Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.     

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.     

Dependences of mucosal dose on photon beams in head-and-neck intensity-modulated radiation therapy: a Monte Carlo study

Dependences of mucosal dose in the oral or nasal cavity on the beam energy, beam angle, multibeam configuration, and mucosal thickness were studied for small photon fields using Monte Carlo simulations (EGSnrc-based code), which were validated by measurements. Cylindrical mucosa phantoms (mucosal thickness = 1, 2, and 3 mm) with and without the bone and air inhomogeneities were irradiated by the 6- and 18-MV photon beams (field size = 1 × 1 cm²) with gantry angles equal to 0°, 90°, and 180°, and multibeam configurations using 2, 4, and 8 photon beams in different orientations around the phantom. Doses along the central beam axis in the mucosal tissue were calculated. The mucosal surface doses were found to decrease slightly (1% for the 6-MV photon beam and 3% for the 18-MV beam) with an increase of mucosal thickness from 1–3 mm, when the beam angle is 0°. The variation of mucosal surface dose with its thickness became insignificant when the beam angle was changed to 180°, but the dose at the bone-mucosa interface was found to increase (28% for the 6-MV photon beam and 20% for the 18-MV beam) with the mucosal thickness. For different multibeam configurations, the dependence of mucosal dose on its thickness became insignificant when the number of photon beams around the mucosal tissue was increased. The mucosal dose with bone was varied with the beam energy, beam angle, multibeam configuration and mucosal thickness for a small segmental photon field. These dosimetric variations are important to consider improving the treatment strategy, so the mucosal complications in head-and-neck intensity-modulated radiation therapy can be minimized.     

Dose Delivery Accuracy of Therapeutic Photon and Electron Beams at Low Monitor Unit Settings

PURPOSE: Dose delivery accuracy at low monitor units (LMU) was evaluated for photon and electron beams. Knowledge of this study is required for few dosimetric applications and to know the dose delivered to the patient when the treatment is delivered with few monitor units (MU). MATERIAL AND METHODS: Dose measurements were carried out for photon and electron beams with 0.6 cm(3) PTW ion chamber in white polystyrene phantom at D(max) with a field size of 10 x 10 cm(2) at 100 cm FSD. The relative dose, which is the ratio of dose delivered per MU at the testing to that of the calibration condition, was found out. RESULTS: Significant deviation (+20% to +25%) in dose delivery was noticed for photon and electron beams (+39% to +45%) at LMU settings. Slightly higher inaccuracy in dose delivery was noticed for 6-MV compared to 18-MV photons. The deviation in dose delivery for electron beams was found to be energy-independent and the pattern of variation was similar for all electron energies. CONCLUSION: The dose delivery accuracy at LMU settings has to be ascertained before implementing conformal and IMRT (intensity- modulated radiotherapy) techniques. When there is dose nonlinearity, the treatment delivered with multiple small MU settings can result in significant error in dose delivery.     

Effects of neurosurgical titanium mesh on radiation dose

The purpose of this study was to determine the dosimetric impact of a neurosurgical titanium mesh in patients treated with 6- and 18-MV photon beams. The effects of a 0.4-mm-thick titanium mesh on the dose profile at 3 regions within a solid water phantom were measured using extended dose range-2 (EDR2) film for 6- and 18-MV photon beams. All measurements were performed with the titanium mesh placed at a depth of 1.5 cm in the phantom. Films were exposed immediately above the mesh, immediately below the mesh, and at a depth of 5 cm from the surface of the phantom. The films were scanned using a scanning densitometer. In the region directly above the titanium mesh, there was an increase in dose of 7.1% for 6-MV photons and 4.9% for 18-MV photons. Directly below the titanium mesh, there was an average decrease in dose of 1.5% for 6-MV photons and an increase of 1.0% for 18-MV photons. At 5-cm depth, for 6- and 18-MV photons, there was a decrease in dose of 2.2% and 0.6%, respectively. We concluded that for cranial irradiation with high-energy photons, the dosimetric impact of a 0.4-mm titanium mesh is small and does not require modification in treatment parameters.     

Dosimetric assessment of the mono and dual-isocentric VMAT technique based on flattening filter-free beams for SBRT with non-contiguous spinal targets

The study aimed to evaluate the planning characteristics of spinal stereotactic body radiotherapy (SBRT) using mono- and dual-isocentric volumetrically modulated arc therapy (VMAT) techniques. The dosimetric indices were compared between different beam arrangement techniques for spinal SBRT planning, including spinal cord avoidance, planning target volume (PTV) dose coverage, conformity, homogeneity, and gradient index. A total of 8 PTVs were contoured on RANDO phantom computed tomography (CT) images, with 4 PTVs per section of the spine (thoracic and lumbar). VMAT plans for each PTV were generated using 4 different beam arrangement techniques with a 6-MV FFF photon beam, two of which were mono-isocentric (MI) and 2 of which were dual-isocentric (DI). Dose calculations for all plans were performed using the Acuros XB algorithm. The study found that when non-contiguous spinal lesions are widely spaced, it may be more effective to use 4-Arcs DI to generate a better homogeneity index and gradient index, whereas 2-Arcs MI was beneficial for closely spaced lesions. Furthermore, the use of more arcs with a dual isocenter reduced the volume of partial cord receiving 10 Gy (V_10Gy), maximum dose to 0.03 cc of partial cord (D_0.03cc), and monitor units (MUs). The results showed that DI has a higher plan quality than MI for treating non-contiguous spine SBRT, with better homogeneity and a lower dose to the spinal cord, as well as comparable tumor coverage, delivery accuracy, and adequate tumour coverage. 4-Arcs DI had the sharpest dose falloff and achieved the lowest overall spinal cord doses at the expense of twice the treatment time as 2Arcs-MI. These results could help figure out which VMAT beam arrangements are best for treating non-contiguous spinal tumors.     

Effects of neurosurgical titanium mesh on radiation dose

The purpose of this study was to determine the dosimetric impact of a neurosurgical titanium mesh in patients treated with 6- and 18-MV photon beams. The effects of a 0.4-mm-thick titanium mesh on the dose profile at 3 regions within a solid water phantom were measured using extended dose range-2 (EDR2) film for 6- and 18-MV photon beams. All measurements were performed with the titanium mesh placed at a depth of 1.5 cm in the phantom. Films were exposed immediately above the mesh, immediately below the mesh, and at a depth of 5 cm from the surface of the phantom. The films were scanned using a scanning densitometer. In the region directly above the titanium mesh, there was an increase in dose of 7.1% for 6-MV photons and 4.9% for 18-MV photons. Directly below the titanium mesh, there was an average decrease in dose of 1.5% for 6-MV photons and an increase of 1.0% for 18-MV photons. At 5-cm depth, for 6- and 18-MV photons, there was a decrease in dose of 2.2% and 0.6%, respectively. We concluded that for cranial irradiation with high-energy photons, the dosimetric impact of a 0.4-mm titanium mesh is small and does not require modification in treatment parameters.     

Monte Carlo study on a flattening filter-free 18-MV photon beam of a medical linear accelerator

PURPOSE: Several studies on the dosimetric properties of unflattened photon beams have shown some advantages for radiotherapy. In this study, the effect of removing the flattening filter from an 18-MV photon beam was investigated using the Monte Carlo method. MATERIALS AND METHODS: The 18-MV photon beam of an Elekta SL25 linear accelerator was simulated using the MCNP4C Monte Carlo (MC) code. Beam dosimetric features, including central axis absorbed doses, beam profiles, and photon energy spectra, were calculated for flattened and unflattened 18-MV photon beams. RESULTS: A 4.24-fold increase in the dose rate was seen for the unflattened beam with a field size of 10 x 10 cm(2). A decrease in the out-of-field dose up to 30% was seen for the unflattened beam. For the unflattened beam, photon energy spectra were softer, and the mean energies of the spectra were higher for a smaller field size. CONCLUSION: Our study showed that the increase in dose rate and lower out-of-field dose can be possible advantages for an unflattened 18-MV beam.     

Entrance dose measurement: a simple and reliable technique.

Verification of tumor dose for patients undergoing external beam radiotherapy is an important part of quality assurance programs in radiation oncology. Among the various methods available, entrance dose in vivo is one reliable method used to verify the tumor dose delivered to a patient. In this work, entrance dose measurements using LiF:Mg;Ti and LiF:Mg;Cu;P thermoluminescent dosimeters (TLDs) without buildup cap was carried out. The TLDs were calibrated at the surface of a water equivalent phantom against the maximum dose, using 6- and 10-MV photon and 9-MeV electron beams. The calibration geometry was such that the TLDs were placed on the surface of the "solid-water" phantom and a calibrated ionization chamber was positioned inside the phantom at calibration depth. The calibrated TLDs were then utilized to measure the entrance dose during the treatment of actual patients. Measurements were also carried out in the same phantom simultaneously to check the stability of the system. The dose measured in the phantom using the TLDs calibrated for entrance dose to 6-and 10-MV photon beams was found to be close to the dose determined by the treatment planning system (TPS) with discrepancies of not more than 4.1% (mean 1.3%). Consequently, the measured entrance dose during dose delivery to the actual patients with a prescribed geometry was found to be compatible with a maximum discrepancy of 5.7% (mean 2.2%) when comparison was made with the dose determined by the TPS. Likewise, the measured entrance dose for electron beams in the phantom and in actual patients using the calibrated TLDs were also found to be close, with maximum discrepancies of 3.2% (mean 2.0%) and 4.8% (mean 2.3%), respectively. Careful implementation of this technique provides vital information with an ability to confidently accept treatment algorithms derived by the TPS or to re-evaluate the parameters when necessary.     

Choice of beam energy and dosimetric implications for radiation treatment in a subpopulation of women with large breasts in the United States and Japan.

Radiation complications are often related to the dose inhomogeneity (hot spot) in breast tissue treated with conservative therapy, especially for large patients. The effect of photon energy on radiation dose distribution is analyzed to provide guidelines for the selection of beam energy when tangential fields and limited slices are used to treat women with large breasts. Forty-eight patients with chest wall separation > 22 cm were selected for dosimetric analysis. We compared the maximum dose in the central axis (CAX) plane (2D) using 6-, 10-, and 18-MV photon beams in all patients and 3D data set for 16 patients. Correlation between hot spot dose (HSD), separation, breast cup size, breast volume, and body weight was derived with beam energy. Among the 48 patients in this study, HSD > 10% in the CAX plane was noted in 98%, 46%, and 4% of the population when 2D dosimetry was performed; however, with 3D study, it was in 50%, 19%, and 6% of the patients with 6-MV, 10-MV and 18-MV beams, respectively. The chest wall separation, body weight, and breast volume were correlated with the HSD in both the 2D and 3D plans. Patient's bra size was not correlated with the hot spot. The chest wall separation was found to be the most important parameter to correlate with hot spot in tangential breast treatment. Simple guidelines are provided for dose uniformity in breast with respect to chest wall separation, body weight, bra size, and breast volume with tangential field irradiations.     

High-energy electron and photon therapy to the parotid bed: radiation dose perturbations with a titanium mandibular implant

Adjuvant radiation therapy to the parotid bed is commonly administered following surgical resection using either a pair of angled wedged photon beams or an ipsilateral mixed-beam portal of electrons and photons. The present study seeks to determine the optimal parotid bed treatment technique in the presence of a titanium mandibular implant by investigating perturbations in the dose distribution deep to this implant for a 15-MeV electron beam and a 6-MV photon beam. A titanium mandibular plate was embedded in a tissue-equivalent phantom, and irradiated with 15-MeV electrons, and 6 MV photons. Radiation doses behind the plate were measured with both thermoluminescent dosimeters and radiographic film. With 15-MeV electrons, there is a clinically significant decrease in the dose beyond the titanium plate, which is most important at 5-mm and 10-mm depths (18-27%). With 6-MV photons the dose at the deep interface of titanium and tissue is reduced by between 15 and 18%, but rapidly drops to < 5% at a depth of 5 mm. In adjuvant treatment to the parotid bed, when the clinical target volume includes tissue positioned deep to a titanium implant, significant underdosage occurs with ipsilateral beam arrangements, especially when electrons are used.     

Comparison of 4 MV photon surface dose among Varian, Siemens, and Elekta linear accelerators for tangential breast treatment: a phantom study

Purpose We have compared the differences in a 4-MV photon surface dose among Varian, Siemens, and Elekta linear accelerators (linacs) with wedges for tangential breast treatment. Materials and methods The wedge factor and the surface dose were measured using a solid water phantom and an ion chamber for each linear accelerator with various field sizes and wedge angles. A tangential treatment plan was applied to an elliptical hollow cylinder water phantom with a radiochromic film placed thereon. A dose was delivered to a simulated target in the phantom, and the resulting dose distribution was analyzed using a film scanner. Results Varian's wedges resulted in the highest wedge factors, ranging from 0.37 to 0.75 depending on the wedge angles. Varian's wedges led to the highest normalized skin doses, ranging between 0.40 and 0.73 depending on the wedge angles and field sizes. In the cylinder phantom test with two tangential beams, the Varian linac provided a nearly 20% higher maximum dose than the Siemens and Elekta linacs. Conclusion The Varian linac resulted in the highest surface doses, and the Elekta linac led to the lowest for nearly all the measurement conditions we employed, including open beams.     

Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT). METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle³ treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0^o with equal separation of 72^o, was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations. RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams. CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.     

Surface and superficial dose dosimetric verification for postmastectomy radiotherapy

In patients given postmastectomy radiotherapy (PMRT), the chest wall is a very thin layer of soft tissue with a low-density lung tissue behind. Chest wall treated in this situation with a high-energy photon beam presents a high dosimetric uncertainty region for both calculation and measurement. The purpose of this study was to measure and to evaluate the surface and superficial doses for patients requiring PMRT with different treatment techniques. An elliptic cylinder cork and superflab boluses were used to simulate the lung and the chest wall, respectively. Sets of computed tomography (CT) images with different chest wall thicknesses were acquired for the study phantom. Hypothetical clinical target volumes (CTVs) were outlined and modified to fit a margin of 1–3 mm, depending on the chest wall thickness, away from the surface for the sets of CT images. The planning target volume (PTV) was initially created by expanding an isotropic 3-mm margin from the CTV, and then a margin of 3 mm was shrunk from the phantom surface to avoid artifact-driven results in the beam-let intensity. Treatment techniques using a pair of tangential wedged fields (TWFs) and 4-field intensity-modulated radiation therapy (IMRT) were designed with a prescribed fraction dose (D_p) of 180 cGy. Superficial dose profiles around the phantom circumference at depths of 0, 1, 2, 3, and 5 mm were obtained for each treatment technique using radiochromic external beam therapy (EBT) films. EBT film exhibits good characteristics for dose measurements in the buildup region. Underdoses at the median and lateral regions of the TWF plans were shown. The dose profiles at shallow depths for the TWF plans show a dose buildup about 3 mm at the median and lateral tangential incident regions with a surface dose of about 52% of D_p. The dose was gradually increased toward the most obliquely tangential angle with a maximum dose of about 118% of D_p. Dose profiles were more uniform in the PTV region for the 4-F IMRT plans. Most of the PTV region had doses >94% of D_p at depths >1 mm. The mean surface dose was about 65% of D_p for the 4-F IMRT plans. The maximum dose for the 4-F IMRT plans was <118.4% of D_p. The application of added bolus has to consider the treatment technique, tumor coverage, and possible skin reactions. For PMRT, if the chest surface and wall are treated adequately, at least 3 mm bolus should be added to the chest wall when tangential beams and 6-MV photon energy are arranged. However, when the surface and superficial regions are not high-risk areas, an IMRT plan with tangential beams and 6-MV photon energy can provide uniform dose distributions within the PTV, spare the skin reaction, and deliver sufficient doses to the chest wall at depths >1 mm.     

A Monte Carlo study on neutron and electron contamination of an unflattened 18-MV photon beam

Recent studies on flattening filter (FF) free beams have shown increased dose rate and less out-of-field dose for unflattened photon beams. On the other hand, changes in contamination electrons and neutron spectra produced through photon (E>10 MV) interactions with linac components have not been completely studied for FF free beams. The objective of this study was to investigate the effect of removing FF on contamination electron and neutron spectra for an 18-MV photon beam using Monte Carlo (MC) method. The 18-MV photon beam of Elekta SL-25 linac was simulated using MCNPX MC code. The photon, electron and neutron spectra at a distance of 100 cm from target and on the central axis of beam were scored for 10×10 and 30×30 cm² fields. Our results showed increase in contamination electron fluence (normalized to photon fluence) up to 1.6 times for FF free beam, which causes more skin dose for patients. Neuron fluence reduction of 54% was observed for unflattened beams. Our study confirmed the previous measurement results, which showed neutron dose reduction for unflattened beams. This feature can lead to less neutron dose for patients treated with unflattened high-energy photon beams.     

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

PURPOSE: To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. MATERIAL AND METHODS: Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. RESULTS: Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). CONCLUSION: Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.     

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

AIM: Since the skin dose becomes the limiting factor while deciding the tumorcidal dose, the detailed analysis of dose distribution in the build-up region is necessary for high-energy photon beams. In this study the beam characteristics affecting the build-up and skin dose for 6- and 18-MV photons are analyzed. MATERIALS AND METHODS: Measurements were made with 6- and 18-MV photons using a PTW parallel-plate ionization chamber (B 23344-036) and a RDM-1F electrometer. Build-up ionization measurements were made with the chamber fitted into a 25 x 25 x 25 cm polystyrene phantom with a fixed SSD of 100 cm. The entrance and build-up dose measurements were made with a polycarbonate and a mesh type metallic shielding tray and a 45 degrees wedge. Exit dose measurements were carried out for the graphite patient supporting assembly table top, 1.0 cm thick piece of wood and the 1.0 cm thick patient supporting perspex base frame for head and neck treatments. RESULTS: It was observed that the dmax decreased slightly with field size as with other accelerators. For both photon energies the surface dose was observed to increase with increase in field size. It was also noticed that the dose in the build-up region increases slightly when the polycarbonate secondary blocking tray is introduced with the increase in surface dose. The data show that the tray perturbation factor (TPF) at surface decreases steadily with tray-surface distance for both photon beams for all field sizes. It was noted that the TPF was more when the polycarbonate tray was introduced at shorter tray-surface distances for both energies. At tray-surface distances above 60 cm the TPF almost remained close to unity for 6-MV photons for all field sizes, whereas the continuous decrease in TPF could be noted for 18-MV photon beams even after the TPF reached unity. CONCLUSION: The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.     

Treatment planning for dose escalation in non-small cell lung cancer (NSCLC).

This study was performed to examine potential field arrangements for irradiating non-small cell lung cancer (NSCLC) on a dose escalation study. An example patient was chosen and 7 coplanar treatment plans were created to treat a NSCLC. Two plans included prophylactic nodal irradiation (PNRT) and 5 did not. Four plans used 4 fields, 2 plans used 5 fields, and 1 plan included dynamic conformal 360 degrees rotational therapy. All plans delivered 80 Gy to the isocenter with 10-MV x-rays. Each plan was initially created without dose inhomogeneity corrections and then was recalculated with these corrections, maintaining the same weighting and number of monitor units. Avoiding PNRT spared a considerable volume of normal tissue from radiation. Plans with 5 fields generally spared normal tissues better than 4-field plans. There was no benefit to the dynamic conformal 360 degrees rotational plan. Inhomogeneity corrections revealed that higher doses were delivered to both the tumor and normal structures. Seven beam arrangements for the treatment of NSCLC were compared to develop potential beam arrangements that would be applicable to treating NSCLC on a multi-institutional dose escalation study. We favor the use of at least 5 beams in most situations. It is possible that the use of more fields would further improve plans up to a point of diminishing returns, as exemplified by the lack of benefit seen with the dynamic conformal 360 degrees rotational plan. It is possible that the use of noncoplanar fields or intensity-modulated radiation therapy (IMRT) may further improve the therapeutic ratio.     

Quality assurance measurements of a-Si EPID performance.

The performance stability of a Varian aS500 amorphous silicon (a-Si) electronic portal imaging device (EPID) was monitored over an 18-month period using a variety of standard quality assurance (QA) tests. The tests were selected to provide ongoing information about image quality and dose response from the time of EPID acceptance into clinical service. To evaluate imaging performance, we made spatial resolution and contrast measurements using both PortalVision and QC-3V phantoms for 6- and 15-MV photon beams at repetition rates of 100, 300, and 400 MU/min in standard scanning mode. To assess operational stability for dosimetry applications, we measured central axis radiation response and beam pulse variability for the same image acquisition modes. Using the QC-3V phantom, values for the critical frequency of 0.435 +/- 0.005 lp/mm for 6 MV and 0.382 +/- 0.003 lp/mm for 15 MV were obtained. The contrast-to-noise ratio was found to be approximately 20% higher for the lower photon energy. Beam pulse variability remained within the tolerance of 3% set by the manufacturer. The central axis pixel response of the EPID remained constant within +/-1% over a 5-month period for the 6-MV beam, but fell approximately 4% over the same period for the 15-MV beam. The Varian aS500 EPID studied exhibited consistent image quality and a stable radiation response. These characteristics render it suitable for quantitative applications such as clinical dose measurement.     

Evaluation of RayStation's delivered dose and accumulated dose features for spine stereotactic radiotherapy

We investigated delivered dose and dose accumulation features in the dose tracking module of RayStation version 11B before potential integration into the spine stereotactic radiosurgery (SSRS) program at our institution. End-to-end testing with a rigid Rando phantom was performed, and 10 retrospective clinical cases were selected for evaluation. Pre-treatment cone beam CTs (pCBCT) were corrected for Hounsfield unit (HU) integrity and contours were rigidly copied from the reference plan. We then calculated the delivered dose to the corrected cone beam CT (cCBCT). A deformable image registration (DIR) was generated between cCBCT and reference planning CT (rCT) using controlling region of interests. Deformed delivered dose to the rCT was summed to generate the accumulated dose for multiple fractions. The end-to-end tests of the phantom study revealed an improvement of cCBCT HU information by > 100 HU compared to the pCBCT. When compared to the reference plan, the delivered dose and deformed delivered dose were within 1.0% for the GTV and CTV and 3.0% for the spinal cord, respectively. Nearly all 10 clinical cases demonstrated delivered dose and accumulated dose deviations < 3.0% from the reference plan. However, 2 patients rendered delivered dose deviations between 3.0% and 4.0%, showing the effectiveness of the module. The dose tracking module in RayStation version 11B could potentially be utilized to aid clinical decision-making for external body shape change or positional deviation in SSRS for rigid target and critical structures. Evaluation before clinical application under one's specific practice is required, and results must be carefully analyzed specially near the high dose gradient area.