Portal Films Research Articles

SU‐E‐T‐347: A Cost‐Effective Technique for Treating Penis Cancer with Delivery Accuracy Verified with In‐Vivo Measurement

Purpose: To report an in‐house cost‐effective, easily made and set up, yet accurate technique for treating penis cancer at a community freestanding cancer. Methods: Recently, a patient with penis cancer was under radiotherapy at our institution. The treated site was his penis only. The patient was simulated in supine position with his penis inserted in a custom supporting tool. This supporting tool is made of a regular candle that can be bought at any retail store. It measures about 10 cm in diameter and has a HU about −100. A hole slightly larger than the patientˈs penis was drilled in the middle with enough buildup wall left for a 6 MV beam used for the treatment plan. For the remaining space not occupied by the patientˈs penis, regular flour was filled in tightly, which has a HU about −200. The plan was done with an opposing lateral conformal field setup to get a uniform dose distribution across the treatment site. It makes essential no difference whether the heterogeneity is turned on or not in the dose calculation with Varianˈs Eclipse TPS. Four MOSFET dosimeters were placed on the surface of the penis for in‐vivo measurement for two consecutive fractions. The prescription was 180 cGy/fx. The mean dose for the skin (3 mm deep region from the surface contoured in Eclipse) was 182.9 cGy with a minimum of 179.3 cGy and a maximum of 185.8 cGy. Results: The in‐vivo measurement was 183.2 cGy +/− 1.4 cGy, including a port film with 2 MU. The measurement was in good agreement with the calculation. Conclusions: In‐vivo measurement proved the in‐house developed supporting tool to be cost‐effective, easily made, and easy to set up on a daily basis for accurately treating penis cancer.

Prospective phase II trial with intra-operative radiotherapy in initial breast cancer using nondedicated linear accelerator.

e11560 Background: To report single-institution series of breast intra-operative radiotherapy (IORT) using nondedicated linear accelerator. Methods: Prospective phaseII trial opened in May 2004. Eligibility criteria: single-focal breast infiltrating ductal carcinoma, age >40 years, size <3cm (after February 2009 >50 years and <2cm) and cN0. Exclusion criteria: carcinoma in situ or lobular types, impossibility of breast-conserving surgery, metastasis or another cancer. Patients underwent classic quadrantectomy with intraoperative margins and SLN evaluation. If both negative, IORT was prescribed (21Gy). Adjuvant treatment considered after definitive pathology report. Endpoints: oncologic, toxicity, aesthetics, and technical issues. Results: From May/2004 to May/2010, 161 patients were enrolled (intention-to-treat analysis) and 34 were excluded. Thus, 127 patients were analyzed. Oncologic analysis involved patients with follow-up ≥24 months (n=83) or breast events. Median age was 56 (40-86) years and median follow-up was 43 (24-72) months. The table summarizes all findings. Conclusions: Single-dose breast IORT presents early oncologic, toxicity, and aesthetic results similar to previously reported trials. Nondedicated facilities have advantages: lower costs; allow higher electron energies; and portal film imaging before treatments. Long-term results will be necessary to strength this data. Summarized results. Parameter Value (%) cT (x, 1, 2) 62.9, 29.9, 7 pT (1,2) 85.8, 14.1 pN (0,1) 86.7, 13.3 Menopausal status (post, pre) 74.5, 25.5 Hormonal receptors (E, P) 84.3, 76 Grade (I, II, III) 33, 24.5, 62.5 Chemotherapy, hormones 15.7, 66.6 3-year survival Overall 97.6 Cause-specific 98.4 Disease-free 96 Recurrences (cumulative) Local 2.4 Loco-regional 1.2 Distant 2.4 Toxicities < 1 month (n) Redness (5), seroma (3),dehiscence (2) ≥ 1 month (n) Fat necrosis (10), fibrosis (20),lymphedema (2), and papillaretraction (3) One-year aesthetic evaluation (RTOG/NSABP score)(excellent, good, regular, bad) 91.7, 4.5, 1.8, 0.9 Electron energies (6, 9, 12, 15, 18 MeV) 7.8, 51.5, 35.1, 3.9, 0.7 Shield repositioning (no, yes) 70.3, 29.7 In-hospital infections None

Setup Accuracy of the Novalis ExacTrac 6DOF System for Frameless Radiosurgery

Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique. A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively). The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique. The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.

Analysis of Dose at the Site of Second Tumor Formation After Radiotherapy to the Central Nervous System

Second tumors are an uncommon complication of multimodality treatment of childhood cancer. The present analysis attempted to correlate the dose received as a component of primary treatment and the site of the eventual development of a second tumor. We retrospectively identified 16 patients who had received radiotherapy to sites in the craniospinal axis and subsequently developed a second tumor. We compared the historical fields and port films of the primary treatment with the modern imaging of the second tumor locations. We classified the location of the second tumors as follows: in the boost field; marginal to the boost field, but in a whole-brain field; in a whole-brain field; marginal to the whole brain/primary treatment field; and distant to the field. We divided the dose received into 3 broad categories: high dose (>45 Gy), moderate dose (20-36 Gy), and low dose (<20 Gy). The most common location of the second tumor was in the whole brain field (57%) and in the moderate-dose range (81%). Our data contradict previous publications that suggested that most second tumors develop in tissues that receive a low radiation dose. Almost all the second tumors in our series occurred in tissue within a target volume in the cranium that had received a moderate dose (20-36 Gy). These findings suggest that a major decrease in the brain volume that receives a moderate radiation dose is the only way to substantially decrease the second tumor rate after central nervous system radiotherapy.

The challenge of maximizing safety in radiation oncology

There is a growing interest in the evolving nature of safety challenges in radiation oncology. Understandably, there has been a great deal of focus on the mechanical and computer aspects of new high-technology treatments (eg, intensity-modulated radiation therapy). However, safety concerns are not limited to dose calculations and data transfer associated with advanced technologies. They also stem from fundamental changes in our workflow (eg, multiple hand-offs), the relative loss of some traditional “end of the line” quality assurance tools (port films and light fields), condensed fractionation schedules, and an under-appreciation for the physical limitations of new techniques. Furthermore, changes in our workspace and tools (eg, electronic records, planning systems), and workloads (eg, billing, insurance, regulations) may have unforeseen effects on safety. Safety initiatives need to acknowledge the multiple factors affecting risk. Our current challenges will not be adequately addressed simply by defining new policies and procedures. Rather, we need to understand the frequency and causes of errors better, particularly those that are most likely to cause harm. Then we can incorporate principles into our workspace that minimize these risks (eg, automation, standardization, checklists, redundancy, and consideration of “human factors” in the design of products and workspaces). Opportunities to enhance safety involve providing support through diligent examinations of staffing, schedules, communications, teamwork, and work environments. We need to develop a culture of safety in which all team members are alerted to the possibility of harm, and they all work together to maximize safety. The goal is not to eliminate every error. Rather, we should focus our attention on conditions (eg, rushing) that can cause real patient harm, and/or those conditions that reflect systemic problems that might lead to errors more likely to cause harm. Ongoing changes in clinical practice mandate continued vigilance to minimize the risks of error, combined with new, nontraditional approaches to create a safer patient environment.

Verification of quality parameters for portal images in radiotherapy

BackgroundThe purpose of the study was to verify different values of quality parameters of portal images in radiotherapy.Materials and methodsWe investigated image qualities of different field verification systems. Four EPIDs (Siemens OptiVue500aSi®, Siemens BeamView Plus®, Elekta iView® and Varian PortalVision™) were investigated with the PTW EPID QC PHANTOM® and compared with two portal film systems (Kodak X-OMAT® cassette with Kodak X-OMAT V® film and Kodak EC-L Lightweight® cassette with Kodak Portal Localisation ReadyPack® film).ResultsA comparison of the f50 and f25 values of the modulation transfer functions (MTFs) belonging to each of the systems revealed that the amorphous silicon EPIDs provided a slightly better high contrast resolution than the Kodak Portal Localisation ReadyPack® film with the EC-L Lightweight® cassette. The Kodak X-OMAT V® film gave a poor low contrast resolution: from the existing 27 holes only 9 were detectable.ConclusionsOn the base of physical characteristics, measured in this work, the authors suggest the use of amorphous-silicon EPIDs producing the best image quality. Parameters of the EPIDs with scanning liquid ionisation chamber (SLIC) were very stable. The disadvantage of older versions of EPIDs like SLIC and VEPID is a poor DICOM implementation, and the modulation transfer function (MTF) values (f50 and f25) are less than that of aSi detectors.

IGRT after Prostatectomy: Evaluation of Corrective Shifts and Toxicity using Online Conebeam CT vs. Weekly Port Films for Target Localization

IGRT after Prostatectomy: Evaluation of Corrective Shifts and Toxicity using Online Conebeam CT vs. Weekly Port Films for Target Localization

SU‐GG‐J‐82: Comparison of Positional Accuracy for KVkV, KVMV, and CBCT OBI for Cranial Localization

Purpose: To quantify and compare the positional accuracy and precision of an OBI device for use as a localization system for cranial alignment. Quantification of these errors will allow for discussion of the efficacy of OBI as a replacement for invasive head ring systems. Methods and Materials: A 6.5mm BB is placed in an anthropomorphic head phantom at five positions located throughout the head. A treatment planning CT is taken and a simple plan and DRR's are generated to precisely define the isocenter. The phantom is then aligned to isocenter using a traditional three‐point setup and kVkV, kVMV, and CBCT images are acquired. These images are then auto‐fused with either setup DRR's (kVkV &amp; kVMV) or with the planning CT (CBCT). Shifts are applied and 2×2cm port films are taken. Measurement of the displacement of the BB from the center of the field in each direction (A‐P, LAT, Sup‐Inf) indicate residual systematic error. Results: Results for displacement in the A‐P, Sup‐Inf, and Lat directions are −1.3±0.7mm (1.5mm RMS), 0.1±0.8mm (0.8mm RMS), and 0.2±1.1mm (1.1mm RMS) for 2D2D kVkV fusion; −1.5±0.7mm (1.7mm RMS), 0.7±0.8mm (1.1mm RMS), 0.0±1.2mm (1.2mm RMS) for 2D2D kVMV fusion; and −1.5±0.5mm (1.6mm RMS), 0.1±0.7mm (0.7mm RMS), and 0.2±0.9mm (0.9mm RMS) for 3D3D CBCT fusion. For each technique positive values indicate the BB is displaced in the anterior, superior, or patient left direction. The average magnitude of the total displacement from isocenter is 1.3±0.6mm, 1.7±0.5mm, and 1.6±0.3mm for kVkV, kVMV, and CBCT respectively. Conclusion: We have shown that OBI imaging can place the target at isocenter with millimeter accuracy in all directions and for each technique. A large ≥ 1.5mm systematic error is present in the posterior direction for each technique. Further investigation is ongoing to isolate the cause of this error.

MO‐FF‐A1‐05: On‐line CT Verification of Needle Applicator Positions in HDR Prostate Brachytherapy

Purpose: Needle applicators have shown displacement between fractions during HDR prostate brachytherapy, which will degrade plan quality. Typically, displacement is determined by analyzing catheters on a verification CT (vCT) or port films obtained before treatment relative to fiducials or bony landmarks. We present a procedure to verify applicator positions to achieve planned dose distributions by registering vCT directly to planning CTs (pCT). Method and Materials: Patients received a vCT prior to each fraction. This was imported to BrachyVision (Varian Medical Systems) and fused to the pCT by rigid‐body registration based on matching urethra and three prostate‐implanted gold fiducials. Then, applicators in the vCT were compared to applicators in the pCT in a reconstructed plane through each catheter and any difference larger than 3mm was manually adjusted by a radiation oncologist before treatment. To assess treatment quality, the prostate volume was copied from pCT to vCT and the planned dwell positions/time was applied to the vCT applicators. Results: Two consecutive patients were treated using our CT registration needle verification method. Both patients required applicator adjustments in the first two fractions. Patient one had an average 7mm cranial‐to‐caudal applicator displacement, corresponding to 23% and 12% fractional drop in V100prostate respectively; Patient two had displacement in both cranial‐to‐caudal and caudal‐to‐cranial directions up to 9mm, but due to minimum loading of affected applicators, no significant decrease in V100prostate was seen. Neither patient required applicator adjustment at the Srd/last fraction. Despite differences in applicator displacement, post‐adjustment, the achieved V100prostate at each fraction deviated from planned value by less than 4% for both patients. The distances between fiducials in vCTs were consistent with those in pCTs within 3mm, confirming that fiducials are appropriate landmarks for registration. Conclusion: We have successfully treated two patients using this method. This technique ensures the treatment quality closely matches plan quality.

Adobe Photoshop images software in the verification of radiation portal

Objective To investigate the value of Adobe Photashop images software in the verifica-tion of radiation portal. Methods The portal and simulation films or CT reconstruction images were impor-ted into computer using a scanner. The image size, gray scale and contrast scale were adjusted with Adobe Photoshop images software, then image registration and measurement were completed. Results By the com-parison between portal image and simulation image, the set-up errors in right-left, superior-inferior and ante-rior-posterior directions were (1.11 ± 1.37) mm, (1.33 ± 1.25) mm and (0.83±0.79) mm in the head and neck;(1.44±1.03) mm,(1.6±1.52) mm and (1.34±1.17) mm in the thorax;(1.53±0.86) mm, (1.83 ± 1.19) mm and (1.67 ± 0.68)mm in the abdomen; (1.93 ± I. 83) mm, (1.59 ± 1.07)mm and (0.85 ± 0.72)mm in the pelvic cavity. Conclusions Accurate radiation portal verification and posi-tion measurement can be completed by using Adobe Photoshop, which is a simple, safe and reliable method. Key words: Radiotherapy; Portal verification; Image processing,computer-assisted

IGRT After Prostatectomy: Evaluation of Corrective Shifts and Toxicity Using Online Cone Beam CT vs. Weekly Port Films for Target Localization

Purpose/Objective(s): Image guidance (IG) may permit higher radiotherapy (RT) doses (>65 Gy) after radical prostatectomy (RP) without increased toxicity, with improved accuracy and smaller margins. Conebeam (CBCT) allows IGRT with volumetric images. This study evaluated CBCT shifts and toxicity after conformal IGRT, compared to RT with port films. American Society for Therapeutic Radiation Oncology (ASTRO) 52nd Annual Meeting October 31 - November 4, San Diego, CA

Base of skull recurrences after treatment of salivary gland cancer with perineural invasion reduced by postoperative radiotherapy

To determine the effect of postoperative radiation therapy for salivary gland carcinomas in the presence of microscopic perineural invasion. Retrospective review at an academic tertiary center. One hundred and forty patients with pathological evidence of perineural invasion at the time of initial surgery for salivary gland carcinomas were analysed. Sixteen patients (11%) had major (named) nerve involvement. Ninety-four patients (67%) received postoperative radiation therapy to the primary site, and the portal films of 65 of these patients were available for review. The incidence of skull base recurrences among patients treated by surgery with or without postoperative radiation therapy. Ten patients experienced skull base recurrences. T4 disease and the omission of postoperative radiation therapy were identified as significant predictors of skull base recurrence. Postoperative radiation therapy reduced the actuarial probability of skull base recurrence from 15% to 5% (P = 0.03). The crude rates of skull base recurrence were 6% (2/35) and 10% (3/30), respectively, for patients whose skull base were and were not confirmed to be encompassed in the irradiation field. The 5-year overall survival for patients who experienced a skull base recurrence was 19% compared to 91% for those who did not (P < 0.001). The use of postoperative radiation therapy significantly reduced the incidence of skull base recurrence among salivary gland carcinoma patients with perineural invasion.

The Variation of Cervical Spine Curvature during Treatment for Nasopharyngeal Cancer

The vague difference of cervical spine curvature (CSC) between simulation film and port films has put doubts on accurate target irradiation. We measured the CSC variation during treatment to identify its clinical importance.

Patterns of Radiation Treatment Planning for Localized Prostate Cancer in Japan: 2003-05 Patterns of Care Study Report

The purpose of this study is to identify the treatment planning process for Japanese patients with localized prostate cancer. The Patterns of Care Study conducted a random survey of 61 institutions nationwide. Detailed information was collected on prostate cancer patients without distant metastases who were irradiated during the periods 2003-05. Radiation treatment planning and delivery were evaluated in 397 patients who were treated radically with external photon beam radiotherapy. Computed tomography data were used for planning in approximately 90% of the patients. Contrast was rarely used for treatment planning. Simulations and treatments were performed in the supine position in almost all patients. Immobilization devices were used in only 15% of the patients. Verification of the treatment fields using portal films or electric portal imaging devices was performed in most of the patients. However, regular or multiple verifications in addition to initial treatment and/or portal volume changes were performed in only 30% of the patients. Typical beam arrangements for treatment of the prostate consisted of a four-field box. Three-dimensional conformal techniques were applied less frequently in non-academic hospitals than in academic ones. Modernized multileaf collimators with leaf widths < or =10 mm were used in about two-thirds of the patients. Although the total doses given to the prostate were affected by the leaf widths, there were no significant differences between leaf widths of 5 and 10 mm. The results of the survey identified certain patterns in the current treatment planning and delivery processes for localized prostate cancer in Japan.

Effect of collimator and couch angle change on breast IMRT dose distributions

Intensity‐modulated tangential photon beams for breast cancer treatment can improve the dose uniformity significantly throughout the whole breast and reduce the dose to the lung and the heart when compared to the conventional technique. Before the first treatment, patient setup may require a change on the collimator angle and/or the couch angle based on the chest wall coverage according to the port films. The objective of this work is to investigate the effects of the collimator and the couch angle change on the dose distribution for breast cancer treatment using intensity‐modulated tangential photon beams, and to determine the clinical acceptable range of the angle change for routine treatment. Ten breast cases treated with intensity‐modulated tangential photon beams were analyzed in this study. Patient‐specific CT data and the radiation therapy planning (RTP) files obtained from our in‐house Monte Carlo based breast IMRT treatment planning system were used for IMRT dose recalculation with collimator or couch angle changes. The isodose distributions and DVHs were compared with the original plans, and the effects of the collimator and couch angle change to breast IMRT dose distributions were evaluated. Our results show that a 4° change in the collimator angle or the couch angle did not affect the dose distribution significantly and it is acceptable in the clinic for patient treatment.PACS number: 87.10.Rt

Clinical Utility of the Modified Segmental Boost Technique for Treatment of the Pelvis and Inguinal Nodes

Low-lying pelvic malignancies often require simultaneous radiation to pelvis and inguinal nodes. We previously reported improved homogeneity with the modified segmental boost technique (MSBT) compared to that with traditional methods, using phantom models. Here we report our institutional clinical experience with MSBT. MSBT patients from May 2001 to March 2007 were evaluated. Parameters analyzed included isocenter/multileaf collimation shifts, time per fraction (four fields), monitor units (MU)/fraction, femoral doses, maximal dose relative to body mass index, and inguinal node depth. In addition, a dosimetric comparison of the MSBT versus intensity modulated radiation therapy (IMRT) was conducted. Of the 37 MSBT patients identified, 32 were evaluable. Port film adjustments were required in 6% of films. Median values for each analyzed parameter were as follows: MU/fraction, 298 (range, 226-348); delivery time, 4 minutes; inguinal depth, 4.5 cm; volume receiving 45 Gy (V45), 7%; V27.5, 87%; body mass index, 25 (range, 16.0-33.8). Inguinal dose was 100% in all cases; in-field inhomogeneity ranged from 111% to 118%. IMRT resulted in significantly decreased dose to normal tissue but required more time for treatment planning and a higher number of MUs (1,184 vs. 313 MU). In our clinical experience, the mono-isocentric MSBT provides a high degree of accuracy, improved homogeneity compared with traditional techniques, ease of simulation, treatment planning, treatment delivery, and acceptable femoral doses for pelvic/inguinal radiation fields requiring 45 to 50.4 Gy. In addition, the MSBT delivers a relatively uniform dose distribution throughout the treatment volume, despite varying body habitus. Clinical scenarios for the use of MSBT vs. intensity-modulated radiation therapy are discussed. To our knowledge, this is the first study reporting the utility of MSBT in the clinical setting.

SU-FF-T-551: From Frame-Based to Frameless Radiosurgery

Purpose: The purpose of this study is to analyze the positioning accuracy of the Robotics couch for longitudinal and lateral rotations and to compare the accuracy of the frameless with the frame‐based radiosurgery technique. Materials and methods: A head phantom with seven lead beads inserted was used for this study. A software tool was developed in‐house to rotate the reference CTimages up to 4degrees in the positive and negative direction and for both the rotation around the longitudinal and lateral axis. The angles measured by ExacTrac were compared to the known rotations. The accuracy of the positioning of the phantom based on the rotated images was measured with a digital waterlevel and portal films. Multiple hidden target tests (HTT) were performed to measure the accuracy of the 2 positioning techniques for radiosurgery.Results: The difference between the angles measured with ExacTrac and the known angles introduced with the in‐house program, was 0.03° (SD0.04°) and 0.14° (SD0.06°), for longitudinal and lateral rotations. The positioning error of the Robotics couch measured with the digital waterlevel was 0.01° (SD0.04°) for both longitudinal and lateral rotations. With the portal films, the positioning error was 0.01° (SD0.06°) and 0.04° (SD0.05°), for longitudinal and lateral rotations, respectively. The overall 3D accuracy defined by the HTT was 1.19mm (SD0.45mm) and 0.87mm (SD0.59mm) for the frame‐based and frameless techniques, respectively. Conclusions: The study showed a high detection accuracy and a sub‐degree positioning accuracy for the Robotics couch. Comparable accuracy with a small advantage for the frameless technique to the frame‐based radiosurgery technique was found.

SU‐FF‐J‐31: Quantitative Analysis of Prostate Motion Using Implanted Fiducial Markers Based On Port Film Image Guidance

Purpose: To assess daily localization of prostate using implanted fiducial markers with port film based image guidance. Methods and Materials: 2640 Prostate shift data for 60 patients were used to analyze the interfraction movement of the prostate. Three non‐migratable gold seeds were implanted approximately at base, apex and mid prostate. Prior to treatment, orthogonal radiographs were taken at 45° and 315° for clear visibility of the markers. The images were imported to dedicated software and the required positional shifts calculated in superior‐inferior, left‐right and anterior‐posterior directions by digitizing seeds. If shift exceeded 3 mm in any direction, the treatment table was moved to required distances and radiographs were repeated to confirm the target was within 3 mm. The dose at isocenter was also measured with port films in a phantom. Results: From the analyzed data, 57% of patients had shifts had &gt;3mm, 28% between 3–5 mm, 25% between 5–10mm and 5% &gt;10mm. The median values of deviation from all data were: 1.7 mm laterally, 2.0 mm longitudinally, and 3.6 mm vertically. From this data there is a risk of underdosing the PTV and delivering higher doses to the rectum and bladder without image guidance. With our treatment protocol of 7740 cGy/43 fractions the repeat portal imaging required for field verification after shift delivers an estimated average additional 287 cGy. Conclusion: From the analysis, image guidance with the appropriate modality is of prime importance for treatment of prostate cancers. The present technique of daily portal imaging gives good beam localization but with higher potential total dose than with kV based cone beam CT published in the literature.

SU-FF-J-85: Evaluation of the Positioning Accuracy in Helical Tomotherapy for Stereotactic Intracranial Radiosurgery

Purpose: To assess the accuracy of patient alignment for intracranial stereotactic radiosurgery(SRS) on Tomotherapy using the standard for linac alignment assessment — hidden target port films. Method and Materials: For each trial, a small 6.5‐mm diameter ball bearing (target) was placed in one of four positions inside a Rando head phantom. A kilovolt computed tomography (kVCT) scan was used for both planning and as the dosimetryquality assurance (DQA) phantom. The DQA capabilities of the treatment planningsoftware were used to position the target to coincide with the isocenteric lasers of the Tomotherapy unit in the axial, sagittal, and coronal planes. Accurate registration of the MVCT to the kVCT should place the target at the isocenter. Lateral and Anterior‐Posterior (AP) port films were taken with only the center two binary multi‐leaf collimators (bMLC) open. The films were analyzed and shifts were measured in the AP, right‐left, and superior‐inferior directions relative to the radiation field. The process was repeated 15 times for each target location. Results:Analysis of 60 pairs of port film yielded the following millimeter errors in the AP, lateral, and average SI directions respectively for each of four trial locations: (1) 0.79±0.27, 0.77±0.11, 0.84±0.29; (2) 0.88±0.28, 0.39±0.14, 0.18±0.25; (3) 1.19±0.23, 0.57±0.19, 0.43±0.39; (4) 1.16±0.18, 0.34±0.35, 0.56±0.35. Across the trials the mean errors were seen to be 1.00±0.30 anterior, 0.50±0.40 superior and 0.52±0.27 to the patient's left. Conclusion: The results show a root‐mean‐square error of 1.33±0.28. The tight standard deviation in each direction suggests there may be systematic error in alignment, which may include measurement error (isocentric placement of phantom) or inaccuracy of MLC alignment.

EVALUATION OF A HEAD‐REPOSITIONER AND Z‐PLATE SYSTEM FOR IMPROVED ACCURACY OF DOSE DELIVERY

Radiation therapy requires accurate dose delivery to targets often identifiable only on computed tomography (CT) images. Translation between the isocenter localized on CT and laser setup for radiation treatment, and interfractional head repositioning are frequent sources of positioning error. The objective was to design a simple, accurate apparatus to eliminate these sources of error. System accuracy was confirmed with phantom and in vivo measurements. A head repositioner that fixates the maxilla via dental mold with fiducial marker Z-plates attached was fabricated to facilitate the connection between the isocenter on CT and laser treatment setup. A phantom study targeting steel balls randomly located within the head repositioner was performed. The center of each ball was marked on a transverse CT slice on which six points of the Z-plate were also visible. Based on the relative position of the six Z-plate points and the ball center, the laser setup position on each Z-plate and a top plate was calculated. Based on these setup marks, orthogonal port films, directed toward each target, were evaluated for accuracy without regard to visual setup. A similar procedure was followed to confirm accuracy of in vivo treatment setups in four dogs using implanted gold seeds. Sequential port films of three dogs were made to confirm interfractional accuracy. Phantom and in vivo measurements confirmed accuracy of 2 mm between isocenter on CT and the center of the treatment dose distribution. Port films confirmed similar accuracy for interfractional treatments. The system reliably connects CT target localization to accurate initial and interfractional radiation treatment setup.