Christopher Stepaniak

Improvements in dose accuracy delivered with static-MLC IMRT on an integrated linear accelerator control system.

PURPOSE Dose accuracy has been shown to vary with dose per segment and dose rate when delivered with static multileaf collimator (SMLC) intensity modulated radiation therapy (IMRT) by Varian C-series MLC controllers. The authors investigated the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of SMLC-IMRT fields on a Varian TrueBeam linear accelerator (LINAC), which delivers dose and manages motion of all components using a single integrated controller. METHODS An SMLC sequence was created consisting of ten identical 10 × 10 cm(2) segments with identical MUs. Beam holding between segments was achieved by moving one out-of-field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 1 to 40 and dose rates of 100-600 MU/min for a 6 MV photon beam (6X) and dose rates of 800-2400 MU/min for a 10 MV flattening-filter free photon (10XFFF) beam. All measurements were made with a Farmer (0.6 cm(3)) ionization chamber placed at the isocenter in a solid-water phantom at 10 cm depth. The measurements were performed on two Varian LINACs: C-series Trilogy and TrueBeam. Each sequence was delivered three times and the dose readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Δ(i)) were compared using F-tests (α = 0.05). RESULTS On the Trilogy, large Δ(i) was observed in small MU segments: at 1 MU/segment, the maximum Δ(i) was 10.1% and 57.9% at 100 MU/min and 600 MU/min, respectively. Also, the first segment of each sequence consistently overshot (Δ(i) > 0), while the last segment consistently undershot (Δ(i) < 0). On the TrueBeam, at 1 MU/segment, Δ(i) ranged from 3.0% to 4.5% at 100 and 600 MU/min; no obvious overshoot/undershoot trend was observed. F-tests showed statistically significant difference [(1 - β) =1.0000] between the Trilogy and the TrueBeam up to 10 MU/segment, at all dose rates greater than 100 MU/min. The linear trend of decreasing dose accuracy as a function of increasing dose rate on the Trilogy is no longer apparent on TrueBeam, even for dose rates as high as 2400 MU/min. Dose inaccuracy averaged over all ten segments in each beam delivery sequence was larger for Trilogy than TrueBeam, with the largest discrepancy (0.2% vs 3%) occurring for 1 MU/segment beams at both 300 and 600 MU/min. CONCLUSIONS Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SMLC-IMRT delivery. Our results confirmed these findings. The dose delivery accuracy for SMLC-IMRT is significantly improved on TrueBeam compared to Trilogy for every combination of low MU/segment (1-10) and high dose rate (200-600 MU/min), in part due to the faster sampling rate (100 vs 20 Hz) and enhanced electronic integration of the MLC controller with the LINAC. SMLC-IMRT can be implemented on TrueBeam with higher dose accuracy per beam (±0.2% vs ±3%) than previous generations of Varian C-series LINACs for 1 MU/segment delivered at 600 MU/min).

Image-guided radiation therapy for prostate cancer: A computed tomography–based assessment of fiducial marker migration between placement and 7 days

PURPOSE This study was conducted to determine whether clinically significant fiducial marker migration occurs immediately after prostatic implantation. METHODS AND MATERIALS One hundred patients with transperineal (n = 39) or transrectal (n = 61) placement of 3 gold fiducial markers underwent computed tomography scans on day 0 (after placement) and day 7 (at radiation planning). Each marker was marked as a point of interest in a treatment planning system. An automated point-based algorithm was then used to coregister the day 0 and day 7 images by matching the markers through rigid translations and rotations. The mean distance between fiducial pairs (d¯) was recorded to assess the degree of seed migration. Prostate contours were delineated, and the day 0 prostate volumes were uniformly expanded by 1, 3, and 5 mm. The percentage of the day 7 prostate volume covered by each day 0 prostate with expansion was calculated to assess whether prostate contours, if performed on day 0, would adequately cover the prostate on day 7. RESULTS The average d¯ for all patients was 0.78 ± 0.45 mm; only 1 patient had d¯ > 2 mm. Placement technique, hormonal therapy, prostate size, and marker distance from the capsule were not associated with d¯ (P > .05). The mean percentages of day 7 prostate volumes covered by the day 0 prostate plus 1, 3, and 5 mm were 98.3%, 99.8%, and 100%, respectively. With an expansion of 3 mm, 98% of men had >95% of day 0 volume covered; with an expansion of 5 mm, 100% of men had 100% of the day 0 volume covered. CONCLUSIONS There is minimal change in the relative positions of fiducial markers (average d¯ < 1.0 mm) 1 week after placement. A 1- to 3-mm expansion would account for the variation in seed position for the vast majority of cases. These results suggest that planning could be performed on the day of implantation without adverse consequence.

A feasibility study of 2-mm bolus for postmastectomy radiation therapy

PURPOSE The purpose of this study was to prospectively evaluate the use of daily 2-mm bolus in patients undergoing postmastectomy radiation without reconstruction using optically stimulated luminescence dosimetry and weekly assessment of skin toxicity. METHODS AND MATERIALS We prospectively collected data from the first 49 women treated with a daily 2-mm Superflab bolus during their postmastectomy radiation therapy from 2013 to 2016 at The University of Chicago Comprehensive Cancer Center at Silver Cross. Within the first 3 days of starting radiation therapy, we measured the surface dose in vivo at 5 anatomical locations under the 2-mm bolus on the chest wall. We assessed weekly the acute skin toxicity during radiation using the National Cancer Institute Common Toxicity Criteria. Patients with reconstruction before radiation therapy were excluded. RESULTS Forty-nine women with a mean age of 54.3 years were treated with daily 2-mm bolus to the chest wall following mastectomy. Median follow-up was 32.7 weeks. The mean percentages of prescribed dose (standard deviation) for the median, central, lateral, superior, and inferior optically stimulated luminescence dosimeters were 100.1% (5.6%), 108.1% (6.7%), 98.1% (6.5%), 102.6% (8.9%), and 106.3% (6.6%), respectively. The majority (71.4%) of women experienced a maximum acute National Cancer Institute Common Toxicity Criteria skin toxicity score of 2, with only 12.2% experiencing a score of 3. There were no grade 4 toxicities. There were no local recurrences during our follow-up period. CONCLUSIONS A daily 2-mm bolus is a feasible regimen for chest wall bolus during postmastectomy radiation therapy with acceptable dose buildup and skin toxicity.

SU‐E‐T‐434: Improvements in Step‐And‐Shoot Dose Delivery Accuracy on Varian TrueBeam

Purpose: To investigate the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of step‐and‐shoot intensity modulated radiation (SS‐IMRT) fields on a TrueBeam LINAC.Methods: A step‐and‐shoot multi‐leaf collimator(MLC) sequence was created consisting of 10 identical 10×10cm segments with identical MUs. Beam holding between segments was achieved by moving one out‐of‐field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 2–40 and dose rates of either 300 or 600MU/min. All measurements were made with a Farmer (0.6cc) ionization chamber placed at the isocenter in a SolidWater phantom at 10cm depth. The measurements were performed on two Varian LINACs: Trilogy and TrueBeam. Each sequence was delivered three times and the charge readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Delta_i) was compared using F‐tests.Results: On the Trilogy, large Delta_i was observed in small MU segments: at 2MU/segment, the maximum Delta_i was 20.0%/26.2% at 300/600MU/min, respectively. Also, the first segment of each sequence consistently over‐shot(Delta_i>0), while the last segment consistently under‐shot(Delta_i 10 MU/segment) MU segments Conclusions: Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SS‐IMRT delivery. Our results appeared to confirm these findings. The dose delivery accuracy in small MU segments and at high dose rate for SS‐IMRT is significantly improved on TrueBeam compared to Trilogy, likely due to the faster sampling rate (100Hz vs. 20Hz).