Jadwiga B. Wojcicka

Dosimetric comparison of three different treatment techniques in extensive scalp lesion irradiation

BACKGROUND AND PURPOSE This study compared lateral photon/electron plan (3DCRT), intensity modulated radiation therapy (IMRT) plan, and high dose rate (HDR) brachytherapy plan for total scalp irradiation. MATERIALS AND METHODS The techniques were planned on a patient with squamous cell carcinoma of the scalp for a prescribed dose of 60 Gy. Conformity indexes and dose volume histograms were used for the comparison. RESULTS Clinical target volume coverage factors for 3DCRT, IMRT, and HDR were 0.976, 0.998, and 0.967, and Conformation Numbers were 0.532, 0.713, and 0.761, respectively. The dose gradient across the target was 59-136%, 91-129%, and 58-242% for 3DCRT, IMRT, and HDR techniques, respectively. The 3DCRT and IMRT techniques produced low optical structure doses. 3DCRT produced hotspots in the brain, while IMRT produced brain sparing. HDR produced the highest integral doses to the brain and optical structures. CONCLUSIONS IMRT provided the best target dose homogeneity and coverage, and delivered clinically acceptable doses to normal structures. HDR produced the most conformal plan, but the total dose delivered is limited by doses to the brain and eyes. HDR is a clinically feasible alternative for less extensive lesions, lower prescription doses, and for patients who cannot lie on the treatment table.

Technical note: on cerrobend shielding for 18-22 MeV electron beams.

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22MeV electron beams, for cone sizes ranging from 6×10cm2to25×25cm2. In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7cm. For the 18MeV beam, the maximum transmission was 6.2% for the 6×10cm2 cone, and 7.4% for the 25×25cm2 cone. For the 22MeV beam, it was 9.5% for the 6×10cm2 cone, and 11.3% for the 25×25cm2 cone. For the 22MeV beam and 15×15cm2 cone, it takes 2.95 and 1.4cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18MeV and greater require additional shielding.

Clinical implementation of stereotaxic brain implant optimization

This optimization method for stereotaxic brain implants is based on seed/strand configurations of the basic type developed for the National Cancer Institute (NCI) atlas of regular brain implants. Irregular target volume shapes are determined from delineation in a stack of contrast enhanced computed tomography scans. The neurosurgeon may then select up to ten directions, or entry points, of surgical approach of which the program finds the optimal one under the criterion of smallest target volume diameter. Target volume cross sections are then reconstructed in 5-mm-spaced planes perpendicular to the implantation direction defined by the entry point and the target volume center. This information is used to define a closed line in an implant cross section along which peripheral seed strands are positioned and which has now an irregular shape. Optimization points are defined opposite peripheral seeds on the target volume surface to which the treatment dose rate is prescribed. Three different optimization algorithms are available: linear least-squares programming, quadratic programming with constraints, and a simplex method. The optimization routine is implemented into a commercial treatment planning system. It generates coordinate and source strength information of the optimized seed configurations for further dose rate distribution calculation with the treatment planning system, and also the coordinate settings for the stereotaxic Brown-Roberts-Wells (BRW) implantation device.

Case study of radiation therapy treatment of a patient with a cardiac ventricular assist device

A patient with a cardiac ventricular assist device (VAD) with computer‐controlled driver presented to our department for radiation therapy. The treatment plan was 4500 cGy to the rectum over 25 fractions with 15MV photon beams. All beams avoided the pump and leads. The response to electromagnetic interference (EMI) was evaluated by observing a duplicate driver in the treatment configuration as the patients fields were delivered to a solid water equivalent phantom. Pretreatment dose assessment included calculations with Pinnacle treatment planning system, AAPM TG36 data analysis, and MOSFET measurements on the surface of the driver during the phantom irradiation. During the first patient treatment, MOSFETs were placed on the pump and leads, approximately 1cm from the left lateral treatment portal. No additional shielding was applied to the VAD. EMI was absent and the VAD operated normally during the pretreatment test and throughout the treatment course. Radiation to the driver was too low to be detected by the MOSFETS. Cumulative dose estimates to the pump were 425cGy to 0. 1cc (DVH), 368cGy (TG36), and 158.5cGy (MOSFET). MOSFET readings to the leads were 70.5cGy. External beam radiation treatment was safely delivered to a VAD dependent patient. The VAD exhibited no adverse response to EMI and doses up to 425 cGy. Our results are based on one case and further study is encouraged. PACS number: 87.53.Dq

Technical Note: On Cerrobend shielding for 18-22MeV electron beams: Cerrobend shielding for 18-22MeV electron beams

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22MeV electron beams, for cone sizes ranging from 6×10cm2to25×25cm2. In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7cm. For the 18MeV beam, the maximum transmission was 6.2% for the 6×10cm2 cone, and 7.4% for the 25×25cm2 cone. For the 22MeV beam, it was 9.5% for the 6×10cm2 cone, and 11.3% for the 25×25cm2 cone. For the 22MeV beam and 15×15cm2 cone, it takes 2.95 and 1.4cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18MeV and greater require additional shielding.

Technical Note: On Cerrobend shielding for electron beams

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22MeV electron beams, for cone sizes ranging from 6×10cm2to25×25cm2. In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7cm. For the 18MeV beam, the maximum transmission was 6.2% for the 6×10cm2 cone, and 7.4% for the 25×25cm2 cone. For the 22MeV beam, it was 9.5% for the 6×10cm2 cone, and 11.3% for the 25×25cm2 cone. For the 22MeV beam and 15×15cm2 cone, it takes 2.95 and 1.4cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18MeV and greater require additional shielding.

SU‐FF‐T‐223: Experience with Thoratec Left Ventricular Assist Device (LVAD) During Radiotherapy Treatment

Purpose: This poster describes the response of a Thoratec left ventricular assist device (LVAD) to the scatter, leakage, and RF environment in a linear accelerator vault. The device contains a mechanical pump attached to the abdominal wall that the supports left ventricle in circulating blood. Cannulae connect the paracorporeal pump to the ventricle. An electric leadrelays pump filling information to the TLC‐II computer‐controlled driver, and a pneumaticlead transfers positive air pressure to the pump and ejects the blood. Method and Materials: The treatment plan was 4500 cGy to the rectum over 25 fractions with three 15MV photon beams on a Varian 2100EX. All beams avoided the pump and leads. The response to EMI was evaluated by observing a duplicate driver in the treatment configuration as the patients fields were delivered to a 30×18×60cm3 solid water equivalent phantom. Pre‐treatment dose assessment included calculations with Pinnacle treatment planning system, AAPM TG36 data analysis, and MOSFET measurements with high sensitivity bias on the surface of the driver during the phantom irradiation. During the first patient treatment, MOSFETs were placed on the pump and leads under 1cm of bolus, approximately 1cm from the left lateral treatment portal. No additional shielding was applied to the LVAD. Results: EMI interference was absent and the LVAD operated normally during the pre‐treatment test and throughout the treatment course. Radiation to the driver was too low to be detected by the MOSFETS. Cumulative dose estimates to the pump were 425cGy to 0.1cc (DVH), 262.8cGy (TG36), and 158.5cGy (MOSFET).MOSFET readings to the leads were 70.5cGy. Conclusion: External beam radiation treatment was safely delivered to a LVAD dependent patient. The Thoratec TLC‐II exhibited no adverse response to EMI and doses up to 425 cGy. Our results are based on one case and further study is encouraged.

SU‐FF‐T‐323: On the Adequacy of Shielding for 15–22 MeV Electron Beams

Purpose: It is customary to define block transmission as a fraction of open field dose at the depth of maximum dose (dmax) for an electron beam. The purpose of this study is to investigate the depth at which transmission should be measured and the adequacy of shielding for 15–22MeV electron beams to reduce the transmission to the regulatory level. Method and Materials: Central axis data were collected in open and blocked beams in water phantom. Measurements were taken for a set of cones from 6×10 to 25×25cm2, 100 cm SSD and 5 cm gap between the cone and water surface. Transmission factors as a function of depth and cone size were calculated. For the 22MeV electron beam, the effect of adding thicknesses of different attenuators (cerrobend, lead, bolus) was investigated. The data were verified independently in three institutions with similar accelerator model and electron energies. Monte Carlo simulations with PENELOPE code were performed to compare with measurements.Results:Photon contamination of 15–22MeV electron beams in the open beam was in agreement with the manufacturers specification. Maximum transmission for the blocked beam for all energies and cones occurred between 4–7 mm depth. The 15MeV electron beam was well attenuated for the standard cerrobend thickness. For 18 and 22MeV beams, maximum transmission varied from 5.9% to 7.5% and from 9.5% to 11% for the 6×10 to 25×25cm2 cones, respectively. For the 15×15cm2 cone, 30 mm thick cerrobend attenuated 22MeV beam to 5%. For 21MeV beam, Monte Carlo simulations showed maximum transmission from 10.6% to 11% for 10×10cm2 and 25×25cm2 cones, respectively. Conclusion: The dmax dose under a blocked beam occurs closer to the surface than for the open beam. In order to decrease transmission to the regulatory level of 5%, 18–22MeV electron beams require additional shielding to supplement the standard cerrobend block.

SU‐FF‐T‐173: Dosimetric Comparison of Three Different Treatment Techniques for Extensive Scalp Lesion Irradiation

Purpose: Homogeneous scalp irradiation poses technical and dosimetric challenges due to the complex shape of the target. The purpose of this study is to compare conventional electron/photon therapy, photonIMRT, and HDR brachytherapy for treatment of extensive scalp lesions. Method and Materials: A 73‐year‐old man presented with extensive squamous cell carcinoma of the scalp. A custom helmet‐shaped mold was made and the target area delineated. The following three treatment methods were evaluated: a 3D plan using lateral photon/electron fields with moving junctions, an IMRT plan using seven coplanar beams, and a brachytherapy plan with catheters placed on the outer side of the mold. All plans were created for the same biological effective doses of 60 Gy. TLDs were used to verify the accuracy of the analyzed techniques. Coverage factors, dose conformity indexes, global conformity indexes, minimum, maximum and mean doses were presented for the scalp and critical structures including the brain and eyes.Results:Dosimetry parameters were normalized to 95% of prescription dose. Clinical target volume coverage factors in electron/photon, IMRT and HDR techniques were 0.976, 0.998, 0.957, and target conformity indexes were 1.98, 1.67, and 1.38, respectively. The dose gradient across the target was 59–136%, 91–129%, and 58–242% for electron/photon, IMRT, and HDR techniques, respectively. The lowest maximum dose to the eyes was achieved with external beam techniques. HDR produced the lowest maximum dose to brain, while IMRT irradiated the least amount of brain above 45 Gy. The measurements agreed within 5% of the calculated doses.Conclusion:IMRT provided better dose homogeneity and target coverage, and delivered lower doses to normal structures than the other techniques. While dose inhomogeneity in the target region was higher in HDR, this technique produced the most conformal plan. Therefore, our department selected IMRT and HDR for extensive scalp lesion treatment.

SU‐FF‐T‐398: Commissioning a 5 Mm Circular Cone for Linac‐Based Stereotactic Radiosurgery Using MicroMOSFET and Polymer Gel

Purpose: The accuracy of measured small cone parameters is important in the treatment of certain disorders like trigeminal neuralgia, where a single large dose is delivered via a small cone. The purpose of this presentation is to identify practical dosimeters for commissioning the cone accurately and efficiently in a community clinic. Method and Materials: Relative output factors for 5, 12.5, and 15 mm cones were measured using microMOSFET, Kodak EDR2 film, and TLD microcubes. TMRs for the 5 mm cone were measured using microMOSFET and BANG®polymer gel. OARs for the 5 mm cone were measured using radiographic and radiochromic films. Results: The output factor for the 5 mm cone measured with microMOSFET was 0.654 for a 6 MV beam and agreed with data published elsewhere. MicroMOSFET measurements agreed with EDR2 film and TLD microcubes measurements within 4.3% and 3.2% respectively for the 5 mm cone. All techniques were within 2.5% agreement for the 12.5 and 15 mm cones. TMR values measured with microMOSFET and polymergel agreed within 3%. Radiographic and radiochromic film off‐axis ratio measurements showed differences not exceeding 1% above the 10% relative dose level. The measurements were verified using a MD Anderson Cancer Center phantom for a single static beam and polymergel for a clinical set of three arcs. The doses reported by the institution and MDACC at dmax and 7.5 cm depth agreed within 4% and 3% respectively. The volumetric doses between the treatment planning system and the polymergel were within 4%. Conclusion: The overall precision and accuracy of microMOSFET‐based measurement techniques are clinically acceptable. The microMOSFET is a feasible alternative with some advantages to TLD microcubes for dosimetric measurements of very small cones and fields. The polymergel was found to be the only commercially available 3D‐dimensional verification dosimeter for these cones.