Evaluation of HyperArc™ using film and portal dosimetry quality assurance

HyperArc™ is a stereotactic radiotherapy modality designed for targeting multiple brain metastases using a single isocenter with multiple non-coplanar arcs. This study aimed to assess the efficacy of two patient-specific quality assurance methods, film and the Varian Portal Dosimetry System with Varian’s HyperArc™ technique and raise important considerations in the customisation of patient-specific quality assurance to accommodate HyperArc™ delivery. Assessment criteria included gamma analysis and mean dose at full width half maximum. The minimum metastasis size, maximum off-axis distance and suitable energy were identified and validated. Patient-specific quality assurance procedures were applied to a range of clinically relevant brain metastasis plans. Initial investigation into energy selection showed no significant differences in gamma pass rates using 6MV, 6MV FFF, or 10MV FFF for metastasis sizes greater than 15 mm diameter at the isocenter. Gamma pass rates (2%/2mm) for 15 mm metastases at the isocenter for all energies were greater than 96.0% for portal dosimetry and greater than 98.7% for film. Fields of size 15 mm placed at various distances (10–70 mm) from the isocenter resulted in a maximum mean dose difference of 1.5% between film and planned. Clinically relevant plans resulted in a maximum mean dose difference for selected metastases of 1.0% between film and plan and a maximum point dose difference of 2.9% between portal dose and plan. Portal dose image prediction was a quick and convenient quality assurance tool for metastases larger than 15 mm near the isocenter but provided diminished geometrical relevance for off-axis metastases. Film QA required exacting procedures but offered the ability to assess the accuracy of geometrical targeting for off-axis metastases and provided dosimetric accuracy for metastases to well below 15 mm diameter.


Introduction
HyperArc™ has provided a single isocentric technique for delivering radiation to multiple brain metastases (BM). One of the driving forces behind this emerging technology is the increase in patients presenting with BM. Demographic studies have shown [1] that up to 8.5% of patients developed BM within 5 years of primary diagnosis. The number of patients presenting with BM increased due to extended life based on curative primary treatment. Treatments for BM [2] currently range from surgery, chemotherapy, use of steroids, and radiation therapy [3,4]. Whole brain radiation therapy was one of the earliest applications of radiation treatment for multiple intracranial tumours and prophylaxis for microscopic disease; however, sparing radiosensitive intracerebral organs at risk was met with limited success [5]. The preferred radiosurgery modality for small BM has been led by Gamma Knife [6] but the modality and the cost have not been widely available to the general public [7], with only a limited number of Gamma Knife machines in Australia [8]. External beam radiation with linacs is becoming more commonly available, with an increasing proportion commissioned for stereotactic radiation therapy (SRT) techniques for single to oligometastatic BM [9]. The number of metastases able to be successfully treated with a single isocenter technique is limited by the complexity of the treatment planning system (TPS) and the machine's ability to deliver the intended technique.

3
HyperArc™ offers a potential solution to the complexities facing multiple BM treatments [4]. It is a stereotactic technique available with TrueBeam™ version 2.7 or higher, offering an optimised single isocenter, MLC-based, noncoplanar multiple target volumes (TV) planning solution. It also provides a clinically lean delivery time for patient comfort and can conform radiation to multiple BM (up to 13 metastases in this project) while sparing organs at risk. This stereotactic radiation technique is an optimised and automated form of volumetric modulated arc therapy with Eclipse's ability to plan MLC conformality to multiple targets over at least one couch kick. The patient is immobilised in a dedicated frameless fixation device by QFix's Encom-pass™ support [10], which is attached to a PerfectPitch™ six degrees of freedom couch (6DoF) [11]. The thermoplastic material provides cranial immobilisation and radiopaque markers used by the TPS during the planning process.
Comprehensive quality assurance (QA) for SRT requires a comprehensive procedure to minimise errors and uncertainty [12] due to the nature of the hypofractionated deliveries to small target volumes with rapid dose fall-off [13]. Intracerebral lesions vary in size, with many presenting diameters less than 10 mm. The small targets raise small field dosimetry challenges. The chamber ionisation measurement is the gold standard for point dose measurements. Still, it is unsuitable for many of these small-sized metastases due to compromised volume averaging of the chamber, lateral charged particle disequilibrium and potential source occlusion [14].
Film offers good dose accuracy and superior resolution, near water equivalence, excellent uniformity, and minimal energy dependence and its use is easily integrated by most phantoms [15]. Film is considered the gold standard for high-resolution dosimetry [15] but is avoided for patientspecific QA in favour of quicker and more convenient alternatives. The inclusion of Varian's portal dose image prediction (PDIP) with patient-specific QA has already been widely used for techniques such as intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) for single metastases [16].
Portal dosimetry has emerged as an extension of the electronic portal imaging device's (EPID) initial imaging intent. In essence, the image data collected by the EPID is calibrated relative to a unit (CU) under calibration conditions of machine output to provide dose equivalence [17]. The collected data were compared to predicted data attained through a modelled algorithm. The EPID solution for dosimetric checks is popular due to its already standard availability for imaging, minimal set-up, immediate 2D high-resolution images and seamless software integration to provide quick analysis. Varian's PDIP solution is already widely used for VMAT patient-specific QA for single lesion sites [16]. The 2D reconstruction model [18] is based on data from the EPID, which is non-inclusive of couch/couch kicks and phantom involvement. This paper assesses the efficacy of these patient-specific QA methods for HyperArc™ and highlights important considerations in customisation.

Methods
Portal Dosimetry (Varian Medical Systems, Palo Alto, USA) was this project's integrated patient-specific QA system. It was used to acquire images of the measured fluence delivered from a Varian TrueBeam Platform version 2.7, while the Eclipse™ (v. 15.6.03, Varian Inc.) planning system was used to generate a modelled dosimetric predicted image. The verification plan was delivered directly to the EPID with the detector's imaging plane at the isocenter. Dosimetric adaptation of the portal imager involved calibrating the acquired fluence to a CU to provide a form of absolute dose equivalence. The calibrated data was then compared to the predicted data attained through a modelled algorithm. The portal dose prediction calculation used the Portal Dose Image Prediction (PDIP) algorithm. Various QA tools for gamma analysis and point/profile dose were available with Aria RTM 16.1 Portal Dosimetry to determine plan validity [18]. Portal dosimetry was used on a Varian TrueBeam™ linear accelerator with an aS1200-II EPID panel. Suitability of the panel for the HyperArc™ technique with flattening filter-free (FFF) energy, lesion size and high number of monitor units were initially checked by its specifications [18,19]. The detector panel had an active area of 40.0 × 40.0 cm 2 with a pixel matrix of 1024 × 1024 pixels attaining a resolution of 2.56 px/mm with the maximum dose rates for 10 MV FFF within the detector signal saturation limits [18]. The EPID detector was calibrated to 1 CU to correspond to 100 MU for a 10 × 10 cm 2 field with an SDD of 100 cm for each energy. Constancy checks for output and uniformity were performed daily for each energy as per AAPM TG-142 [18] report recommendation. Varian's Machine Performance Check [20] was used for these daily checks and verified beam constancy for PDIP verification plans.
The patient-specific QA procedure for portal dosimetry required a verification plan to be created from the patient plan. Test cases were planned with the TPS algorithm Acuros (v 15.6.03), from which PDIP (v 15.6.03) verification plans were generated. The EPID had been commissioned for PDIP with checks covering variable dose rates, linearity, ghosting, output factors and intensity profile accuracy.
Film QA used GafChromic EBT3 and EBT-XD, with the latter used where the dose per fraction exceeded 10 Gy [21]. Films were exposed using LAP's Easy Cube [22], where the modular inserts made it a useful tool for stereotactic dosimetry. The CIRS 30 × 30 × 30 cm 3 slabs (Model PW, CIRS Inc, Norfolk, VA, USA) were initially trialled since they are commonly available in most physics departments and water equivalent; however, due to their size excluded the use of the Encompass™ couch. The Easy Cube's smaller size enabled it to be placed in the Encompass™ couch. Its 18 cm-sided cubic volume was large enough to encompass the geometric range of intracranial metastases. The Easy Cube's polystyrene RW3 composition was not water equivalent; however, the polystyrene composition was taken into account by the Acuros algorithm along with a reference dose calibrated to the TPS. The built-in radiopaque markers allowed for image guidance localisation. Films were scanned with an EPSON 1100 XL flatbed scanner following well-documented QA procedures [23][24][25][26][27]. A calibration file for each batch of EBT-XD and EBT3 GafChromic film was acquired using CIRS solid water slabs. The pixel value function [26] was dosimetrically validated with independent reference fields and rescaled during the patient QA checks with plan-specific reference fields [26,28].
The HyperArc™ patient verification plans were calculated with dose to water (D w ) by the Acuros algorithm with grid resolution and CT slice thickness at 1 mm. D w in transport medium was used with Acuros since the film calibration was based on dose to water calculations. Coronal plane profiles were exported with a pixel resolution of 5.13px/ mm. This project's delivery of HyperArcTM patient plans consisted of four arcs with 3 couch-kicks ± 45° and 90° from the sagittal plane. The kV/MV isocenter coincidence was checked through the Winston-Lutz (WL) test [29] before HyperArc™ delivery. AAPM TG 142 [30] was referenced for the tolerances on the mechanical checks. Film was placed parallel to the coronal plane in the Easy Cube, marked with fiducials for laser alignment. No CBCT registration was used after confirming fractional millimetre shifts between laser alignment and CBCT registration. A reference film accompanied every treatment plan delivery as per protocol outlined by Lewis et al. [24] to account for variation in machine daily output and film scanning conditions.
The 48-bit film images were scanned with a resolution of 2.79 px/mm and analysed using Ashland's FilmQAPro 2015 software. Each BM was initially analysed by comparing the FWHM and mean dose to validate dosimetric equivalence for the metastases at various off-axis (OA) distances. Gamma analysis was then acquired for each metastasis with gamma pass rate criteria set at a dose difference of 2%, distance to agreement (DTA) set to 2 mm with a 10% threshold. The region of interest (ROI) was localised to a rectangular area inclusive of a single BM with a margin encompassing the 10% minimum dose threshold.
The methods outlined in this section cover validation of film and PDIP dosimetry for multi-metastases, geometrical and dosimetric uncertainties in the QA process for single and multi-metastases HyperArc™ analysis and BM case studies.
Although film and PDIP patient-specific QA procedures were already established in the department, 1 validation of a higher dose range for stereotactic radiosurgery (SRS) techniques and scanning of multi-metastases was required. SRS plans generated maximum dose in excess of fractionated SRT dose per fraction plans and required extended dosimetric validation of calibration files for both EBT-XD and EBT3. Known issues as addressed by Chen et al. [31] regarding variable scanning response in the lateral direction were potentially an issue for scanning multiple metastases on a single film. Checks were done by scanning a range of exposed film for EBT3 and EBT-XD over 50 mm from the central longitudinal axis of the EPSON 1100 XL flatbed scanner. The ROI measuring the pixel values (PV) for each film strip was measured using the red and green channels. Percentage differences between the average 50 mm lateral offset PV were compared to the central axis PV. Based on these results, recommendations were made for scanning films with multiple off-axis metastases and the selection of an appropriate colour channel for all other measurements pertaining to the remainder of this project.
A range of tolerances and criteria for SRT/ SRS were investigated by Miften et al. [30] for SRT/SRS techniques from which dosimetric and geometric tolerances were nominally set 2%/2mm and 3%/1mm. These were selected to test QA processes, provide baselines for future development and were within the scope of machine deliverability. Film QA processes required investigating to determine the sources of most significant uncertainty due to the inclusion of extended SRS dose range and multi-metastases film exposures. A film calibration curve model will vary from data measured due to the nature of the curve fitting [27] and may differ by up to 2% at key points. Variances in machine output, scanner temperature and film development time caused further dosimetric uncertainty; however, these uncertainties were minimised by including reference films [23]. Checks were performed to determine the maximum variation of a range of exposed films of known dose over the range of the film calibration. Checks were performed to determine the geometric shifts required in the film QA gamma optimisation process for multi-metastases. WL checks were recorded over eight months with the requirement that tests be repeated if a maximum delta shift was above 1 mm.
Single metastasis plans were created using the Hyper-Arc™ planning technique for metastases 10-25 mm in diameter at the isocenter with energies 6MV, 6FFF, and 10FFF. Each plan was prescribed to a single dose of 10 Gy, with each plan using four non-coplanar arcs. Gamma values were 1 3 acquired from both PDIP and Film QA Pro 2015. FWHM comparisons were measured for film and compared with the TPS. The FWHM was not a relevant measure with PDIP due to the portal 2D reconstruction generating smeared fluence for OA BM. Based on the outcome of these results, target volume (TV) size limits would be recommended for PDIP and film for the remainder of this project.
Multi-metastases plans were created with HyperArc™ for 20 mm TVs spaced at diagonal distances of 10 mm, 20 mm, 50 mm and 70 mm (see Fig. 1) from the isocenter. The TVs were placed diagonally to investigate off-axis distances (OAD) in both in-plane and cross-plane axes. Each TV in each single fraction plan had a prescription dose of 10 Gy. The diagonal OA metastases plans were delivered using 6MV, 6MV FFF, and 10 MV FFF energies. Checks were performed to determine if there was any dosimetric dependency on the distance of the TV from the isocenter. PDIP and film gamma analysis were acquired based on ROI relative to the TV with 2%/2mm criteria. FWHM and mean dose were measured for each TV for film and compared to the TPS plan. The FWHM was measured by the average of the in-plane and cross-plane profiles of the TV's maximum dose. Based on the results, recommendations were made for beam energy and OAD limits for both PDIP and film for the remainder of this report.
Relative dosimetric and absolute DTA uncertainty were then established based on results from single and multiple BM studies. The DTA uncertainty was established by a combination of WL and setup margins for film and the automatic panel alignment feature in the Portal dosimetry software.
Case studies were sampled from a set of verification plans from a sister site. 2 The plans were re-planned using HyperArc™. Four were selected from the eleven case studies to represent a range of possible multi-metastasis scenarios. Having determined patient-specific QA guidelines on size limits for TVs, and off-axis geometry, each of the four representative plans was initially screened against these guidelines. Each verification plan included an analysis of the size and location of each BM. Metastases not able to be delineated by the width of a single 5 mm pair of MLC leaves were planned as a single TV. Film QA was measured in the coronal plane of the TV's maximum dose. Gamma analyses were acquired for both film and PDIP QA with margins of 2%/2 mm for SRT while 3%/1 mm margins were acquired as per SRS guidelines [11]. The phantom was set up in the Encompass™ couch and registered using CBCT. Due to the limited number of slab combinations, some plans were required to be delivered more than once to have films placed in the correct coronal plane.

Results
The percentage difference between pixel value variation for a range of exposed films placed 50 mm laterally from the centre of the scanner resulted in the green channel varying by less than 0.7% for either XD and EBT3 film, while the red channel varied by less than 3.6% (see Fig. 2). The PV variation did not increase for the green channel based on an increase in exposure for either XD or EBT3 films. Although EBT3 can be used beyond the vendor's recommended optimal dosimetric range of 10 Gy, XD was used for the remainder of the project.
Recommendations for project scope limited scanning to within 50 mm of the central longitudinal axis of the scanner bed using the green channel. If distances between BM ranged beyond 100 mm in the scanning plane, it was recommended that each BM be scanned separately along the central longitudinal axis.
The max dosimetric variation with the green channel for EBT3 film was 0.8%, and for XD, 0.7%. The inclusion of a reference film accounted for daily output and scanning deviations enabling the film QA workflow to have less than 2% uncertainty. Geometric shifts were monitored over multi-metastases delivered in the in-plane and cross-plane directions during the film QA gamma optimisation for multi-metastases. Shifts of 0.8 ± 0.4 mm in the in-plane and 0.9 ± 0.6 mm in the cross-plane were noted for gamma results to account for film setup error. Single metastasis analyses were (see Fig. 3) based on a 95% gamma pass rate using 2%/2mm criteria. All energies showed dosimetric agreement for lesions down to a size of 15 mm diameter. The lowest gamma value for film was 98.7%, corresponding 10 MV FFF beam with a 10 mm TV. PDIP results started to deteriorate with TVs less than the nominal 15 mm.
The FWHM differences between TPS and film were averaged for measurements in the in-plane and cross-plane directions and resulted in agreement to within 1 mm for 10MV FFF for all TVs.
Multi-metastases gamma results for all energies with film were greater than 97%; however, PDIP produced mixed results with the flattened 6 MV beam greater than 91.6%. The gamma results for the unflattened beams deteriorated with increasing OAD. Results for the difference in FWHM for film and TPS (see Fig. 4) indicated that TV coverage was slightly greater for film than planned for unflattened beams and that this difference increased more noticeably for larger OA distances.
The mean dose difference between film and TPS (see Fig. 5) indicated that the measured film dose increased with OAD for all three energies tested.
Based on results from multi-metastases analyses, 10 FFF was selected for use in the case studies. The maximum change in FWHM (see Fig. 4) was less than 1.0 mm, with gamma analyses greater than 98.8% for TVs down to 10 mm.  Relative dose uncertainty is collated in Table 1, and absolute DTA is collated in Table 2. Values of the coverage factor of 2, correspond to confidence limits of about 95%.
Case study results are for BM (see Table 3) of average diameters ranging from 7 to 26 mm. Each case study contained BMs below the diameter guideline of 15 mm, and test case 2 only had metastases below the suggested guideline from the single metastasis results. PDIP peak CU differences were averaged over a range of peaks from each plan, with only one plan registering a CU difference less than 2%. Gamma analysis included 3%/1 mm to allow for a greater dosimetric margin but tighter DTA as per for each PDIP plan was calculated based on the whole plane. Localising ROI of individual metastases was problematic due to fluence smearing with all results out of tolerance. Gamma analyses for film were all greater than 98.4% for each TV, and the difference in FWHM was less than 1 mm. The mean dose difference between TPS and film was 1.0%.

Discussion
Variable lateral scanner response is a well-documented effect [31] and needed to be investigated due to the possibility of scanning multiple BM in the one coronal plane and as a matter of instilling confidence in established film analysis procedures. The higher sensitivity of the red channel [32] made it more prone to lateral scanning factors, especially in the higher dose regions. The differences in the response with GafChromic's EBT3 and XD can be attributed to EBT3's greater sensitivity and the corresponding effect of amplifying the lateral artefact response, as noted by Lewis et al. [33]. The lateral variation response across the red and green channels was considered with multi-metastases scans. However, lateral factors were mitigated by scanning films along the same longitudinal axis where the calibration films set were scanned. In addition, ROI pertaining to each BM could be analysed individually, thereby enabling each BM to be scanned along the central longitudinal axis of the scanner. The processes for film QA are well documented by Lewis et al. [24], and these should be systematically adhered to so as to lessen the dosimetric measurement uncertainty. Point dose and mean dose measurements could be analysed by film and to a lesser meaningful extent by PDIP as a measure of peak CU due to the smearing of the fluence. Comparison of peak dose with measured and predicted for PDIP also proved ambiguous due to varying outcomes based on point selection. Determination of point dose and mean dose comparisons were important to confirm minimum dose coverage to a BM. They could be used to determine the maximum dose to adjacent organs at risk, like the brain stem. The film QA process could determine systematic over and under dose estimation at specific regions by the planning system; however, this ability was lost in PDIP due to the 2D fluence integration process. Max dose with HyperArc™ may depend on how the prescription is applied with steeper dose gradients and higher dose maximums attained by prescribing to a lower isodose [3]. Steeper gradients to the TVs allowed for less dose wash to nearby OAR. The FWHM metric, as outlined by Miften et al. [34], provided quantitative analysis of coverage which is an important metric for high-grade tumour control and is measurable in film but not with PDIP. Along with FWHM, PDIP's lack of geometric relevance was a concern when the BM being analysed was near an OAR. Bresciani et al. [35] notes that PDIP's 2D reconstruction algorithm lacked anatomical or geometrical relevance; however, this could be addressed by a 3D forward projection technique. The nature of maximum dose points may also not be a leading concern where lesion ablation is one of the goals in stereotactic treatment. In this case, dosimetric tolerances may opt for a greater margin than 2%, as canvased by Miften et al. [34]. Likewise, DTA could have been reduced to 1 mm to provide greater sensitivity for QA outcomes for metastases with varying OAD, morphology or neighbouring OAR.
Gamma analysis covered the essential dosimetric and geometric properties for clinical delivery of SRS/SRT [23]. These techniques require tight margins for TV and consequently rely on dedicated patient immobilisation offered by systems such as the Encompass™, superior patient positioning such as 6DoF, SRT compliant Linac tolerances such as kV/MV coincidence and a tight radiation isocenter sphere. Halvorsen et al. [13] states the main criteria for implementation of SRS besides hypofractionation is the approximate 1 mm accuracy margin requirement for intracranial targets. Crowe, S et al. follows in their technical note [36] that the application of γ metrics for dosimetry and DTA should be applied in such a way as to prioritise the property of most significant clinical importance. Steep dose gradients, for example, can easily fail the dosimetric quality of the γ test, where a 1 mm shift can amount to a 20% dose difference. The 2 mm DTA provided a good basis to canvas optimal beam energy use, BM minimum size and OAD for the single and mulit-BM. These results however may provide over-estimated γ results which could overlook coverage of the target volume. The 1 mm DTA was more a suitable metric for SRS requirements with the option of widening the dosimetric margin due to the need to at least ensure ablation in the TV. The multi-metastases analyses included investigating the FWHM (see Fig. 4) and mean dose (see Fig. 5) for each TV in the TPS HyperArc™ plan. It was important to investigate the properties of mean dose and FWHM consistency across OA distances since any variation could point to potential issues with the unflattened beam model or mechanical tolerance of couch rotation or kV/MV isocenter tolerances. These properties lost their dosimetric relevance when measured with PDIP. The portal image in Fig. 6 for a three metastases plan single arc highlighted the spread of fluence captured by the 2D integrated process. A peak CU comparison for the TV placed a the isocenter was feasible in this case since high dose regions were visible in the red region and could provide useful comparisons of percentage over/under dose; however, the peak CU characteristic was further compromised when the TVs were further off-axis with no visible concentration of peak CU. In contrast, film fluence map (see Fig. 1) provided the ability to determine the mean dose and coverage with FWHM. The SRS techniques are ablative by design and may not consider the maximum dose in the TV as important as minimum coverage. The FWHM comparisons did indicate (see Fig. 4) that TPS coverage was greater when measured with unflattened beams for off-axis TVs. This feature would require further investigation and may not be detectable in gamma analysis results where DTA is 2 mm.
The selection of 10FFF for the case studies was partly due to the verification of single and multi-metastases analyses and the lower beam on time. The 10FFF high dose rate of 2400 MU/min reduced the risk of intrafraction movement, and the filter-free beams reduced scatter to the patient, as noted by Miften et al. [34]. The FFF beam's non-uniformity potentially counteracted the benefit of lower beam-on time, especially for those metastases with larger OAD; however, film QA was able to confirm acceptable dose distributions for OA metastases and the use of unflattened beams was supported in studies by Smith et al. [37]. An added complication to the FFF dose non-uniformity resulted in the varying modulating required by the TPS to deliver the same prescription to a range of metastases at varying off-axis distances. To account for the non-uniformity for OAD metastases, central metastases required more modulation to collimate excess dose, and this was a consideration when selecting metastases to analyse. This also prompted the isocenter to be optimally placed such that off-axis distances to metastases were overall minimal. No noticeable differences were identified in the individual BMs analysed from the cases that were analysed.
The PDIP QA process was far less time-consuming than film QA and gave immediate results. The software tools allowed for individual and composited field analysis, and ROI could be localised around a target metastasis. The lack of geometric relevance (see Fig. 7a compared to its film equivalent in Fig. 7b) for multi-OAD metastases made localising a ROI an unproductive task. The PDIP QA process  excluded beam attenuation and scatter through phantom and the mechanical variation generated by the HyperArc™ plan's three couch kicks. Based on the single and multimetastases results, the case studies (see Table 3) resulted in largely out-of-tolerance gamma analyses and peak dose comparisons due to a large number of small metastases. The individual arcs for PDIP were also analysed with 2 out of 16 arcs resulting in a peak CU difference of less than 2%. The multi-metastases study alluded to potential issues with the prediction model for small fields or with FFF intensity fluence maps, which required further investigation outside this project's scope. Out of the original eleven case studies, four cases were selected; however, none of these cases would have been treated if the minimum TV size of 15 mm was the deciding metric. All cases had small lesions, with case study 3 having a 7 mm diameter.
The film mean dose percentage differences (see Table 3) varied by a minimal amount and are likely to be a product of controlled film procedures and of localised film BM scans.

Conclusion
PDIP potentially provided a quick tool for HyperArc™ patient QA checks for SRS/SRT. However, due to a lack of geometric relevance for off-axis metastases, restriction in lesion size, and the non-inclusion of geometric uncertainty of couch-kicks, PDIP does not provide a sufficient basis for meaningful patient QA. PDIP would require development with optimising the prediction algorithm and detector fluence factors along with supplementary machine QA to ensure the integrity of kV/MV isocenter tolerance. Film QA provided high-resolution geometric relevance and meaningful γ analysis for small lesions when a strict methodology was followed and suitable for use for HyperArc™ patient QA.