Treatments of exceptionally large prostate cancer patients with low-energy intensity-modulated photons

Comparison of the dose distribution of the VMAT radiotherapy technique depending on the beam used: FFF-X10MV and FFF-X15MV

Background

The aim of the study was to answer the question of whether flattening filter (FF) and flattening filter-free (FFF) beams can be used alternately in the volumetric modulated arc therapy (VMAT) treatment technique, regardless of the size of the irradiated volume [small (S) or large (L) planning target volume (PTV)].

Materials and methods

Two groups of patients were examined: a group with a S-PTV-laryngeal cancer and a group with a L-PTV - gynecological volume. For each patient, two treatment plans were made for beams (energies): FFF-X10MV and FF-X15MV. Then, a statistical analysis, nonparametric test, and independent groups were performed, comparing the beams' impact on the analyzed treatment plans.

Results

In the case of laryngeal irradiation (S-PTV), there are no statistically significant differences between the energy used and the assessed parameters of the plan. In the case of gynecological volume (L-PTV), only statistically significant differences were noted for the number of monitor units depending on the energy used. For a large irradiated volume (gynecological case), the use of FFF beams increases the number of monitor units by 39,4% in relation to the FF beam.

Conclusions

In the case of gynecological neoplasms, statistically significant differences were found in the number of monitor units. Therefore, in the case of irradiation of L-PTV, it is recommended that flattening-filtering beams are used due to the smaller number of monitors. In the case of S-PTV, no statistically significant differences were found between the types of beams used (FF or FFF) and the treatment plan parameters analyzed in the study.

VMAT partial arc technique decreases dose to organs at risk in whole pelvic radiotherapy for prostate cancer when compared to full arc VMAT and IMRT

Whole pelvic radiotherapy (WPRT) can sterilize microscopic lymph node metastases in treatment of prostate cancer. WPRT, compared to prostate only radiotherapy (PORT), is associated with increased acute gastrointestinal, and hematological toxicities. To further explore minimizing normal tissue toxicities associated with WPRT in definitive IMRT for prostate cancer, this planning study compared dosimetric differences between static 9-field-IMRT, full arc VMAT, and mixed partial-full arc VMAT techniques. In this retrospective study, 12 prostate cancer patients who met the criteria for WPRT were randomly selected for this study. The initial volume, PTV46, included the prostate, seminal vesicles, and pelvic nodes with margin and was prescribed to 4600 cGy. The cone-down volume, PTV78, included the prostate and proximal seminal vesicles with margin to a total dose of 7800 cGy. For each CT image set, 3 plans were generated for each of the PTVs: an IMRT plan, a full arc (FA) VMAT plan, and a mixed partial-full arc (PFA) VMAT plan, using 6MV photons energy. According to RTOG protocols none of the plans had a major Conformity Index (CI) violation by any of the 3 planning techniques. PFA plan had the best mean CI index of 1.00 and significantly better than IMRT (p = 0.03) and FA (p = 0.007). For equivalent PTV coverage, the average composite gradient index of the PFA plans was better than the IMRT and the FA plans with values 1.92, 2.03, and 2.01 respectively. The defference was statistically significant between PFA/IMRT and PFA/FA, with p- values of < 0.001. The IMRT plans and the PFA plans provided very similar doses to the rectum, bladder, sigmoid colon, and femoral heads, which were lower than the dose in the FA plans. There was a significant decrease in the mean dose to the rectum from 4524 cGy with the FA to 4182 cGy with the PFA and 4091 cGy with IMRT (p < 0.001). The percent of rectum receiving 4000 cGy was also the highest with FA at 66.1% compared to 49.9% (PFA) and 47.5% (IMRT). There was a significant decrease in the mean dose to the bladder from 3922 cGy (FA) to 3551 cGy (PFA) and 3612 cGy (IMRT) (p < 0.001). The percent of bladder receiving 4000 cGy was also the highest with FA at 45.4% compared to 36.6% (PFA) and 37.4% (IMRT). The average mean dose to the sigmoid colon decreased from 4177 cGy (FA) to 3893 cGy (PFA) and 3819 cGy (IMRT). The average mean dose to the femoral heads decreased from 2091 cGy (FA) to 2026 cGy (PFA) and 1987 cGy (IMRT). Considering the improvement in plan quality indices recorded in this study including the dose gradient and the dose to organs at risk, mixed partial-full arc plans may be the preferred VMAT treatment technique over full arc plans for prostate cancer treatments that include nodal volumes.

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation

Exposure to ionizing radiation can occur during medical treatments, from naturally occurring sources in the environment, or as the result of a nuclear accident or thermonuclear war. The severity of cellular damage from ionizing radiation exposure is dependent upon a number of factors including the absorbed radiation dose of the exposure (energy absorbed per unit mass of the exposure), dose rate, area and volume of tissue exposed, type of radiation (e.g., X-rays, high-energy gamma rays, protons, or neutrons) and linear energy transfer. While the dose, the dose rate, and dose distribution in tissue are aspects of a radiation exposure that can be varied experimentally or in medical treatments, the LET and eV are inherent characteristics of the type of radiation. High-LET radiation deposits a higher concentration of energy in a shorter distance when traversing tissue compared with low-LET radiation. The different biological effects of high and low LET with similar energies have been documented in vivo in animal models and in cultured cells. High-LET results in intense macromolecular damage and more cell death. Findings indicate that while both low- and high-LET radiation activate non-homologous end-joining DNA repair activity, efficient repair of high-LET radiation requires the homologous recombination repair pathway. Low- and high-LET radiation activate p53 transcription factor activity in most cells, but high LET activates NF-kB transcription factor at lower radiation doses than low-LET radiation. Here we review the development, uses, and current understanding of the cellular effects of low- and high-LET radiation exposure.

Impact of nominal photon energies on normal tissue sparing in knowledge-based radiotherapy treatment planning for rectal cancer patients

The interactive adjustment of the optimization objectives during the treatment planning process has made it difficult to evaluate the impact of beam quality exclusively in radiotherapy. Without consensus in the published results, the arbitrary selection of photon energies increased the probability of suboptimal plans. This work aims to evaluate the dosimetric impact of various photon energies on the sparing of normal tissues by applying a preconfigured knowledge-based planning (RapidPlan) model to various clinically available photon energies for rectal cancer patients, based on model-generated optimization objectives, which provide a comparison basis with less human interference. A RapidPlan model based on 81 historical VMAT plans for pre-surgical rectal cancer patients using 10MV flattened beam (10X) was used to generate patient-specific objectives for the automated optimization of other 20 patients using 6X, 8X, 10X (reference), 6MV flattening-filter-free (6F) and 10F beams respectively on a TrueBeam accelerator. It was observed that flattened beams produced very comparable target dose coverage yet the conformity index using 6F and 10F were clinically unacceptable (>1.29). Therefore, dose to organs-at-risk (OARs) and normal tissues were only evaluated for flattened beams. RapidPlan-generated objectives for 6X and 8X beams can achieve comparable target dose coverage as that of 10X, yet the dose to normal tissues increased monotonically with decreased energies. Differences were statistically significant except femoral heads. From the radiological perspective of view, higher beam energy is still preferable for deep seated tumors, even if multiple field entries such as VMAT technique can accumulate enough dose to the target using lower energies, as reported in the literature. In conclusion, RapidPlan model configured for flattened beams cannot optimize un-flattened beams before adjusting the target objectives, yet works for flattened beams of other energies. For the investigated 10X, 8X and 6X photons, higher energies provide better normal tissue sparing.

Investigation on using high-energy proton beam for total body irradiation (TBI)

This work investigated the possibility of using proton beam for total body irradiation (TBI). We hypothesized the broad‐slow‐rising entrance dose from a monoenergetic proton beam can deliver a uniform dose to patient with varied thickness. Comparing to photon‐based TBI, it would not require any patient‐specific compensator or beam spoiler. The hypothesis was first tested by simulating 250 MeV, 275 MeV, and 300 MeV protons irradiating a wedge‐shaped water phantom in a paired opposing arrangement using Monte Carlo (MC) method. To allow ±7.5% dose variation, the maximum water equivalent thickness (WET) of a treatable patient separation was 29 cm for 250 MeV proton, and >40 cm for 275 MeV and 300 MeV proton. The compared 6 MV photon can only treat patients with up to 15.5 cm water‐equivalent separation. In the second step, we simulated the dose deposition from the same beams on a patient's whole‐body CT scan. The maximum patient separation in WET was 23 cm. The calculated whole‐body dose variations were ±8.9%,±9.0%, ±9.6%, and ±14% for 250 MeV proton, 275 MeV proton, 300 MeV proton, and 6 MV photon. At last, we tested the current machine capability to deliver a monoenergetic proton beam with a large uniform field. Experiments were performed on a compact double scattering single‐gantry proton system. With its C‐shaped gantry design, the source‐to‐surface distance (SSD) reached 7 m. The measured dose deposition curve had 22 cm relatively flat entrance region. The full width half maximum field size was measured 105 cm. The current scatter filter had to be redesigned to produce a uniform intensity at such treatment distance. In conclusion, this work demonstrated the possibility of using proton beam for TBI. The current commercially available proton machines would soon be ready for such task.

PACS number(s): 87.53.Bn, 87.55.K‐, 87.55.‐x, 87.56.‐v

The dosimetric significance of using 10 MV photons for volumetric modulated arc therapy for post-prostatectomy irradiation of the prostate bed

BACKGROUND

The purpose of the study was to analyse the dosimetric differences when using 10 MV instead of 6 MV for VMAT treatment plans for post-prostatectomy irradiation of the prostate bed.

METHODS AND MATERIALS

Ten post-prostatectomy prostate bed irradiation cases previously treated using 6 MV with volumetric modulated arc therapy (VMAT) were re-planned using 10 MV with VMAT. Prescription dose was 66.6 Gy with 1.8 Gy per fraction for 37 daily fractions. The same structure set, number of arcs, field sizes, and minimum dose to the Planning Target Volume (PTV) were used for both 6 MV and 10 MV plans. Results were collected for dose to Organs at Risk (OAR) constraints, dose to the target structures, number of monitor units for each arc, Body V5, Conformity Index, and Integral Dose. The mean values were used to compare the 6 MV and 10 MV results. To determine the statistical significance of the results, a paired Student t test and power analysis was performed.

RESULTS

Statistically significant lower mean values were observed for the OAR dose constraints for the rectum, bladder-Clinical Target Volume (bladder-CTV), left femoral head, and right femoral head. Also, statistically significant lower mean values were observed for the Body V5, Conformity Index, and Integral Dose.

CONCLUSIONS

Several dosimetric benefits were observed when using 10 MV instead of 6 MV for VMAT based treatment plans. Benefits include sparing more dose from the OAR while still maintaining the same dose coverage to the PTV. Other benefits include lower Body V 5,Conformity Index, and Integral Dose.

Monte Carlo investigation of collapsed versus rotated IMRT plan verification

IMRT QA requires, among other tests, a time-consuming process of measuring the absorbed dose, at least to a point, in a high-dose, low-dose-gradient region. Some clinics use a technique of measuring this dose with all beams delivered at a single gantry angle (collapsed delivery), as opposed to the beams delivered at the planned gantry angle (rotated delivery). We examined, established, and optimized Monte Carlo simulations of the dosimetry for IMRT verification of treatment plans for these two different delivery modes (collapsed versus rotated). The results of the simulations were compared to the treatment planning system dose calculations for the two delivery modes, as well as to measurements taken. This was done in order to investigate the validity of the use of a collapsed delivery technique for IMRT QA. The BEAMnrc, DOSXYZnrc, and egs_chamber codes were utilized for the Monte Carlo simulations along with the MMCTP system. A number of different plan complexity metrics were also used in the analysis of the dose distributions in a bid to qualify why verification in a collapsed delivery may or may not be optimal for IMRT QA. Following the Alfonso et al. formalism, the kfclin,frefQclin,Q correction factor was calculated to correct the deviation of small fields from the reference conditions used for beam calibration. We report on the results obtained for a cohort of 20 patients. The plan complexity was investigated for each plan using the complexity metrics of homogeneity index, conformity index, modulation complexity score, and the fraction of beams from a particular plan that intersect the chamber when performing the QA. Rotated QA gives more consistent results than the collapsed QA technique. The kfclin,frefQclin,Qfactor deviates less from 1 for rotated QA than for collapsed QA. If the homogeneity index is less than 0.05 then the kfclin,frefQclin,Q factor does not deviate from unity by more than 1%. A value this low for the homogeneity index can only be obtained with the rotated QA technique.

Automated generation of IMRT treatment plans for prostate cancer patients with metal hip prostheses: comparison of different planning strategies

PURPOSE

To compare IMRT planning strategies for prostate cancer patients with metal hip prostheses.

METHODS

All plans were generated fully automatically (i.e., no human trial-and-error interactions) using iCycle, the authors' in-house developed algorithm for multicriterial selection of beam angles and optimization of fluence profiles, allowing objective comparison of planning strategies. For 18 prostate cancer patients (eight with bilateral hip prostheses, ten with a right-sided unilateral prosthesis), two planning strategies were evaluated: (i) full exclusion of beams containing beamlets that would deliver dose to the target after passing a prosthesis (IMRT remove) and (ii) exclusion of those beamlets only (IMRT cut). Plans with optimized coplanar and noncoplanar beam arrangements were generated. Differences in PTV coverage and sparing of organs at risk (OARs) were quantified. The impact of beam number on plan quality was evaluated.

RESULTS

Especially for patients with bilateral hip prostheses, IMRT cut significantly improved rectum and bladder sparing compared to IMRT remove. For 9-beam coplanar plans, rectum V60 Gy reduced by 17.5% ± 15.0% (maximum 37.4%, p = 0.036) and rectum D mean by 9.4% ± 7.8% (maximum 19.8%, p = 0.036). Further improvements in OAR sparing were achievable by using noncoplanar beam setups, reducing rectum V 60Gy by another 4.6% ± 4.9% (p = 0.012) for noncoplanar 9-beam IMRT cut plans. Large reductions in rectum dose delivery were also observed when increasing the number of beam directions in the plans. For bilateral implants, the rectum V 60Gy was 37.3% ± 12.1% for coplanar 7-beam plans and reduced on average by 13.5% (maximum 30.1%, p = 0.012) for 15 directions.

CONCLUSIONS

iCycle was able to automatically generate high quality plans for prostate cancer patients with prostheses. Excluding only beamlets that passed through the prostheses (IMRTcut strategy) significantly improved OAR sparing. Noncoplanar beam arrangements and, to a larger extent, increasing the number of treatment beams further improved plan quality.

Photon energy-modulated radiotherapy: Monte Carlo simulation and treatment planning study

PURPOSE

To demonstrate the feasibility of photon energy-modulated radiotherapy during beam-on time.

METHODS

A cylindrical device made of aluminum was conceptually proposed as an energy modulator. The frame of the device was connected with 20 tubes through which mercury could be injected or drained to adjust the thickness of mercury along the beam axis. In Monte Carlo (MC) simulations, a flattening filter of 6 or 10 MV linac was replaced with the device. The thickness of mercury inside the device varied from 0 to 40 mm at the field sizes of 5 × 5 cm(2) (FS5), 10 × 10 cm(2) (FS10), and 20 × 20 cm(2) (FS20). At least 5 billion histories were followed for each simulation to create phase space files at 100 cm source to surface distance (SSD). In-water beam data were acquired by additional MC simulations using the above phase space files. A treatment planning system (TPS) was commissioned to generate a virtual machine using the MC-generated beam data. Intensity modulated radiation therapy (IMRT) plans for six clinical cases were generated using conventional 6 MV, 6 MV flattening filter free, and energy-modulated photon beams of the virtual machine.

RESULTS

As increasing the thickness of mercury, Percentage depth doses (PDD) of modulated 6 and 10 MV after the depth of dose maximum were continuously increased. The amount of PDD increase at the depth of 10 and 20 cm for modulated 6 MV was 4.8% and 5.2% at FS5, 3.9% and 5.0% at FS10 and 3.2%-4.9% at FS20 as increasing the thickness of mercury from 0 to 20 mm. The same for modulated 10 MV was 4.5% and 5.0% at FS5, 3.8% and 4.7% at FS10 and 4.1% and 4.8% at FS20 as increasing the thickness of mercury from 0 to 25 mm. The outputs of modulated 6 MV with 20 mm mercury and of modulated 10 MV with 25 mm mercury were reduced into 30%, and 56% of conventional linac, respectively. The energy-modulated IMRT plans had less integral doses than 6 MV IMRT or 6 MV flattening filter free plans for tumors located in the periphery while maintaining the similar quality of target coverage, homogeneity, and conformity.

CONCLUSIONS

The MC study for the designed energy modulator demonstrated the feasibility of energy-modulated photon beams available during beam-on time. The planning study showed an advantage of energy-and intensity modulated radiotherapy in terms of integral dose without sacrificing any quality of IMRT plan.

The dosimetric effect of mixed-energy IMRT plans for prostate cancer

We investigated the effect of mixing high‐ and low‐energy photon beams on the quality of intensity‐modulated radiation therapy (IMRT) plans for patients with prostate cancer. Three different plans for each of twenty patients were generated using either 6 MV or 15 MV alone, and both 6 and 15 MV beams. All the planning parameters, goals, and constraints were set to be identical except beam energy. The dose distributions were similar in terms of target coverage, conformity, and homogeneity regardless of beam energy. The V70Gy of rectal wall in 6 MV, 15 MV and mixed‐energy plans was 16.7%, 17.9%, and 16.3%, respectively, while V40Gy was 55.6%, 53.2%, and 50%. The mean dose to femoral heads in 6 MV, 15 MV, and mixed‐energy plans were 31.7 Gy, 26.3 Gy, and 26.2 Gy, respectively. The integral dose of 6 MV plans was 7% larger than those of 15 MV or mixed‐energy plans. These results indicated that mixed‐energy IMRT plans could take advantage of the dosimetric characteristics of low‐ and high‐energy beams. Even though the reduction of dose to the organs at risk may not be clinically relevant, mixing energy in an IMRT plan for deep‐seated tumors can improve the overall plan quality.

PACS number: 87.55.ne

Influence of photon energy on the quality of prostate intensity modulated radiation therapy plans based on analysis of physical indices

The goal of the present study was to study the effects of low- and high-energy intensity-modulated photon beams on the planning of target volume and the critical organs in cases of localized prostate tumors in a cohort of 8 patients. To ensure that the difference between the plans is due to energy alone, all other parameters were kept constant. A mean dose volume histogram (DVH) for each value of energy and for each contoured structure was created and was considered as completely representative for all patients. To facilitate comparison between 6-MV and 15-MV beams, the DVH-s were normalized. The different parameters that were compared for 6-MV and 15-MV beams included mean DVH, different homogeneity indices, conformity index, etc. Analysis of several indices depicts more homogeneous dose for 15-MV beam and more conformity for 6-MV beam. Comparison of all these parameters showed that there was little difference between the 6-MV and 15-MV beams. For rectum, 2 to 4 % more volume received high dose with the 6-MV beam in comparison with the 15-MV beam, which was not clinically significant, since in practice much tighter constraints are maintained, such that Normal Tissue Complication Probability (NTCP) is kept within 5 %. Such tighter constraints might increase the dose to other regions and other critical organs but are unlikely to increase their complication probabilities. Hence the slight advantages of 15-MV beam in providing benefits of better normal-tissue sparing and better coverage cannot be considered to outweigh its well-known risk of non-negligible neutron production.

A dosimetric analysis of 6 MV versus 15 MV photon energy plans for intensity modulated radiation therapy (IMRT) of carcinoma of cervix

BACKGROUND

Intensity modulated radiotherapy (IMRT) is being used to treat carcinoma of cervix (Ca Cx). Integral dose to normal tissue and increased leakage are the concern about IMRT. 6 MV photon beam is a good choice of energy for Ca Cx IMRT treatment.

AIM

The objective of this study was to compare intensity modulated radiotherapy (IMRT) plans generated by 6 MV and 15 MV photon energies for carcinoma of cervix (Ca Cx) with regards to dosimetric parameters of planning target volume (PTV) and organs at risk (OAR), homogeneity index (HI), conformity index at 98% level (CI 98%), integral dose to normal tissue (NTID) and total number of monitor units (MUs).

MATERIAL AND METHODS

A cohort of 16 patients was selected for this study. All patients were to receive a dose of 50 Gy in 25 fractions. IMRT plans were generated for both energies using same dose-volume constraints.

RESULTS

Our results show a comparable coverage of planning target volume (PTV) for both energies. Volume of PTV receiving a prescription dose is 97.8 ± 0.5% and 98.8 ± 0.4% for the 6 MV and the 15 MV plans. Volume of PTV receiving a dose of 107% is 4.4 ± 7.8% and 16.1 ± 22.2%. Bladder and rectum mean doses for the 6 MV and the 15 MV photon plans were 39.8 ± 3.0 Gy and 40.0 ± 3.2 Gy, and 35.8 ± 3.1 Gy and 36.0 ± 3.1 Gy, respectively. Homogeneity index (HI) for both energies was 1.04. The conformity indices at 98% isodose (CI 98%) were 1.3 ± 0.1 and 1.4 ± 0.1 for 6 MV and 15 MV photon plans, respectively.

CONCLUSIONS

We conclude that a 6 MV photon is a good choice for Ca Cx IMRT as it produces a highly conformal, homogeneous plan with superior target coverage and better OAR sparing.

Comparison of intensity-modulated radiotherapy and forward-planning dynamic arc therapy techniques for prostate cancer

We compare an inverse‐planning intensity‐modulated radiotherapy (IMRT) technique with three previously published forward‐planning dynamic arc therapy techniques and a newly implemented technique for treatment of prostate only. The three previously published dynamic arc techniques are dynamic arc therapy (DAT), two‐axis dynamic arc therapy (2A‐DAT), and modified dynamic arc therapy (M‐DAT). The newly implemented technique is the bilateral wedged dynamic arc (BW‐DAT). In all dynamic arcs, the multileaf collimator is moving during rotation to fit the prostate, except that, in 2A‐DAT, it is fitting two separate symmetrical rhombi including the prostate. The rectum is shielded during rotation only in the cases of M‐DAT and BW‐DAT.

The results obtained indicate that the BW‐DAT, M‐DAT, and DAT techniques provide the intended dose coverage of the prescribed dose to the planning target volume (PTV)—that is, 95% of the PTV is covered by 100% of the dose. The maximum dose to a 3‐cm margin of healthy tissue that surrounds the PTV is lower by 2.5% in the case of IMRT than in both BW‐DAT and M‐DAT, but it is lower by 5.0% than that in both DAT and 2A‐DAT. The maximum dose to the rest of the healthy tissue in the case of BW‐DAT is 33.2Gy±2.2Gy. This dose covers percentage healthy body volumes of 8%±3.2% with IMRT, 4%±1.5% with DAT, and 6%±1.2% with both 2A‐DAT and M‐DAT. Also, this dose is much lower than the accepted maximum dose (52 Gy) to the femoral heads and necks according to Report 62 from the International Commission on Radiation Units and Measurements. Accordingly, it would be possible to neglect delineation of the femoral heads and necks as organs at risk in cases of BW‐DAT.

Doses to 15%, 25%, 35%, and 50% (D15%, D25%, D35%, and D50%) of the rectum volume in the case of BW‐DAT were 43.5Gy±8.6Gy, 24.2Gy±8.7Gy, 13.2Gy±4.2Gy, and 5.7Gy±2.1Gy respectively. The D15% of rectum in the case of IMRT was lower than that in BW‐DAT, M‐DAT, 2A‐DAT, and DAT by 7.3%, 10.3%, 33.0%, and 17.6% of the prescribed dose (78 Gy in 39 fractions) respectively. The D25%, D35%, and D50% of the rectum volume in the cases of IMRT and DAT were comparable (with a maximum variation of 4.5%); they were similarly comparable in the cases of M‐DAT and BW‐DAT (with maximum variation of 1.5%). These same doses in BW‐DAT were lower than those in IMRT by 8.7%, 10.6%, and 6.2% respectively, but they were quite lower than those in 2A‐DAT, because the average variation was 41.6% (with a maximum of 44.0%).

The D15%, D25%, D35%, and D50% of the bladder volume in the case of BW‐DAT were 33.2Gy±10.9Gy, 17.4Gy±7.9Gy, 6.5Gy±4.3Gy, and 4.2Gy±3.5Gy respectively. The D15% and D25% of the bladder in the cases of IMRT, M‐DAT, and BW‐DAT were comparable (with a maximum variation of 2.2% and 3.6% respectively), and the mean values of each dose were lower in DAT by 14.3% and 11.7% respectively. However, the values of D35% and D50% in the four techniques were comparable, with maximum variations of 5.1% and 2.7% respectively. The D15%, D25%, D35%, and D50% of the bladder in the case of DAT were lower than those in 2A‐DAT by 20.1%, 26.9%, 16.0%, and 2.7% respectively.

Ion chamber measurements showed good agreement between the calculated and measured isocentric doses (maximum deviation: 3.2%). Accuracy of the dose distribution calculation for BW‐DAT was evaluated by film dosimetry using a gamma index, allowing 3% dose variation and 3 mm distance to agreement as the individual acceptance criteria. We found that fewer than 6.5% of the pixels in the dose distributions of the scanned and calculated area of 10×10 cm failed the acceptance criteria.

We conclude that, in addition to simplicity of the dose calculation, the BW‐DAT technique provides the intended concave dose distribution for treatment of the prostate only. Compared with IMRT, it produces better dose protection to the most of the rectum volume and to the healthy tissue outside the treatment volume. Also, as compared with the other forward planning dynamic arc techniques, it gives the most favorable isodose distributions to the prostate and rectum.

PACS number: 87.53.Tf

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.