Direction Modulated Brachytherapy Tandem Model Applicators for Treatment Planning of Multi-Institutional Cervical Cancer Cases

PURPOSE/OBJECTIVE(S)

Direction Modulated Brachytherapy (DMBT) offers a means of utilizing an anisotropic source to create more conformal dose distributions when integrated with image-guided adaptive brachytherapy (IGABT). Authors sought to validate the implementation of nine unique six-channeled, MRI-compatible, novel DMBT tandem applicators of varying physical dimensions within Varian's BrachyVision® (v16.1) treatment planning system (BV-TPS).

MATERIALS/METHODS

A total of 110 retrospective clinically delivered high-dose-rate (HDR) cervical cancer brachytherapy plans, from three institutions, were re-planned for each of the nine DMBT tandem models within the BV-TPS, using the latest VEGO® inverse optimization algorithm, with dose heterogeneity accounted for through AcurosBV®. Plans consisted of both intracavitary (77 plans) and interstitial (33 plans) cases with an average prescription dose and high-risk clinical target volumes (CTVHR) of 607±113 cGy and 26.96±14.95 [range 6.70-69.58] cm3, respectively. During re-planning, the conventional tandems were replaced by one of the nine DMBT tandem models while leaving ovoids or rings, and needles (if present), in place. A two-step inverse optimization process was performed such that the lowest possible organs at risk (OAR) D2cc doses could be achieved while 1) keeping equivalent target coverage (ΔCTVHR-D90 to within ±0.5%) and, at the same time, 2) maintaining the general pear-shape dose distribution of the original plans.

RESULTS

Noteworthy improvements in plan quality were achieved by all nine DMBT tandem models, which are presented in Table 1. Irrespective of the model, about ∼50 cGy reduction in D2cc across all OARs appear feasible. There is also a general trend of D2cc reductions' magnitude becoming smaller as the CTVHR volume increased due to loss in modulation at distance. Additionally, D2cc reductions in terms of EQD2 [Gy] were calculated assuming each re-plan was delivered throughout the course of treatment, which includes the external beam radiotherapy dose of 45 Gy and showed significant reductions of -6.29±4.38 Gy, -3.80±2.06 Gy, and -4.86±3.02 Gy for the bladder, rectum, and sigmoid, respectively, for DMBT model #9 for example.

CONCLUSION

We have successfully incorporated nine DMBT tandem models into a commercial TPS and re-planned 110 cases, to a total of 990 plans. All nine DMBT tandem models were each able to generate notable D2cc reductions to OARs (∼50 cGy), without compromising target coverage, across plans from multiple institutions with various clinical/optimization practices. The results indicate both a promising impact and smooth integration of DMBT tandem technology into modern clinical IGABT workflow.

Direction Modulated Brachytherapy Tandem Model Applicators for Treatment Planning of Multi-Institutional Cervical Cancer Cases: Removing Needles in Intracavitary-Interstitial Techniques

PURPOSE/OBJECTIVE(S)

To evaluate the potential of nine direction modulated brachytherapy (DMBT) tandem applicator models of various designs to obviate the need for needles during intracavitary-interstitial (IC-IS) cervical cancer brachytherapy.

MATERIALS/METHODS

A cohort of 33 retrospective clinical high dose-rate (HDR) brachytherapy plans, from three institutions, were re-planned with Varian's BrachyVision® (v16.1) treatment planning system (BV-TPS), using the latest VEGO® inverse optimization algorithm, with dose heterogeneity accounted for through the AcurosBV® model-based dose calculation algorithm. All plans consisted of IC-IS cases, with a range of 2-4 freehand-loaded needles, with an average prescription dose of 706±54 cGy and average high-risk clinical target volume (HRCTV) of 36.0±17.4 [range 9.8-69.6] cm3. Nine novel DMBT tandem models of varying physical dimensions were integrated for the first time into the BV-TPS, with thicknesses (4-8 mm). During re-planning, the conventional tandems and all of the needles were replaced by one of the nine DMBT tandem models while leaving the ovoids/rings in place. An optimization process was performed such that the lowest possible organs at risk (OAR) D2cc doses could be achieved while keeping equivalent target coverage (ΔHRCTV-D90 to within ±0.5%) and maintaining a pear-shape dose distribution. The process was repeated for each of the nine DMBT tandem models resulting in (33 × 9 =) 297 plans.

RESULTS

Average ΔHRCTV-D90 was +0.35±0.39% (+2.8±3.1 cGy). OAR D2cc reductions were achieved by all models for all plans. The performance of the thickest DMBT model (8 mm) was the best in terms of achieving the lowest D2cc for all OARs, with 31 out of 33 plans (94%) achieving lower D2cc doses for all three OARs. The two cases in which the D2cc doses could not be reduced had HRCTV volumes ranging between 50 cm3 and 60 cm3. Additionally, D2cc reductions in terms of EQD2 [Gy] were calculated assuming each re-plan was delivered throughout the entire course of a patient's treatment, which included the external beam radiotherapy dose of 45 Gy, showed significant reductions of -2.64±2.67 Gy, -1.65±1.97 Gy, and -2.80±2.20 Gy for bladder, rectum, and sigmoid, respectively.

CONCLUSION

According to the results, it is clinically feasible to replace the conventional IC-IS cases, with 2-4 freehand-loaded needles, with the DMBT tandem technology, effectively avoiding the need for needle involvement.

Combining Novel Direction Modulated Brachytherapy Tandem-and-Ovoids Applicators for Treatment Planning of Multi-Institutional Cervical Cancer Cases: Removing Needles in Intracavitary-Interstitial Techniques

PURPOSE/OBJECTIVE(S)

Lack of standard guidelines for optimal needle insertion during high-dose-rate (HDR) intracavitary-interstitial (IC-IS) brachytherapy of the cervix means a sophisticated and technical skillset of inserting needles next to IC applicators must be developed to enhance plan quality. This study sought to evaluate the performance of two separate direction modulated brachytherapy (DMBT) tandem applicators used in conjunction with one set of novel DMBT ovoids, uniquely designed to effectively obviate the need for IS needles.

MATERIALS/METHODS

A cohort of 32 retrospective clinical HDR brachytherapy plans, from three institutions, were re-planned with Varian's BrachyVision® (v16.1) treatment planning system (BV-TPS), using the latest VEGO® inverse optimization algorithm, with dose heterogeneity accounted for through the AcurosBV®. All plans consisted of IC-IS cases, with a range of 2-4 freehand-loaded needles, with an average prescription dose of 709±53 cGy, and with an average high-risk clinical target volume (HRCTV) of 36.73±17.15 [range 9.8-69.6] cm3. Two DMBT tandem models of 5.4-mm and 8.0-mm thicknesses along with a novel DMBT ovoids design, introduced for the first time, with 9 equi-angled grooves and 10-mm-diameter thickness. During re-planning, the conventional tandems, ovoids/rings, and all of the needles were replaced by one of the two DMBT tandem models and a set of DMBT ovoids. A two-step inverse optimization process was performed to achieve the lowest possible OAR D2cc doses while 1) keeping equivalent target coverage (ΔHRCTV-D90 to within ±0.5%) and 2) maintaining the general pear-shape dose distribution used by the original plans. For all plans, this process was repeated using each of the two DMBT tandem-and-ovoids combinations for a total re-planning of (32×2 =) 64 cases.

RESULTS

On average, -47.15±29.61 (-40.40±34.90) cGy, -42.98±26.58 (-41.70±27.40) cGy, and -40.47±25.05 (-32.55±25.30) cGy reductions in D2cc across bladder, rectum, and sigmoid, respectively, were achieved for the 8-mm (5.4-mm) DMBT tandem-and-ovoids combinations while the average ΔHRCTV-D90 was +4.3±2.9 cGy (+0.5%±0.4%). Additionally, D2cc reductions in terms of EQD2 [Gy] were calculated and showed significant reductions of -4.05±2.47 (-3.37±2.83) Gy, -2.71±1.79 (-2.59±1.74) Gy, and -3.27±1.96 (-2.65±2.06) Gy for bladder, rectum, and sigmoid, respectively with an average net increase in total dwell times of 241.0±87.6 seconds at the luxury of avoiding IS needle insertions.

CONCLUSION

It is clinically feasible to obviate the need for IS needles by incorporating the DMBT tandem-and-ovoids while producing lower OAR D2cc doses and maintaining equivalent target coverage.

Technical Report: Evaluation of peripheral dose for flattening filter free photon beams

PURPOSE

To develop a comprehensive peripheral dose (PD) dataset for the two unflattened beams of nominal energy 6 and 10 MV for use in clinical care.

METHODS

Measurements were made in a 40 × 120 × 20 cm(3) (width × length × depth) stack of solid water using an ionization chamber at varying depths (dmax, 5, and 10 cm), field sizes (3 × 3 to 30 × 30 cm(2)), and distances from the field edge (5-40 cm). The effects of the multileaf collimator (MLC) and collimator rotation were also evaluated for a 10 × 10 cm(2) field. Using the same phantom geometry, the accuracy of the analytic anisotropic algorithm (AAA) and Acuros dose calculation algorithm was assessed and compared to the measured values.

RESULTS

The PDs for both the 6 flattening filter free (FFF) and 10 FFF photon beams were found to decrease with increasing distance from the radiation field edge and the decreasing field size. The measured PD was observed to be higher for the 6 FFF than for the 10 FFF for all field sizes and depths. The impact of collimator rotation was not found to be clinically significant when used in conjunction with MLCs. AAA and Acuros algorithms both underestimated the PD with average errors of -13.6% and -7.8%, respectively, for all field sizes and depths at distances of 5 and 10 cm from the field edge, but the average error was found to increase to nearly -69% at greater distances.

CONCLUSIONS

Given the known inaccuracies of peripheral dose calculations, this comprehensive dataset can be used to estimate the out-of-field dose to regions of interest such as organs at risk, electronic implantable devices, and a fetus. While the impact of collimator rotation was not found to significantly decrease PD when used in conjunction with MLCs, results are expected to be machine model and beam energy dependent. It is not recommended to use a treatment planning system to estimate PD due to the underestimation of the out-of-field dose and the inability to calculate dose at extended distances due to the limits of the dose calculation matrix.

Utilizing an electronic portal imaging device to monitor light and radiation field congruence

A method to investigate light and radiation field congruence utilizing a commercially available amorphous silicon electronic portal imaging device (EPID) was developed. This method employed an EPID, the associated EPI software, and a diamond‐shaped template. The template was constructed from a block tray in which Sn/Pb wires, 1 mm in diameter, were embedded into a diamond shaped groove milled into the tray. The collimator jaws of the linac were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertexes of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on the films and EPIs. Three linear accelerators were tracked for a period ranging from 2–8 months. Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 0.6 mm, well within the allowed 2‐mm tolerance. A disparity was observed in the magnitude of the detected light field. The X and Y dimensions of the light field measured with film differed by less than or equal to 1.4 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 2.5 mm. Based on these findings, EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses.

PACS number(s): 87.53.Xd, 87.56.Fc, 87.53.Oq, 87.52.–g, 87.53.–j

First On-line beta-NMR on oriented nuclei: magnetic dipole moments of the (nup(1/2))(-1) 1/2(-) ground state in 67Ni and (pip(3/2))(+1) 3/2(-) ground state in 69Cu

The first fully on-line use of the angular distribution of beta emission in detection of NMR of nuclei oriented at low temperatures is reported. The magnetic moments of the single valence particle, intermediate mass, isotopes 67Ni(nup(-1)(1/2);1/2(-)) and 69Cu(pip(1)(3/2);3/2(-)) are measured to be +0.601(5) &mgr;(N) and +2.84(1) &mgr;(N), respectively, revealing only a small deviation from the neutron p(1/2) single-particle value in the former and a large deviation from the proton p(3/2) single-particle value in the latter. Quantitative interpretation is given in terms of core polarization and meson-exchange currents.