Purpose: Robust detection of implanted fiducials is essential for monitoring intra‐fractional motion during hypo‐fractionated treatment. We developed a plan optimization strategy to ensure clear visibility of implanted fiducials, and facilitate 3D localization during volumetric modulated arc therapy (VMAT). Method: Periodic kilovoltage (kV) images were acquired at 20° gantry intervals, and paired with simultaneously acquired 4.4° short arc megavoltage digital tomosynthesis (MV‐DTS) to localize three fiducials during VMAT delivery for prostate cancer. Beginning with the original optimized plan, control point segments where fiducials (with 1cm margin to account for potential motion) were consistently blocked by MLC within each 4.4° MV‐DTS interval were first identified. For each segment, MLC apertures were edited to expose the fiducial that led to the least increase in a dosimetric cost function. Subsequently, MLC apertures of all control points not involved with fiducial visualization were re‐optimized to compensate for plan quality losses and match the original DVH. MV dose for each MV‐DTS was also kept above 0.4 MU to ensure acceptable image quality. Different imaging (gantry) intervals and visibility margins around fiducials were also evaluated. Result: Fiducials were consistently blocked by the MLC for, on average, 38% of the imaging control points for five hypofractionated prostate VMAT plans but properly exposed after re‐optimization. Optimization resulted in negligible dosimetric differences compared with original plans, and outperformed simple aperture editing: on average, PTV D98 recovered from 87% to 94% of prescription, and PTV dose homogeneity improved from 9% to 7%. Without violating plan objectives and compromising delivery efficiency, the highest imaging frequency and largest margin that can be achieved are a 10° gantry interval, and 15mm, respectively. Conclusion: VMAT plans can be made to accommodate MV‐kV imaging of fiducials. Fiducial visualization rate and workflow efficiency are significantly improved with an automatic modification and re‐optimization approach. Research supported by Varian Medical System
A comparison of the AAPM “Protocol for the determination of absorbed dose from high-energy photon and electron beams” (TG21) with currently used protocols for electron and photon dosimetry is presented. These protocols are the International Commission on Radiation Units and Measurements Report 21, “Radiation Dosimetry: Electrons with Initial Energies Between 1 and 50 MeV” (ICRU21), and the AAPM “Protocol for the Dosimetry of X- and Gamma Ray Beams with Maximum Energies Between 0.6 and 50 MeV” (SCRAD). Assuming a given radiation exposure and chamber parameters, doses to water at dmax for electron beams and at 5 g/cm2 for photon beams are calculated using the three protocols and then compared. The doses for photon beams calculated using the TG21 and SCRAD protocols are found to differ by 3% or less at energies below 10 MeV. The largest differences occur in photon doses at high energies where the dose calculated with the TG21 protocol is as much as 5.5% greater than that calculated with the SCRAD protocol for a typical thimble ionization chamber. For low electron beam energies, the doses calculated with the ICRU21 protocol are as much as 5% less than TG21 doses when using thimble chambers constructed of tissue-equivalent materials in a water phantom. If dosimetry measurements are performed in polystyrene, the dose calculated using TG21 may be greater than the ICRU21 dose, depending on chamber size and composition. An explanation for some of the differences between the protocols is presented emphasizing the dependence on chamber geometry, chamber composition, and phantom composition.
Cancers arising on nasal skin or in the nasal cavity being treated high energy photons or electrons may require bolus to ensure adequate superficial dose deposition. Creating a conformal bolus material on this irregular surface can pose a challenge. In this study, we evaluated the clinical feasibility, benefits and workflow of using 3D-printed bolus that is made of water-equivalent soft polymer materials and customized to patient anatomy by a 3D printer for nasal radiotherapy. We compared 3D-printed bolus with a conventional flat bolus or multi-layer thermoplastic bolus in 9 patients undergoing radiotherapy for nasal cancer. The 3D-printed polymer bolus was soft and had water-equivalency, which was measured dosimetrically by printing a 10x10x1 cm3 flab using a 3D printer and compared with the conventional flat bolus. Using a planning system, a bolus contour conformal to the skin was converted to a mesh structure using a plug-in script and exported as a STL file to the 3D printer. The treatment isocenter was either printed on the bolus with a creation of 5 dots, or identified by unique bolus edges using the planning system and marked in black. During treatment, a conventional bolus or the 3D-printed bolus was used and skin dose was measured using two optically stimulated luminescence (OSL: 10x10x1 mm3) dosimeters taped under the boluses. The skin-bolus airgap was visualized with cone-beam computed tomography (CBCT), while the pictures of bolus conditions were taken in each case. To quantify the mean airgap distance in mm, the airgap was contoured and its volume was calculated. Then the skin was evenly expanded with 1mm interval and the expansion volume under the bolus was calculated. The match between the volume of the airgap and the expanded region defines the mean airgap distance. The mean airgap distance, surface dose, and clinical efforts and time were compared. The bolus-to-skin conformality of a 3D-printed bolus is much improved with the mean (<2mm) and maximum (5-8mm) gap distances, reduced by 50%-150% from the conventional boluses based on CBCT. The conformality is reduced when two OSL dosimeters were placed underneath the bolus due to the chip size, yielding a similar airgap and similar skin dose (p=0.7) near the prescription dose (100%) in all cases (104%±11% for 3D-printed bolus and 105%±7% for other boluses). The 3D-printed bolus provides (1) minimal nasal alteration without forcing the bolus to retain conformal using adhesive tape; (2) minimal efforts at setup with only gentle waggling to fit patient facial surface; and (3) minimal time spending in bolus preparation and at patient setup. The bolus printing is streamlined in parallel to treatment planning. The 3D-printed bolus is dosimetrically equivalent to the conventional bolus, providing a safe, highly conformal, and convenient option for clinical use. With modification, similar procedure and clinical workflow may be applied to other anatomic sites. More clinical evaluations are on-going.
To determine the impact of using fiducial match for daily image-guidance on pelvic lymph node (PLN) coverage for prostate cancer patients receiving stereotactic body radiation therapy (SBRT).Thirty patients underwent SBRT treatment to the prostate and PLN from 2014 to 2016. Each patient received either 800cGy × 5 or 500cGy × 5 to the prostate and 500cGy × 5 to the PLN. A 5 mm clinical target volume (CTV)-to-planning target volume (PTV) margin around the PLN was used for planning. Two registrations with planning computed tomography (PCT) for each of the daily cone beam CTs (CBCTs) were performed: a rigid registration to fiducials and to the bony anatomy. The average translational difference between fiducial and bony match as well as percentage of fractions with differences > 5mm were calculated. Changes in bladder and rectal volume as well as center-of-mass (COM) position from simulation parameters, and their correlation with translational difference were also evaluated. The dosimetric impact of the translational differences was calculated by shifting the plan isocenter.The average translational difference between fiducial and bony match was 0.06 ± 0.82, 2.1 ± 4.1, -2.8 ± 4.3, and 5.5 ± 4.2 mm for lateral, vertical, longitudinal, and vector directions. The average change in bladder and rectal volume from simulation was -67.2 ± 163.04 cc (-12 ± 52%) and -1.6 ± 18.75 (-2 ± 30%) cc. The average change in COM of bladder from the simulation position was 0.34 ± 2.49, 4.4 ± 8.1, and -3.9 ± 7.5 mm along the LR, AP, and SI directions. The corresponding COM change for the rectum was 0.17 ± 1.9, 1.34 ± 3.5, and -0.6 ± 5.2 mm.The 5 mm margin covered ~75% of fractions receiving PLN irradiation with SBRT, daily CBCT and fiducial-guided setup. The dosimetric impact on PLN coverage was significant in 19% of fractions or 25% of patients. A larger translational shift was due to variation in rectal volume and changes in COM position of the bladder and rectum. A consistent bladder positioning and/or rectum filling compared with presimulation volume were essential for adequate coverage of PLN in a hypofractionated treatment regime.
12 Background: To compare toxicity profiles and biochemical tumor control outcomes between patients treated with high-dose image-guided radiotherapy (IGRT) and high-dose intensity-modulated radiotherapy (IMRT) for clinically localized prostate cancer. Methods: 186 patients with prostate cancer were treated with IGRT to a dose of 86.4 Gy with daily correction of the target position based upon kilovoltage imaging of implanted prostatic fiducial markers. This group of patients was retrospectively compared with a similar cohort of 190 patients who were treated with IMRT to the same prescription dose without, implanted fiducial markers in place (non-IGRT). In both groups the margins used for the prostate were the same. The median follow-up time was 2.8 years (range, 2-4 years). Results: A significant reduction in late urinary toxicity was observed for IGRT patients compared with the non-IGRT patients. The 3-year likelihood of urinary toxicity for the IGRT and non-IGRT cohorts were 10.4% and 20.0%, respectively (p=0.02).Multivariate analysis identifying predictors for late urinary toxicity demonstrated that, in addition to the baseline IPSS, IGRT was associated with significantly less late urinary toxicity compared with non-IGRT. The incidence of late rectal toxicity was low for both treatment groups (1.0% and 1.6%, respectively; p = 0.81). No differences in prostate-specific antigen relapse–free survival outcomes were observed for low- and intermediate-risk patients when treated with IGRT and non-IGRT. For high-risk patients a significant improvement was observed at 3-years for patients treated with IGRT compared with non-IGRT. Conclusions: IGRT is associated with a reduction in late urinary toxicity and improvement in biochemical tumor control after definitive high-dose external beam radiotherapy compared with high-dose IMRT. These data suggest that, for definitive radiotherapy, the placement of fiducial markers and daily tracking of target positioning should be the preferred mode of external beam radiotherapy delivery for the treatment of prostate cancer.
