R Grant

Comparison of an anthropomorphic PRESAGE® dosimeter and radiochromic film with a commercial radiation treatment planning system for breast IMRT: a feasibility study.

This work presents a comparison of an anthropomorphic PRESAGE® dosimeter and radiochromic film measurements with a commercial treatment planning system to determine the feasibility of PRESAGE® for 3D dosimetry in breast IMRT. An anthropomorphic PRESAGE® phantom was created in the shape of a breast phantom. A five-field IMRT plan was generated with a commercially available treatment planning system and delivered to the PRESAGE® phantom. The anthropomorphic PRESAGE® was scanned with the Duke midsized optical CT scanner (DMOS-RPC) and the OD distribution was converted to dose. Comparisons were performed between the dose distribution calculated with the Pinnacle3 treatment planning system, PRESAGE®, and EBT2 film measurements. DVHs, gamma maps, and line profiles were used to evaluate the agreement. Gamma map comparisons showed that Pinnacle3 agreed with PRESAGE® as greater than 95% of comparison points for the PTV passed a ±3%/±3mm criterion when the outer 8 mm of phantom data were discluded. Edge artifacts were observed in the optical CT reconstruction, from the surface to approximately 8 mm depth. These artifacts resulted in dose differences between Pinnacle3 and PRESAGE® of up to 5% between the surface and a depth of 8 mm and decreased with increasing depth in the phantom. Line profile comparisons between all three independent measurements yielded a maximum difference of 2% within the central 80% of the field width. For the breast IMRT plan studied, the Pinnacle3 calculations agreed with PRESAGE® measurements to within the ±3%/±3mm gamma criterion. This work demonstrates the feasibility of the PRESAGE® to be fashioned into anthropomorphic shape, and establishes the accuracy of Pinnacle3 for breast IMRT. Furthermore, these data have established the groundwork for future investigations into 3D dosimetry with more complex anthropomorphic phantoms. PACS number: 87.53.Jw, 87.55.D-, 87.55.dk.This work presents a comparison of an anthropomorphic PRESAGE® dosimeter and radiochromic film measurements with a commercial treatment planning system to determine the feasibility of PRESAGE® for 3D dosimetry in breast IMRT. An anthropomorphic PRESAGE® phantom was created in the shape of a breast phantom. A five‐field IMRT plan was generated with a commercially available treatment planning system and delivered to the PRESAGE® phantom. The anthropomorphic PRESAGE® was scanned with the Duke midsized optical CT scanner (DMOS‐RPC) and the OD distribution was converted to dose. Comparisons were performed between the dose distribution calculated with the Pinnacle3 treatment planning system, PRESAGE®, and EBT2 film measurements. DVHs, gamma maps, and line profiles were used to evaluate the agreement. Gamma map comparisons showed that Pinnacle3 agreed with PRESAGE® as greater than 95% of comparison points for the PTV passed a ±3%/±3mm criterion when the outer 8 mm of phantom data were discluded. Edge artifacts were observed in the optical CT reconstruction, from the surface to approximately 8 mm depth. These artifacts resulted in dose differences between Pinnacle3 and PRESAGE® of up to 5% between the surface and a depth of 8 mm and decreased with increasing depth in the phantom. Line profile comparisons between all three independent measurements yielded a maximum difference of 2% within the central 80% of the field width. For the breast IMRT plan studied, the Pinnacle3 calculations agreed with PRESAGE® measurements to within the ±3%/±3mm gamma criterion. This work demonstrates the feasibility of the PRESAGE® to be fashioned into anthropomorphic shape, and establishes the accuracy of Pinnacle3 for breast IMRT. Furthermore, these data have established the groundwork for future investigations into 3D dosimetry with more complex anthropomorphic phantoms. PACS number: 87.53.Jw, 87.55.D‐, 87.55.dk

Relative stopping power measurements to aid in the design of anthropomorphic phantoms for proton radiotherapy

The delivery of accurate proton dose for clinical trials requires that the appropriate conversion function from Hounsfield unit (HU) to relative linear stopping power (RLSP) be used in proton treatment planning systems (TPS). One way of verifying that the TPS is calculating the correct dose is an end‐to‐end test using an anthropomorphic phantom containing tissue‐equivalent materials and dosimeters. Many of the phantoms in use for such end‐to‐end tests were originally designed using tissue‐equivalent materials that had physical characteristics to match patient tissues when irradiated with megavoltage photon beams. The aim of this study was to measure the RLSP of materials used in the phantoms, as well as alternative materials to enable modifying phantoms for use at proton therapy centers. Samples of materials used and projected for use in the phantoms were measured and compared to the HU assigned by the treatment planning system. A percent difference in RLSP of 5% was used as the cutoff for materials deemed acceptable for use in proton therapy (i.e., proton equivalent). Until proper tissue‐substitute materials are identified and incorporated, institutions that conduct end‐to‐end tests with the phantoms are instructed to override the TPS with the measured stopping powers we provide. To date, the RLSPs of 18 materials have been measured using a water phantom and/or multilayer ion chamber (MLIC). Nine materials were identified as acceptable for use in anthropomorphic phantoms. Some of the failing tissue substitute materials are still used in the current phantoms. Further investigation for additional appropriate tissue substitute materials in proton beams is ongoing. Until all anthropomorphic phantoms are constructed of appropriate materials, a unique HU‐RLSP phantom has been developed to be used during site visits to verify the proton facilitys treatment planning HU‐RLSP calibration curve. PACS number: 87.53.Bn

Independent dose per monitor unit review of eight U.S.A. proton treatment facilities

PURPOSE Compare the dose per monitor unit at different proton treatment facilities using three different dosimetry methods. METHODS Measurements of dose per monitor unit were performed by a single group at eight facilities using 11 test beams and up to six different clinical portal treatment sites. These measurements were compared to the facility reported dose per monitor unit values. RESULTS Agreement between the measured and reported doses was similar using any of the three dosimetry methods. Use of the ICRU 59 ND,w based method gave results approximately 3% higher than both the ICRU 59 NX and ICRU 78 (TRS-398) ND,w based methods. CONCLUSIONS Any single dosimetry method could be used for multi-institution trials with similar conformity between facilities. A multi-institutional trial could support facilities using both the ICRU 59 NX based and ICRU 78 (TRS-398) ND,w based methods but use of the ICRU 59 ND,w based method should not be allowed simultaneously with the other two until the difference is resolved.

SU‐FF‐T‐370: Evaluation of An Anthropomorphic Pelvis Phantom for Proton Therapy

Purpose: To design and implement an anthropomorphic pelvis phantom to audit proton therapy treatment procedures. Method and Materials: A pelvis phantom already in use for independent audits of photonIMRT treatments was retrofitted for use with protons. The relative stopping power of each material used to construct the phantom was measured. Hounsfield Units were determined for each material with a clinical CT scanner. The tissue equivalence of the materials was determined by comparing with the CTcalibration curve used clinically for human tissues. A CT simulation of the phantom was then performed and a proton treatment plan was devised. TLD and radiochromic film were inserted in the phantom and the treatment plan was delivered. The measurements from the TLD and film were compared to calculations made with the treatment planning system. Profile plots through the coronal and sagittal planes were compared to confirm agreement between the treatment plan and delivery. Results: Measured relative stopping powers differed by as much as 10% from values used by the planning system. The comparison between the plan and the TLD showed a difference in dose of less than 3%. The film showed a 2 mm shift in the anterior‐posterior profile and a 6 mm shift in the superior‐inferior profile. The width of the delivered high‐dose region shown by the right‐left film profile differed by as much as 8 mm compared to the plan profile. Conclusion: Preliminary results show the phantom will be able to confirm agreement between measured and calculated dose within 5%/3mm. Stopping power differences between tissue and phantom materials might account for discrepancies between the treatment plan and delivery in the left‐right direction, as lateral fields were used. Conflict of Interest: Work supported by PHS CA010953 and CA081647, awarded by NCI, DHHS, and funds from the RTOG.

TH‐C‐144‐11: Radiological Physics Center (RPC) Approval of Proton Centers for NCI‐Sponsored Clinical Trials

PURPOSE To describe the approval process for enrolling patients on NCI-funded cooperative group clinical trials with proton therapy and the impact of data obtained from the process. METHODS Institutions interested in participating in clinical trials with proton therapy must first complete five RPC-coordinated approval steps for each proton delivery Method: a) completion of the proton facility questionnaire; b) annual monitoring of beam output; c) electronic transfer of treatment plans; d) successful completion of baseline anthropomorphic proton phantoms; and e) successful completion of a dosimetry review visit and corresponding site visit report. The RPC implemented a new fee structure for proton therapy site visits on 1/1/2013, establishing a cost-share between the NCI/MGH Federal Share Funds and the institutions visited. These visits consist of QA review as well as dosimetric measurements with an ion chamber, 2D ion chamber array, and multi-layer ion chamber to evaluate treatment delivery. Once an institution is approved for clinical trials with protons, there may be additional credentialing for specific trials, such as irradiations of the RPCs protocol-specific anthropomorphic proton phantoms. RESULTS To date, the RPC has approved eight proton centers for use of proton therapy in clinical trials. These approvals cover scattered, uniform scanning, and spot scanning proton therapy delivery techniques. The RPC has performed 12 proton therapy site visits and has consensus data to establish acceptance criteria for site visit measurements that will ensure clinical trial consistency. With some institutions going through the approval process for a second or third delivery modality, the RPC anticipates eight proton centers will have 12 new beam delivery systems approved for use of proton therapy in clinical trials by the end of 2013. CONCLUSION The RPCs proton therapy approval process and credentialing requirements help ensure consistency across clinical trial participants. Work supported by grants CA10953, CA059267, and CA81647 (NCI, DHHS).

Investigation of photon and proton overlapping fields in PRESAGE? dosimeters

To evaluate the effects of overlapping dose volumes for varying field arrangements in PRESAGE®, several sequential beam irradiations were delivered each to formulations intended for, and irradiated with, proton beams as well as photon beams. The dosimeters were irradiated within timespans consistent of overlapping fields in clinical treatment plans. Dose profiles taken along the beam direction indicated slight over-responses in higher dose regions relative to similar irradiations given in a single fraction. These results will aid future measurements of overlapping field treatment plans delivered to PRESAGE® for treatment verification of proton and photon 3D dosimetry.

Investigation of 3D dosimetry for an anthropomorphic spine phantom

A new dosimetry insert for the Radiological Physics Centers spine phantom was designed to hold a specially molded dosimeter. The phantom was irradiated with the traditional insert loaded with radiochromic film and TLD, and then with the new 3D dosimetry insert. A comparison with the calculated dose distribution showed that PRESAGE® dosimeter, as well as the film and TLD system, agreed to within ±2mm. Further analysis of the 3D dosimeter, including a measured dose volume histogram, demonstrated the advantages of 3D dosimetry in a clinical environment.

SU‐E‐T‐132: Investigation of Photon and Proton Overlapping Fields in PRESAGE‐ Dosimeters

PURPOSE To evaluate the effects of overlapping dose volumes for varying field arrangements in two formulations of PRESAGE®: one intended for, and irradiated with, proton beams and the other photon beams. METHODS For each treatment modality (photon, proton), three overlapping field setups were performed. These included a stationary dosimeter irradiated over six fractions, a dosimeter shifted laterally to the field to deliver a dose plateau in two fractions, and a dosimeter rotated on its axis to deliver a two-field (for protons) and four-field (for photons) box treatment overlapping in the center of the dosimeter. All subsequent fractions were given within ten minutes and never less than one minute apart. Two cylindrical PRESAGE® dosimeters approximately 7.5 cm in length by 7.5 cm in diameter were irradiated for each setup. The dosimeters were paired, with one dosimeter given total dose by a single fraction while the other followed one of the overlapping field setups. The dosimeters were analyzed using an optical CT scanner and exported to the CERR environment where the doses were compared between paired dosimeters. RESULTS Dose profile comparisons showed relative dose agreement between paired dosimeters within 5% along the SOBP region of the proton formulation. In the case of the fractionated proton irradiation, there was an over-response while other setups resulted in under-responses. Dose agreement between the photon dosimeter treated with six fractions showed a dose under-response within 11% and never less than 5%. Future measurements will include the remaining field setups. CONCLUSIONS The proton formulation of PRESAGE® showed good dose agreement between single and multiple field irradiations. While the photon formulation had slightly less agreement, additional field setup comparisons may show improved results. These results will aid future measurements of overlapping field treatment plans delivered to PRESAGE® for treatment verification for proton and photon 3D dosimetry.

SU‐E‐T‐95: Investigation of 3D Dosimetry for an Anthropomorphic Spine Phantom

PURPOSE To evaluate 3D dosimetry for a spinal cord treatment plan delivery using the Radiological Physics Centers (RPC) anthropomorphic spine phantom. METHODS The RPCs spine phantom currently uses radiochromic film and thermoluminescent dosimeters (TLD) to evaluate spinal metastases treatments. A second dosimetry insert for the phantom was created to hold a PRESAGE® 3D dosimeter which matched the location of the TLD and film in the original insert. The phantom was CT imaged with each insert and an IMRT treatment plan was developed. The IMRT plan was delivered to the phantom twice; once with each insert. The film and PRESAGE® were scanned on a CCD microdensitometer and optical-CT system, reconstructed to a 2 mm slice width, respectively. The measured dose distributions were compared to the treatment plan calculated dose distribution using RPC in-house developed software or the Computational Environment for Radiotherapy Research (CERR). Film and PRESAGE® dose profiles were taken across several planes and compared for agreement. The distance to agreement (DTA) between the measured data and treatment plan, within the high dose gradient region, was quantified. RESULTS The PRESAGE® and plan dose profiles agreed to within 2and 1 mm in the AP and SI directions, respectively. The film and plan also agreed to within 2 mm across all profiles. CONCLUSIONS The PRESAGE® 3D dosimeter, based on these preliminary data, shows potential as a dosimeter for the RPCs phantom irradiation studies. Future work will add markers to the PRESAGE® insert to allow for a reproducible registration in CERR and a an optical-CT system, reconstructed to a 2 mm slice width dose calibration protocol will be created. CA 100835.

MO‐D‐BRB‐03: Comparison of Proton Therapy Institutional Data Collected by the RPC

PURPOSE To detail and compare data collected during RPC onsite dosimetry review visits at proton therapy centers. METHODS The RPC has established a complete review process for proton therapy institutions wishing to participate in NCI-funded clinical trials that includes an on-site dosimetry review visit performed by the RPC. During the visit, the RPC takes measurements that include CT# vs. relative stopping power (RSP) conversion, beam output, depth dose, lateral profiles, QA procedure reviews and anthropomorphic phantom irradiations. The RPC reviewed beam output, depth dose and lateral profiles for 5 specific anatomic treatment sites, reference, prostate, lung, brain, and spine as compared to the institutions measured or treatment planning-derived values. In addition, the RPC has compared results from each institutions proton prostate phantom irradiation. RESULTS All of the institutions visited had RPC/Institution output ratios that ranged between 0.95 - 1.02 where the acceptance criterion was ±5%. For the CT# to RSP comparison, there was a larger variability. Only two institutions agreed within five percent of the recommended values, while the other five institutions had disagreements of up to 20 percent in the high density (high CT number) region of the conversion curve that may have a clinical impact on dose delivery. For the prostate phantom irradiation, 3 institutions failed to meet the RPCs ±7%/4mm acceptance criteria on the initial attempt, but in the end all 7 sites met the criteria. CONCLUSIONS The proton beam output for 7 proton centers, as measured by the RPC, is comparable (±5%), however, there are large discrepancies in the CT# vs RSP conversion curves used from institution to institution. As a result of the RPC onsite dosimetry review visits, several institutions have modified their procedures and dosimetry parameters to improve proton therapy delivery for NCI funded clinical trials. Work supported by grants CA10953, CA059267, and CA81647 (NCI, DHHS).