Physician review of image registration and normal structure delineation.

INTRODUCTION: Image registration and delineation of organs at risk (OARs) are key components of three-dimensional conformal (3DCRT) and intensity-modulated radiotherapy (IMRT) treatment planning. This study hypothesized that image registration and OAR delineation are often performed by medical physicists and/or dosimetrists and are not routinely reviewed by treating physicians. METHODS: An anonymous, internet-based survey of medical physicists and dosimetrists was distributed via the MEDPHYS and MEDDOS listserv groups. Participants were asked to characterize standard practices for completion and review of OAR contouring, target volume contouring, and image registration at their institution along with their personal training in these areas and level of comfort performing these tasks. Likert-type scales are reported as Median [Interquartile range] with scores ranging from 1 = "Extremely/All of the time" to 5 = "Not at all/Never." RESULTS: Two hundred and ninety-seven individuals responded to the survey. Overall, respondents indicated significantly less frequent physician review (3 [2-4] vs 2 [1-3]), and less confidence in the thoroughness of physician review (3 [2-4] vs 2 [1-3], P < 0.01) of OAR contours compared to image registration. Only 19% (95% CI 14-24%) of respondents reported a formal process by which OAR volumes are reviewed by physicians in their clinic. The presence of a formal review process was also associated with significantly higher perceived thoroughness of review of OAR volumes compared to clinics with no formal review process (2 [2-3] vs 3 [2-4], P < 0.01). CONCLUSION: Despite the critical role of OAR delineation and image registration in the 3DCRT and IMRT treatment planning process, physician review of these tasks is not always optimal. Radiotherapy clinics should consider implementation of formal processes to promote adequate physician review of OARs and image registrations to ensure the quality and safety of radiotherapy treatment plans.

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.

A Prostate Fossa Contouring Instructional Module: Implementation and Evaluation.

PURPOSE/OBJECTIVE: Radiation oncology trainees frequently learn to contour through clinical experience and lectures. A hands-on contouring module was developed to teach delineation of the postoperative prostate clinical target volume (CTV) and improve contouring accuracy. METHODS: Medical students independently contoured a prostate fossa CTV before and after receiving educational materials and live instruction detailing the RTOG approach to contouring this CTV. Metrics for volume overlap and surface distance (Dice similarity coefficient, Hausdorff distance (HD), and mean distance) determined discordance between student and consensus contours. An evaluation assessed perception of session efficacy (1 = "not at all" to 5 = "extremely"; reported as median[interquartile range]). Non-parametric statistical tests were used. RESULTS: Twenty-four students at two institutions completed the module, and 21 completed the evaluation (88% response). The content was rated as "quite" important (4[3.5-5]). The module improved comfort contouring a prostate fossa (pre 1[1-2] vs. post 4[3-4], p<.01), ability to find references (pre 2[1-3] vs. post 4[3.5-4], p<0.01), knowledge of CT prostate/pelvis anatomy (pre 2[1.5-3] vs. post 3[3-4], p<.01), and ability to use contouring software tools (pre 2[2-3.5] vs. post 3[3-4], p=.01). After intervention, mean DSC increased (0.29 to 0.68, p<0.01) and HD and mean distance both decreased, respectively (42.8 to 30.0, p<.01; 11.5 to 1.9, p<.01). CONCLUSIONS: A hands-on module to teach CTV delineation to medical students was developed and implemented. Student and expert contours exhibited near "excellent agreement" (as defined in the literature) after intervention. Additional modules to teach target delineation to all educational levels can be developed using this model.

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.

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 - ß) =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).

Intra- and interfractional variations for prone breast irradiation: an indication for image-guided radiotherapy.

PURPOSE: Intra- and interfractional errors for breast cancer patients undergoing breast irradiation in the prone position were analyzed. METHODS AND MATERIALS: To assess intrafractional error resulting from respiratory motion, four-dimensional computed tomography scans were acquired for 3 prone and 3 supine patients, and the respiratory motion was compared for the two positions. To assess the interfractional error caused by daily set-up variations, daily electronic portal images of one of the treatment beams were taken for 15 prone-positioned patients. Portal images were then overlaid with images from the planning system that included the breast contour and the isocenter, treatment beam portal, and isocenter. The shift between the planned and actual isocenter was recorded for each portal image, and descriptive statistics were collected for each patient. The margins were calculated using the 2Sigma + 0.7sigma recipe, as well as 95% confidence interval based on the pooled standard deviation of the datasets. RESULTS: Respiratory motion of the chest wall is drastically reduced from 2.3 +/- 0.9 mm in supine position to -0.1 +/- 0.4 mm in prone position. The daily set-up errors vary in magnitude from 0.0 cm to 1.65 cm and are patient dependent. The margins were defined by considering only the standard deviation to be 1.1 cm, and 2.0 cm when the systematic errors were considered using the 2Sigma + 0.7sigma recipe. CONCLUSIONS: Prone positioning of patients for breast irradiation significantly reduces the uncertainty introduced by intrafractional respiratory motion. The presence of large systematic error in the interfractional variations necessitates a large clinical target volume-to-planning target volume margin and indicates the importance of image guidance for partial breast irradiation in the prone position, particularly using imaging modality capable of identifying the lumpectomy cavity.

Technical and dosimetric aspects of respiratory gating using a pressure-sensor motion monitoring system.

This work introduces a gating technique that uses 4DCT to determine gating parameters and to plan gated treatment, and employs a Siemens linear accelerator to deliver the gated treatment. Because of technology incompatibility, the 4DCT scanner (LightSpeed, GE) and the Siemens accelerator require two different motion-monitoring systems. The motion monitoring system (AZ-773V, Anzai Med.) used for the gated delivery utilizes a pressure sensor to detect the external respiratory motion (pressure change) in real time. Another system (RPM, Varian) used for the 4DCT scanner (LightSpeed, GE) is based on an infrared camera to detect motion of external markers. These two motion monitoring systems (RPM and Anzai systems) were found to correlate well with each other. The depth doses and profile measured for gated delivery (with a duty cycle of 25% or 50%) were found to agree within 1.0% with those measured for ungated delivery, indicating that gating did not significantly alter beam characteristics. The measurement verified also that the MU linearity and beam output remained unchanged (within 0.3%). A practical method of using 4DCT to plan a gated treatment was developed. The duty cycle for either phase or amplitude gating can be determined based on 4DCT with consideration of set-up error and delivery efficiency. The close-loop measurement involving the entire gating process (imaging, planning, and delivery) showed that the measured isodose distributions agreed with those intended, validating the accuracy and reliability of the gating technique. Based these observations, we conclude that the gating technique introduced in this work, integrating Siemens linear accelerator and Anzai pressure sensor device with GE/Varian RPM 4DCT, is reliable and effective, and it can be used clinically to account for respiratory motion during radiation therapy.