In vitro quality assurance of three-dimensional conformal radiotherapy mono-isocentric plan for simultaneous treatment of two targets

Introduction

Solid tumors often present with multiple bone metastases necessitate the simultaneous radiotherapeutic treatment of the targets. Such treatments are executed using modern radiotherapy techniques (e.g., Intensity-modulated Radiotherapy, stereotactic body radiation therapy, Rapid Arc, Cyberknife^® & Tomotherapy) where the treatment planning is carried out with treatment planning systems having Inverse planning algorithms. In convention, multiple targets can be treated with beam sets having their isocenters. This results in more scatter dose to the surrounding normal tissue and prolongation of treatment time (starting from quality assurance, patient positioning, setup corrections, and treatment delivery). A simpler method is to have a common isocenter, around which the gantry rotates and delivers the radiation to the multiple target sites. This method can considerably reduce the scatter dose to the surrounding normal tissues and reduce the time of pre-treatment plan quality assurance and execution of that plan significantly. Many investigators have verified the dosimetric quality of a common isocentric plan to treat multiple tumors, especially in brain metastatic lesions. Abdel-Hakim et al. have reviewed the merits, limitations, and recent approaches to optimize match line dose in monoisocentric technique in conventional and intensity-modulated radiotherapy for head and neck cancers.[1] Potter et al., have investigated the treatment of multiple brain tumors with 4 bank micro multileaf collimators and have found that the single isocentric was as good as the multiple isocentric plan dosimetrically, which can reduce a considerable amount of time in quality assurance and treatment.[2] Wadasadawala et al. reported that the monoisocentric technique resulted in better dose homogeneity at the field junctions and reduced mean heart dose as compared to the dual isocentric technique in the clinical practice for breast cancer treatment.[3] Luxton et al., in their study have treated non-spherical targets in the brain with a single isocenter and achieved good dose conformity.[4] Clark et al., have verified the feasibility of a single isocenter in Volume modulated arc therapy radiosurgery, for the treatment of multiple brain metastasis and concluded it as a better method.[5] Huang et al., have compared the quality of target coverage and dose conformity with single isocentric volume modulated arc therapy- stereotactic radiosurgery plans to dynamic conformal arc therapy and concluded that the latter may result in larger low dose regions.[6] VanderSpek et al.; Ebert et al.; Shtraus et al.; have extensively studied the validity of single isocentric plans and concluded it to be a better option in saving pre-treatment quality assurance time as well as treatment delivery time with less systematic errors.[7-9] Marks et al., have confirmed through their investigation that three-dimensional conformal radiotherapy can be a possible alternative to radiosurgery with fixed shaped coplanar or non-coplanar wedged radiation fields having individually shaped beams conformed to irregularly shaped intracranial lesions, as the goal of both the techniques is to achieve better dose conformity.[10-12] A similar kind of logic can be used to treat multiple lesions simultaneously with different/single beam sets confirmed using three-dimensional radiation therapy to different lesion sites elsewhere extracrainally which can yield less low dose bath to normal tissue, save pre-treatment quality assurance time and treatment time appreciably. Planning these techniques with a three-dimensional radiation therapy treatment planning system requires a highly logistic approach using different beam sets conformed to multiple lesions sharing a common isocenter, having different weight points, the feasibility/flexibility to use different wedge angles, to obtain a better conformal dose coverage.[13]

Materials and Methods

A primary known case of carcinoma right lung with bone metastasis at two different locations (two femoral heads and shafts) was planned using a common isocenter. Planning of this kind is based on the three-dimensional radiation therapy technique possible with a linear accelerator, having a dose rate of greater than 400 MU/min, with 40 pair tungsten multileaf collimator leaves in two banks and motorized wedge, with superposition algorithm.

Treatment planning with common isocenter

To create a plan with good conformity and 95% isodose cloud at two distant target levels a combined clinical treatment volume (CTV) structure with a 5 mm margin was created. Beams with common isocenter were then conformed to the two different targets, two pairs of beams were directed anterio-posteriorly, covering the right femur as clinical target volume 1 (CTV1) and left femur as clinical target volume 2 (CTV2), and another pair bilaterally directed to cover both the targets simultaneously, completely sparing the organs in between the two targets. Additional sub-beams with weight points at different desired locations, within the target were created to get dose uniformity inside the target. The isodose curves and dose-volume histograms were analyzed to check the extent of dose overlapping of the two targets, as both the targets were aimed to get 300cGy per fraction. Figure 1 shows the isodose distribution for the treatment planning with the monoisocenteric technique.

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Figure 1

Treatment planning with common isocenter using 3DCRT technique

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Composite dosimetric verification

For the point dose verification (Composite dosimetry) of this plan same patient geometry was simulated by a phantom comprising of two identical tissue phantoms placed side by side, having surface fiducials, with two ionization chambers simulating the targets, and a planning computed tomography scan of slice thickness 3 mm was acquired as shown in Figure 2. The serially scanned images were transferred to the contouring station, and the external contouring was performed, the two chambers simulating the two targets were contoured with a margin of 2 mm around them and named ionization chamber 1 and ionization chamber 2. The contoured Dicom data set was then transferred to the planning system. For treatment planning, ionization chamber 1 and ionization chamber 2 were combined to generate ionization chamber 3 with a 5 mm margin. The reference point was assigned as a common isocenter and the same beam template as planned for the actual patient was imported on this phantom simulating the actual patient. The beams were conformed to the ionization chamber 1 and ionization chamber 2 with a margin of 2 cm around the targets as shown in Figure 3. The isodose curves and dose statistics were analyzed to check the dose uniformity and the extent of dose overlapping of the two targets, for 300cGy/#. The plan was scheduled and executed with the dosimeters simulating the targets. An online record of cumulative charge from both the ionization chambers was recorded which was later converted to absorbed dose to the targets using International Atomic Energy Agency-Technical Report Series 398.

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Figure 2

Computed tomography simulation of the phantom with ionization chambers and its DRR view

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Figure 3

Phantom treatment planning and its execution at the linear accelerator

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2D fluence verification

The same plan template used in the phantom study was executed on a 2D ion chamber array detector (I’mRTMatriXX) for 2D fluence verification using solid phantom-34 30 cm × 30 cm × 30 cm with gantry different angles, as shown in Figure 4.

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Figure 4

Application of the treatment plan on IMatriXX and its execution

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Results

The electrometer average meter reading (M) in nanocoloumbs(nC) obtained from composite dosimetry with two 0.6 cc Farmer type Ionization Chambers (ionization chamber 1 and ionization chamber 2) are shown in Table 1. The final dose measurements were obtained using N_DW based formalism using International Atomic Energy Agency-Technical Report Series 398.[14,15] Table 2 displays the percent deviation between the measured (point dosimetry) and treatment planning system calculated dose. The minimum, maximum, and mean dose in centiGray (cGy) covered by the ionization chamber 1 and ionization chamber 2 volumes were 295, 307 and 303 as per the dose statistics from the generated treatment plan. The global dose max obtained from the plan was 307.4 cGy. Measured point doses to both lesions were within ±2%. Figure 5, shows the validity of the gamma index criterion, within 3%, 3 mm (Dose difference and distance to the agreement) on fluence verification.

Table 1 Cumulative charge collected (*nC) Dose Measurements for IC-1 and IC-2 using point dosimetry

Table 2 Percentage deviation from the calculated dose, by composite point dosimetry

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Figure 5

3% Dose difference, 3 mm distance to agreement, validity by two dimensional fluence verifications

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Discussion

The radiotherapy treatment planning for the treatment of multiple targets usually results in increased scatter dose, that is, increased low dose bath to the surrounding normal tissues if the different targets are planned individually with multiple isocenters for separate targets as described by the international commission on radiation units and measurements- ICRU. It also results in overall increased treatment planning time, patient setuptime as well as plan execution time. In this study, treatment planning of two targets in the pelvic region with a common isocenter was verified dosimetrically and the results obtained were well within the tolerance doses as prescribed by the international commission on radiation units and measurements- ICRU. Earlier a similar kind of attempt has already been made by several investigators for the treatment of multiple lesions/targets especially intracranial using highly sophisticated state of art of radiotherapy with special computerized algorithms based on inverse planning.[16-18] Another group of investigators has tried to achieve the same target using conformal techniques to treat multiple intracranial lesions with coplanar and non-coplanar beams. Keeping the ultimate goal of radiotherapeutic treatment in view, that is, to achieve maximum target dose coverage (95–107% isodose coverage to Target Volume) and conformity with maximum possible normal tissue sparing, the idea of treating two/multiple targets (intra or extracranial) simultaneously having a common isocenter is always appreciable.[19] It usually offers a low dose bath to normal surrounding tissues, with the least possible time consumed in pre-treatment quality assurance. The treatment delivery time also decreases appreciably and therefore the setup errors also are reduced.[20,21] On the contrary, the treatment plans with multiple isocenters are time-consuming, error-prone, and may attribute many uncertainties in setup and positioning ending up with many systematic errors in the treatment delivery.

Conclusions

The results obtained for the composite point dosimetry and the two-dimensional fluence verification during the plan execution, evaluation, and analysis were in agreement with the treatment planning system calculated dose. Thereby validating the technique using monoisocenter to treat multiple targets simultaneously. This technique had resulted in decreasing the scatted dose appreciably. It also helped to save the beam on time as well as the whole radiotherapeutic procedure. Monoisocentric technique has proved dosimetrically as well as radiobiologically better than multiple isocenters. In conclusion, we would recommend the monoisocentric plans to be planned and executed wherever and whenever possible to treat multiple targets within the same patients to save time and achieve better tumor control.

Authors’ Declaration Statements

Authors’ Contributions

Misba Hamid Baba: Worked during the experimental process as well as prepared the manuscript; Benoy Kumar Singh: Provided overall guidance in the preparation of manuscript.

ORCID of submitting author: 0000-0001-7994-5004