Faculty Bibliography

Purpose: For the Cyberknife (Accuray Inc. Sunnyvale CA) stereotactic radiosurgery and stereotactic body radiotherapy treatment unit, a series of collimating cones are used with diameters ranging from 5 mm to 60 mm. The measurement of output factors (OFs) in these small radiation fields is challenging due to limitations in detector size. Currently multiple detectors are used for the measurement of output factors, but if not water equivalent, Monte Carlo corrections are required. The Exradin W2 (Standard Imaging Inc., Middleton, WI) plastic scintillation device (PSD) has an active volume of 0.0008 cm³ and is water equivalent. This work reports OFs and scanning data measured by the W2 PSD and a diode in Cyberknife.
Method(s): A Cyberknife M6 and a W2 PSD in a MP3 water phantom (PTW, Freiberg, Germany) were used to take TPR, profile, and OF measurements. The W2 comes with a specialized electrometer, MAX SD, used to measure the Cerenkov light ratio using the manufacturer's recommendation. Profiles, tissue phantom ratios (TPR), and OF values were taken using the clinically relevant (5, 7.5 and 10) mm cones, with OFs normalized to the 60 mm cone and scanning measurements normalized to maximum dose. All results were compared with PTW 60018 diode measurements taken during commissioning.
Result(s): The W2 scintillator required the slowest scan speed (0.1 mm/s) in order to eliminate noise, and TPR data was smoothed using a moving average filter. Comparison between diode and W2 profiles and TPR data showed good agreement. Monte Carlo adjusted diode OFs and W2 measured OFs were within 2.6%.
Conclusion(s): This work demonstrated the feasibility of using a water equivalent, small volume commercial W2 PSD in Cyberknife. Scanning data was near equivalent, but showed significant noise and required smoothing. W2 measured OFs agreed with MC-corrected diode measurements taken during commissioning

Purpose: Radiation induced interstitial pneumonitis and late renal dysfunction are major concerns for patients undergoing total body irradiation (TBI). The purpose of this work is to evaluate the dosimetry of VMAT-based TBI plans generated using Varian Eclipse scripting.
Method(s): Three full-body CT datasets (two patients, one anthropomorphic CIRS phantom) were used. An in-house Eclipse script was developed to generate optimized field arrangements using the body contour, user origin, and couch longitudinal travel. Plans consisted of a lower-body AP/PA portion and an upper-body VMAT portion (8 full arcs with 4-isocenters). Treatment plans to 1320 cGy (165 cGy x 8fx) were generated with dose directives: [PTV V100% >=90- 95%; Total lung Dmean <900 cGy; Kidneys Dmean <1100 cGy]. All plans used 6MV photons and were calculated using the AAA algorithm. Upperbody VMAT plan dosimetry was evaluated 'in-phantom' placing 12 OSLDs in different key locations (lung, kidneys, bone, and soft tissue). Additionally, dosimetric verification was performed for the three plans using Varian portal dosimetry, PerFraction(SNC) and ArcCheck(SNC) with a global gamma criterion of 2%/2 mm.
Result(s): Planning objectives were met for the three treatment plans with the following averages: PTV V100% = 94.02%, total lung Dmean = 872.9 cGy, and kidneys Dmean = 1075.8 cGy. The dose deviation between Eclipse and the OSLDs (relative to the prescribed dose) averaged 0.98%, with each individual dose deviation within +/-4%. Dose ranged between 52.5 cGy (lung) and 187.5 cGy (bone) for OSLD measurements. The average passing rate for all 24 fields (8 per plan) was 98.0%, 99.76% and 98.6% for portal dosimetry, PerFraction and ArcCheck respectively. The lowest passing rate of any individual field was 95.4%, 99.0% and 91.8% for portal dosimetry, PerFraction and ArcCheck respectively.
Conclusion(s): Eclipse scripting can assist in creating robust multi-isocentric VMATbased TBI treatment plans to block lungs and kidneys without compromising target coverage. Dosimetric accuracy and deliverability was confirmed using in-phantom OSLD dosimetry, Varian portal dosimetry, PerFraction and Arc-Check verification

BACKGROUND:High b-value diffusion-weighted imaging has application in the detection of cancerous tissue across multiple body sites. Diffusional kurtosis and bi-exponential modeling are two popular model-based techniques, whose performance in relation to each other has yet to be fully explored. PURPOSE/OBJECTIVE:To determine the relationship between excess kurtosis and signal fractions derived from bi-exponential modeling in the detection of suspicious prostate lesions. MATERIAL AND METHODS/METHODS:This retrospective study analyzed patients with normal prostate tissue (n = 12) or suspicious lesions (n = 13, one lesion per patient), as determined by a radiologist whose clinical care included a high b-value diffusion series. The observed signal intensity was modeled using a bi-exponential decay, from which the signal fraction of the slow-moving component was derived ( SFs). In addition, the excess kurtosis was calculated using the signal fractions and ADCs of the two exponentials ( KCOMP). As a comparison, the kurtosis was also calculated using the cumulant expansion for the diffusion signal ( KCE). RESULTS:Both K and KCE were found to increase with SFs within the range of SFs commonly found within the prostate. Voxel-wise receiver operating characteristic performance of SFs, KCE, and KCOMP in discriminating between suspicious lesions and normal prostate tissue was 0.86 (95% confidence interval [CI] = 0.85 - 0.87), 0.69 (95% CI = 0.68-0.70), and 0.86 (95% CI = 0.86-0.87), respectively. CONCLUSION/CONCLUSIONS:In a two-component diffusion environment, KCOMP is a scaled value of SFs and is thus able to discriminate suspicious lesions with equal precision . KCE provides a computationally inexpensive approximation of kurtosis but does not provide the same discriminatory abilities as SFs and KCOMP.

Purpose: TBI treatment at our institution has moved from traditional hand calculation to CT-based planning to incorporate dose heterogeneities and organs at risk dose limits. The main objective of this work is to report our institutional experience with CT-based TBI and to show a comparison with the traditional approach. Methods: Ten patients were CT simulated supine with arms immobilized for lung shielding. Legs are separated to achieve a width similar to umbilicus separation; rice bags were placed between the legs for compensation. Four plans (P1, P2, P3 and P4) were created for each patient, all prescribed at midplane-umbilicus. The first three plans use lateral 15X beams, with head compensation. P1 was planned using a hand calculation. P2 includes heterogeneity corrections and inferior subfield to improve coverage. P3 includes heterogeneity corrections, inferior subfield, and adjustment of field weights to maintain coverage while keeping mean lung doses below 10.5 Gy (prescription dose 12 Gy). P4 uses AP-PA 6X beams. Dose to target (mean, max, D98%, D95%, min), mean lung and liver doses are calculated for all plans; reported doses when unitless and normalized to prescription dose. Results: Coverage of the target (Body-2 cm), indicated by D98% was 84.1 +/- 2.8, 84.7 +/- 3.9, 81.0 +/- 1.8, and 92.2 +/- 1.9 whereas the maximum doses were 123 +/- 5, 135 +/- 4, 129 +/- 4, and 124 +/- 5 for P1, P2, P3, and P4 respectively. The mean relative lung and liver doses were lowest for P3 with values of 87.8 +/- 0.5 and 89.8 +/- 3.4. The largest mean lung dose (12.5 Gy) was observed for P4 plan as expected, showing the necessity of using lung shielding. Conclusion: We are able to achieve target coverage of D98% >80%, keeping the mean lung and liver doses <90% of prescription using optimal arm positioning and subfields. This approach is easy to implement without the complexity of introducing lung shielding required with the use of 6X AP-PAbeams