N Hernandez

Credentialing results from IMRT irradiations of an anthropomorphic head and neck phantom

PURPOSE This study was performed to report and analyze the results of the Radiological Physics Centers head and neck intensity-modulated radiation therapy (IMRT) phantom irradiations done by institutions seeking to be credentialed for participation in clinical trials using intensity modulated radiation therapy. METHODS The Radiological Physics Centers anthropomorphic head and neck phantom was sent to institutions seeking to participate in multi-institutional clinical trials. The phantom contained two planning target volume (PTV) structures and an organ at risk (OAR). Thermoluminescent dosimeters (TLD) and film dosimeters were imbedded in the PTV. Institutions were asked to image, plan, and treat the phantom as they would treat a patient. The treatment plan should cover at least 95% of the primary PTV with 6.6 Gy and at least 95% of the secondary PTV with 5.4 Gy. The plan should limit the dose to the OAR to less than 4.5 Gy. The passing criteria were ±7% for the TLD in the PTVs and a distance to agreement of 4 mm in the high dose gradient area between the PTV and the OAR. Pass rates for different delivery types, treatment planning systems (TPS), linear accelerators, and linear accelerator-planning system combinations were compared. RESULTS The phantom was irradiated 1139 times by 763 institutions from 2001 through 2011. 929 (81.6%) of the irradiations passed the criteria. 156 (13.7%) irradiations failed only the TLD criteria, 21 (1.8%) failed only the film criteria, and 33 (2.9%) failed both sets of criteria. Only 69% of the irradiations passed a narrowed TLD criterion of ±5%. Varian-Elipse and TomoTherapy-HiArt combinations had the highest pass rates, ranging from 90% to 93%. Varian-Pinnacle(3), Varian-XiO, Siemens-Pinnacle(3), and Elekta-Pinnacle(3) combinations had pass rates that ranged from 66% to 81%. CONCLUSIONS The head and neck phantom is a useful credentialing tool for multi-institutional IMRT clinical trials. The most commonly represented linear accelerator-planning system combinations can all pass the phantom, though some combinations had higher passing percentages than others. Tightening the criteria would significantly reduce the number of institutions passing the credentialing criteria. Causes for failures include incorrect data entered into the TPS, inexact beam modeling, and software and hardware failures.

SU‐FF‐T‐148: IMRT Head and Neck Phantom Irradiations: Correlation of Results with Institution Size

Purpose: To analyze the results from 136 IMRT HN distance‐to‐agreement in the high dose gradient region near the OAR − ⩽4 mm. The failure rate of institutions that housed 3 or fewer megavoltage therapy machines was compared to that of larger institutions. Results: 41 irradiations failed to meet one or more of the criteria. 24 of the failures were dose discrepancies measured with TLD, 5 were dose distribution discrepancies measured with radiochromic film and 12 were disagreements in both TLD and film measurements. There was a 38% discrepancy rate in first‐time irradiations at the institutions with 3 or fewer machines and a 26% rate at the larger institutions. All of the institutions that failed multiple times were smaller institutions. Conclusion: Institutions of all sizes are capable of making mistakes in IMRT treatments. Sufficient physics coverage is an important aspect of IMRTquality assurance.Conflict of Interest: The investigation was supported by PHS grants CA10953 and CA81647 awarded by the NCI, DHHS.

TU‐C‐AUD B‐03: A Comparison of Heterogeneity Correction Algorithms

Purpose: To compare the measured dose distribution to the planned distribution over a PTV centrally located within the lung when heterogeneity corrections are taken into account. Method and Materials: The Radiological Physics Center has constructed an anthropomorphic thorax phantom that includes a target (∼ 1 g/cm3) centrally located in the left lung (∼ 0.33 g/cm3). The phantom was irradiated by 33 institutions with results that meet criteria established for the RTOG 0236 protocol. TLDs and radiochromic films were used as dosimeters within the target region. Institutions were asked to design plans using 3D‐CRT and IMRT techniques. The TLDdose was compared to the dose calculated by the TPS at the center of the target. Film response was normalized to TLDdoses and a 2D‐gamma analysis comparison to the planned distribution was performed. Institutions whose irradiation of the phantom did not meet the RTOG 0236 criteria were not included. Results: A 2D‐Gamma analysis was done in the axial, sagittal and coronal planes through the PTV. 5% / 5mm criteria were applied. Due to limitations of the analysis software only the comparison over the axial plane is reported. 21 of the cases were planned with a superposition/convolution or AAA algorithm. For these cases, 92% ± 12% of the pixels in the analyzed region met the criteria. 12 of the cases were planned with a pencil beam or Clarkson algorithm. 74% ± 25% of the pixels meet the criteria for these cases. Conclusion: The superposition convolution heterogeneity correction algorithm showed better agreement with the measured dose distribution across the PTV than the pencil beam and Clarkson algorithms. Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

MO‐D‐AUD‐04: A Comparison of Heterogeneity Correction Algorithms Within a Lung PTV

Purpose: To compare the measured dose distribution with the planned distribution over a PTV centrally located within the lung when heterogeneity corrections are taken into account. Method and Materials: The Radiological Physics Centers anthropomorphic thorax phantom includes a target (∼ 1 g/cm3) centrally located in the left lung (∼ 0.33 g/cm3). The phantom was sent to 25 institutions, each of which was instructed to design and deliver a stereotactic treatment plan. The plan was intended to deliver 20Gy (homogeneous calculation) to ⩾ 95% of the PTV and limit the lungdose at point 2 cm from the PTV edge to 11.7 Gy. The institutions were asked to recalculate the dose distribution with the heterogeneity correction using the monitor units determined from the homogeneous calculated plan. TLD and radiochromic films were used as dosimeters within the target region. Results: A total of 17 institutions met the phantom irradiation criteria: +/− 5% for DTLD/DInst, and +/−5mm DTA on all sides of the PTV, based on the heterogeneous calculated plan. For these irradiations, the delivered doses over the central 80% of the PTV were compared to the planned doses along 3 orthogonal profiles through the PTV. An average of 85% of the points in the profiles from the cases calculated with the superposition/convolution algorithm were within 5% of the calculation, while only 69% of the points from the plans using pencil beam and Clarkson were within the 5% of the plan. Conclusions: The superposition/convolution heterogeneity correction algorithm showed better agreement with the measured dose distribution across the PTV than the pencil beam and Clarkson algorithms because it more accurately accounted for the lack of lateral scatter. Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

TU‐E‐224A‐01: Evaluation of Heterogeneity Corrections Algorithms Through the Irradiation of a Lung Phantom

Purpose: To evaluate the impact of applying heterogeneity corrections to the calculation of prescribed doses to a target located within the lung.Method and Materials: The Radiological Physics Centers (RPC) anthropomorphic lung phantom was sent to institutions nationwide. This phantom simulates a patient not only in dimensions but also in densities for imaging and treatment purposes. This design includes two lungs with density of 0.33 g/cm3 and a target centrally located in the left lung with density near 1 g/cm3. TLD and radiochromic films were used as dosimeters within and near the target region. Institutions that received the phantom were requested to image, plan and treat the phantom as if a patient. The prescription dose, based on a stereotactic plan, was 20 Gy to the target, calculated without applying heterogeneity corrections. The institutions were asked to submit both the homogeneous and heterogeneity corrected treatment plans using the same number of monitor units. Results: Twenty‐one irradiations, mostly with 6 MV x‐rays, were analyzed from 7 different Treatment Planning Systems (TPS). The ratio of dose to the target from the plan with to the plan without heterogeneity corrections was calculated and analyzed based on the algorithms used for the heterogeneity correction. A comparison of corrected dose given by the TPS and dose given by TLD was performed. The average ratio between dose with to dose without the heterogeneity correction was 1.18 with values ranging from 1.12 to 1.21. The superposition convolution algorithms agreed better with measurements than the other algorithms studied. The average TLD/Inst dose ratio in the target was 0.97 ranging from 0.92 to 0.99. Conclusions: There continues to be a differences in the heterogeneity corrected tumordoses within the lung from different planning systems. Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

TU‐FF‐A1‐01: Characterization of EBT Versus MD55 Gafchromic® Films for Relative Dosimetry Measurements

Purpose: To evaluate the EBT® radio‐chromic film for relative dosimetry in comparison to MD55®. Method and Materials: For dose‐response study, EBT and MD55 films were irradiated to dose ranges 0–15 and 0–40 Gy respectively. Photon beam energies of 6 and 18 MV were used to study energy dependence. Films were scanned using a scanner from Micro‐densitometer Photoelectron Corporation. The scanner uses a CCDcamera. The two diffused‐light beds using light‐emitting diodes, operating at 636 and 665 nm, were used for EBT and MD55 films respectively. For flatfield subtraction at start of a scanning session, in case of EBT, an unirradiated film was scanned employing a black mask covering the light boxs area outside the film. In case of MD55, un‐masked light box without a film was scanned for the flat field subtraction. For fading study, films were read over a period 2–20 days after irradiation. For light sensitivity, un‐irradiated films were exposed to florescent light to 6 hour maximum. Results: Unlike MD55, light sensitivity of EBT is found to decrease with pre‐irradiation level. With 2Gy pre‐irradiation, it is comparable to MD55. Fading of EBT is comparable to MD55. The small energy dependence observed beyond 8Gy for EBT is considered negligible. Percent uncertainty in relative‐dose determination from two OD values is obviously expected to increase with separation between the OD values, and to be higher at lower OD levels. It is comparable for both films, and is typically estimated to be 0.8% for determination of 50% of 2Gy. Conclusions:Dosimetry characteristics of EBT are comparable to MD55. Its higher sensitivity to radiation and availability in larger size makes it preferable as a relative dosimeter for RPC use. Work supported by PHS grant CA10953 and CA81647 awarded by NCI.

Predictability of electron cone ratios with respect to linac make and model

In the past, the Radiological Physics Center (RPC) has developed standard sets of photon depth‐dose and wedge‐factor data, specific to the make, model, and wedge design of the linear accelerator (linac). In this paper, the RPC extends the same concept to electron‐cone ratios. Since 1987, the RPC has measured and documented cone‐ratio (CR) values during on‐site dosimetry review visits to institutions participating in National Cancer Institute cooperative clinical trials. Data have been collected for approximately 500 electron beams from a wide spectrum of linac models. The analysis presented in this paper indicates that CR values are predictable to 2% to 3% (two standard deviations) for a given make and model of linac with a few exceptions. The analysis also revealed some other interesting systematics. For some models, such as the Varian Clinac 2500 and the Elekta/Philips SL18, SL20, and SL25, CR values were nearly identical for cone sizes 15 cm×15 cm (or 14 cm×14 cm) and 20 cm×20 cm across the range of available energies. Certain models of the same make of linac, such as the Mevatron MD, KD, and 6700 series models or the Clinac 2100 and 2300 models, exhibited indistinguishable CRs. Irrespective of linac model, two consistent general trends were observed: namely, an increase in CR value with incident beam energy for cone sizes smaller than 10 cm×10 cm and a decrease with energy for cone sizes larger than 10 cm×10 cm. These data are valuable to the RPC as a quality assurance remote‐monitoring tool to identify potential dosimetry errors. The physics community will also find the data useful in several ways: as a redundant check for clinical values in use, to validate the values measured during commissioning of new machines or to ensure consistency of values measured during annual quality assurance procedures. PACS number(s): 87.54.–n, 87.53.–j

TH‐C‐BRB‐01: Credentialing Results from a Spine Anthropomorphic Phantom

Purpose: To use an anthropomorphic spine phantom for dosimetric credentialing purposes in National Cancer Institute sponsored clinical trials. Methods: An anthropomorphic spine phantom that consists of lungs, a heart, an esophagus, a spinal cord, vertebrae and an abutting planning target volume (PTV) was sent to institutions interested in becoming credentialed for a radiosurgery protocol for spine metastasis. The phantom contained film in 2 planes and TLD in the PTV. Institutions were asked to fill the phantom with water, image the phantom, create an IMRT plan for the spinal SBRT, perform image‐guidance for the target localization, and to irradiate the phantom. They were also asked to perform the type of patient specific measurements that they would for an actual patient. TLD inside the PTV are required to be within ±7%. 85% of the analyzed area of film was required to pass a gamma analysis of ±5%/3 mm. Results: 125 phantom treatment plans from 88 institutions were analyzed since 2009. Only 83 (66%) of the phantom irradiations passed the acceptance criteria. Of the 42 failures, 14 of the irradiations failed both the TLD and film criteria, 27 failed only the film criteria and 1 failed only the TLD criteria. Six institutions passed the criteria after including the couch in the calculation. The most represented planning systems, machine manufacturers and algorithm types have the ability to pass the phantom. Both pencil beam and superposition/convolution type algorithms can adequately account for the bone heterogeneity present in this phantom. Conclusion: The phantom test is a critical credentialing tool to secure the high quality protocol study. Treatment couch attenuation could affect tumor dose delivery accuracy, especially in instances where the majority of the beams pass through the couch. Supported by PHS grants CA10953 and CA81647 (NCI, DHHS). The investigation was supported by PHS grants CA10953 and CA81647 (NCI, DHHS).

SU‐E‐T‐180: The Radiological Physics Center's Anthropomorphic Quality Assurance Phantom Program

PURPOSE To describe the phantoms, program logistics and current results for the Radiological Physics Centers (RPC) anthropomorphic QA phantom program for credentialing institutions for participation in NCI-sponsored advanced technology clinical trials. METHODS The RPC has developed an extensive phantom credentialing program consisting of four different phantoms designs: H&N, pelvis, lung and spine. These QA phantoms are water-filled plastic shells with imageable targets, avoidance structures, and heterogeneities that contain TLD and radiochromic film dosimeters. Institutions wishing to be credentialed request a phantom and are prioritized for delivery. At the institution, the phantom is imaged, a treatment plan is developed, the phantom is positioned on the treatment couch and the treatment is delivered. The phantom is returned and the measured dose distributions are compared to the institutions electronically submitted treatment plan dosimetry data. RESULTS The RPC currently has an inventory of 31 H&N, 10 pelvis, 9 lung, and 8 spine phantoms that are mailed to institutions nationally and internationally. In 2011, 444 of these phantoms were mailed out for credentialing. Once the phantom is sent, it takes the institution an average of 26 days to return it to the RPC. On average the dosimeters are analyzed within 17 days and the report is sent 21 days after receipt of the phantom data. In 2011 the percent of phantoms meeting the acceptance criteria increased by 12, 13 and 6 percentage points for the H&N, spine and lung phantoms, respectively. It fell by 5 percentage points for the pelvis phantom. CONCLUSIONS The RPCs QA phantom program has been an effective and responsive QA tool for assessing the use of advanced technologies in NCI sponsored clinical trials. The RPC has been efficient in its mailing of phantoms, and analyzing and reporting results. Work supported by PHS grant CA10953 and CA081647 (NCI, DHHS).

TU‐E‐BRB‐06: Results from Multiple Radiations of an Anthropomorphic Spine Phantom

Purpose: To credential institutions to participate in an IMRT‐dose painting spine metastasis protocol. Method and Materials: A mailable anthropomorphic spine phantom was developed and shipped to 21 institutions interested in participating in protocols sponsored by the National Cancer Institute (NCI). The phantom was a modification of the Radiological Physics Centers lung‐thorax phantom. An insert was designed to simulate a solid water spinal cord surrounded by a high impact polystyrene bone structure. An acrylic planning target volume is adjacent to the bone structure. The insert houses radiochromic film and TLD. The phantom also contains an esophagus, a heart and two lungs. Institutions were asked to image the phantom, create an IMRT plan to deliver 6 Gy to at least 90% of the PTV, perform their normal IMRT QA, and deliver the treatment plan to the phantom. The spinal cord dose was limited to 3.75 Gy to a volume of less than 0.35 cc and 2.63 Gy to a volume of less than 1.2 cc. The following criteria were applied: PTV TLD ±7%, ≥85% of the pixels in regions of interest in the axial and sagittal planes must pass a gamma criteria of 5%, 3 mm, and the plan must comply with the protocol. Results: 21 institutions irradiated the phantom 24 times. 8 of the irradiations failed the criteria. 4 of the failing irradiations passed the TLD criterion, but did not pass the gamma criterion. 4 irradiations failed both the TLD and gamma criteria. One of these also failed the plan criteria. The 3 institutions that irradiated the phantom twice all passed on the second irradiation. Conclusion: The phantom is a useful tool in credentialing institutions to participate in protocols. The investigation was supported by PHS grants CA10953 and CA81647 awarded by the NCI, DHHS.