensitive to disagreement between TPS and delivery. Despite PASS rates agreeing for both devices, after dose-scaling corrections were applied using 2mm/2% @ 95% PASS criteria, 13% of un-scaled fields requiring dose scaling adjustment would not be detected with the coarser array. Similar trends are observed with different analysis and PASS criteria. Dose scaling magnitudes ranging from 0.1 - 5.9 % were within 0.8 ± 0.3 % when using 1-3% / 1-3mm criteria and within 0.5 ± 0.3 % when excluding 2-3% / 1mm. However, using the tighter criteria of 1-2% / 2mm was optimal to assess dose differences. Conclusion: Our results provided confidence in using the high-resolution array for small fields and we were able to reduce our minimum PBS field diameter from 40 mm to 15-20 mm FWHM. Since testing this device, we have measure 240 clinical PBS fields at various depths using the new device.
