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Dosimetric impact of random spot positioning errors in intensity modulated proton therapy plans of small and large volume tumors

Published Online:https://doi.org/10.1259/bjr.20201031

Abstract

Objective

To study dosimetric impact of random spot positioning errors on the clinical pencil beam scanning proton therapy plans.

Methods and materials

IMPT plans of 10 patients who underwent proton therapy for tumors in brain or pelvic regions representing small and large volumes, respectively, were included in the study. Spot positioning errors of 1 mm, −1 mm or ±1 mm were introduced in these clinical plans by modifying the geometrical co-ordinates of proton spots using a script in the MATLAB programming environment. Positioning errors were simulated to certain numbers of (20%, 40%, 60%, 80%) randomly chosen spots in each layer of these treatment plans. Treatment plans with simulated errors were then imported back to the Raystation (Version 7) treatment planning system and the resultant dose distribution was calculated using Monte-Carlo dose calculation algorithm.

Dosimetric plan evaluation parameters for target and critical organs of nominal treatment plans delivered for clinical treatments were compared with that of positioning error simulated treatment plans. For targets, D95% and D2% were used for the analysis. Dose received by optic nerve, chiasm, brainstem, rectum, sigmoid, and bowel were analyzed using relevant plan evaluation parameters depending on the critical structure. In case of intracranial lesions, the dose received by 0.03 cm3 volume (D0.03 cm3) was analyzed for optic nerve, chiasm and brainstem. In rectum, the volume of it receiving a dose of 65 Gy(RBE) (V65) and 40 Gy(RBE) (V40) were compared between the nominal and error introduced plans. Similarly, V65 and V63 were analyzed for Sigmoid and V50 and V15 were analyzed for bowel.

Results

The maximum dose variation in PTV D95% (1.88 %) was observed in a brain plan in which the target volume was the smallest (2.7 cm3) among all 10 plans included in the study. This variation in D95% drops down to 0.3% for a sacral chordoma plan in which the PTV volume is significantly higher at 672 cm3. The maximum difference in OARs in terms of absolute dose (D0.03 cm3) was found in left optic nerve (9.81%) and the minimum difference was observed in brainstem (2.48%). Overall, the magnitude of dose errors in chordoma plans were less significant in comparison to brain plans.

Conclusion

The dosimetric impact of different error scenarios in spot positioning becomes more prominent for treatment plans involving smaller target volume compared to plans involving larger target volumes.

Advances in knowledge

Provides information on the dosimetric impact of various possible spot positioning errors and its dependence on the tumor volume in intensity modulated proton therapy.

Introduction

Proton beam therapy offers significant improvement in dose conformation with reduced toxicity in comparison to even advanced forms of photon beam therapy and hence has become the preferred choice of radiation therapy for many clinical indications.1 The ability of proton beam therapy to tightly conform radiation dose around the target is attributable to the Bragg peak characteristic of proton beams and to the technically advanced delivery methods used in the proton therapy system gantries. For long time since inception, proton therapy systems employed double scattering technique (DST) as the delivery method in which the proton beam is shaped using customized collimators to conform the dose around the target in the lateral direction.2 DST also requires field-specific tissue compensators to conform the dose to distal end of the target in addition to range modulators that creates spread out Bragg peaks in the longitudinal direction of the beam. Design and fabrication of these custom made collimators and range compensators in DST are cumbersome procedures. Also the presence of these hardware materials in the proton beamline creates radiation safety concern, as it increases the production of secondary photons and neutron radiation. Recent technical advancements in proton beam delivery techniques in the form of pencil beam scanning (PBS) mitigates the requirement of these hardware materials.3 Unlike DST, PBS delivery technique uses narrow proton beams with spot-shaped Bragg peaks of different intensities resulting in Intensity Modulated Proton Therapy (IMPT). Each treatment field in PBS consists of multiple spots with modulated intensity to deliver a three-dimensional conformal dose distribution around the target. A scanning magnet system with two pairs of electro magnets in the gantry is used to position these spots in predefined positions within the target by scanning the proton beam in the lateral direction.

Characteristics of these proton spots including size, range and output are measured during commissioning of the proton therapy system and modeled in the treatment planning system (TPS).4 Successful treatment delivery depends on the ability of the proton therapy system in consistently maintaining the stability of these spot characteristics and in precise positioning of spots to the planned location within the stipulated tolerance.5,6 Studies have reported the dosimetric impact of errors in spot characteristics and that of inaccurate positioning of spots.4,7 Lateral spot positioning error (SPE) is reported to be the most vulnerable amongst possible uncertainties related to spot characteristics.8 The spot positioning accuracy of the PBS system is verified during commissioning and during periodical quality assurance checks to ensure that it is within the set tolerance of ±1 mm. However, various factors contribute to the SPE and there is also a possibility that the proton delivery system itself poses a greater uncertainty with lateral spot position accuracy because of the steering magnetic fields. Misalignment of the beam before it enters the scanning nozzle can also cause SPEs. Due to these various possibilities of occurrence of SPE, it becomes important to study the dosimetric impact of it in detail for the PBS-based proton beam therapy.

In an already published report, Yu et al have studied to quantify the impact of SPE in the quality of plan delivery and in patient safety.8 However, this report deals mostly with a phantom study and is limited to only two clinical cases representing intracranial tumors alone. With Proton therapy being increasingly indicated for many clinical sites, it become necessary to study the dosimetric impact of SPEs for a range of clinical target volumes. Also, the multiple spot positioning error scenario considered in Yu et al’s study is simulated only in the deepest layer. However, it holds good that, if spots in the deepest layer of a dose volume are delivered inaccurately, the same error might propagate to subsequent layers toward the proximal end as well. Hence, it becomes important to study the dosimetric impact of various possible SPE scenario in detail, to verify whether the set tolerance value for spot positioning accuracy is adequately safe or not. In this study, we have attempted to investigate the dosimetric impact of SPE for a range of targets that vary in smaller volume to larger volume.

Methods and materials

Retrospectively chosen IMPT plans of 10 patients who underwent proton therapy in our center for tumors in brain or pelvic sites (Intracranial tumor - 5, Cranial Chordoma -1 and Sacral Chordoma - 4) representing small and large volume targets respectively were included in the study. To simulate SPE in these plans, digital imaging and communications (DICOM) files of all these plans were extracted from the TPS and imported in the Matrix Laboratory (MATLAB) program (The Math works Inc., Natwick, MA, USA). A set of SPEs were introduced in the DICOM files of these plans using an in-house scripted MATLAB code. The SPE introduced plans were then imported back to the TPS and the resultant dose distributions were recalculated using Monte-Carlo dose calculation algorithm. Dosimetric impact of SPEs were then investigated by comparing the nominal plan and error simulated plans using critical plan evaluation parameters relevant to planning target volume (PTV) and organs at risk (OARs).

Treatment plan

All treatment plans included in this study were generated using RayStation (RaySearch Laboratories AB, Stockholm, Sweden) treatment planning system (TPS) on image sets of 2 mm slice thickness acquired using a wide bore (85 cm) multislice AcquilonLB (M/S Canon Medical Systems, USA) CT system. Contouring of tumor and relevant critical structures were outlined in the planning CT images following relevant guidelines. A set of protocol established and practiced in the department in line with the published literature was followed in the treatment planning and in plan evaluation for all ten patients. Treatment plans included in this study were made with two to four fields arrangement. The number of fields and field orientations were decided on case by case basis, mainly considering the tumor geometry and location, nearby critical structures and the presence of any proton beam-related uncertainties. RayStation Monte-Carlo algorithm was used both in the optimization as well as in final dose calculation of each treatment plan.

In two patients (patient nos. 8 and 10), multifield optimization (MFO) was used as the optimization technique and treatment plans for all other patients were optimized using singlefield optimization (SFO) technique. The final three-dimensional dose distribution was calculated for a grid spacing of 3 mm. All treatment plans were robustly optimized to the clinical target volume (CTV) to deliver a dose coverage of D99 >95% against ±3 mm setup errors and ±3.5% range uncertainty. Criteria for setup errors and range uncertainties are combined together during the optimization process resulting in a combination of 16 different error scenarios to which the plan robustness is evaluated. PTV structure was created with a 3 mm margin around the CTV and was used as a dose reporting structure in line with the recommendations by “International commission on radiation units & measurements” in its report no.78.9 Various spot-related parameters including spot spacing, layer spacing and number of layers, number of spots per layer, monitor units (MU) per layer were determined by the TPS as part of the optimization process. “Automatic with scale” option in Raystation TPS was chosen as the setting for spot and energy spacing within a field. With this option, the spot spacing is determined as 1.06 times of spot size (one sigma) in each layer. For energy spacing, the system chooses energy layers in such a way that, between each pair of two adjacent Bragg peaks, the distal 80% dose in the first layer coincides with the proximal 80% dose in the subsequent layer. The number of energy layers in all the treatment plans included in the study varied from 11 to 61. The number of spots per layer were in the range of 6 to 854. All treatment plans were delivered with a Proteus®PLUS-235 proton therapy system (IBA, Louvain-La-Neuve, Belgium) equipped with volumetric image guidance, robotic treatment table and dedicated nozzle for PBS delivery.

Error simulation

An in-house MATLAB code was generated as part of the study to introduce 1 mm of SPE either in X or Y direction (perpendicular to the beam direction) in the DICOM file of the treatment plan. To start with, the SPE was introduced on 20% of the spots that were chosen randomly in each layer. Three different scenarios of SPEs were applied to these randomly chosen spots. In the first SPE introduced plan, all 20% spots were subjected to an error of 1 mm in the forward direction (1 mm). Similarly, in the second SPE introduced plan, randomly selected spots were subjected to the error in the reverse direction (−1 mm) and then in the final plan the SPE was introduced either in the forward or reverse direction (±1 mm). In all three SPE scenario, positioning errors were introduced at the level of isocenter plane since spot position coordinates of the treatment plan are usually defined with respect to isocenter in the TPS. The selection of spots that have to undergo the SPE simulation process is made by the MATLAB script in a random nature. This is verified by repeated running of the script as it was found that the group spots that underwent the SPE simulation process were not the same when the script with a certain error scenario is repeated for the second time. Proper functioning of this MATLAB code was validated by importing the SPE plans in the TPS and cross-checking the geometric co-ordinates of the proton spots between the nominal plan and SPE introduced plans. The same set of error scenario was then extended to more number of (40%, 60% and 80%) randomly chosen spots in each layer. Thus, for each nominal plan, a set of 12 SPE introduced plans were generated using the MATLAB code. All error simulated plans were imported back to the TPS and the resultant dose distribution was calculated without adjusting any other planning parameters.

Dosimetric comparison

To study the dosimetric impact of SPE in the treatment plan quality, SPE-introduced proton therapy plans were evaluated against nominal plans for clinically used dose volume histogram parameters. For both small and large volume tumors (brain and chordoma), the change in dose received by 95% (D95%) and 2% (D2%) of the PTV were compared between the nominal and SPE-introduced plans. In routine clinical practice, dose received by OARs are evaluated using various dosimetric parameters based on the anatomical site involved. In this study, in intracranial lesions, the dose received by 0.03 cm3 volume (D0.03 cm3) was analyzed for optic nerve, chiasm, and brainstem. In chordoma, the dosimetric impact of SPE on critical structures was studied for rectum, bowel and sigmoid. In rectum, the volume of it receiving a dose of 65 Gy(RBE) (V65) and 40 Gy(RBE) (V40) were compared between the nominal and error simulated plans. Similarly, V65 and V63 were analyzed for Sigmoid and V50 and V15 were analyzed for bowel.

Results

Analysis of treatment plans

The treatment plan parameters of all 10 plans used in the study are listed in Table 1. Most of the treatment plans contain three proton beams except for three treatment plans. Out of these three plans, one treatment plan which was used for the treatment of a brain patient had two beams and the two other plans had four beams each, out of which one plan was employed in the treatment of a brain patient and another one in the treatment of a chordoma patient. Treatment plans for the brain patients had proton beams with low-to-medium energy ranges (70.18 to 202 MeV), whereas treatment plans for the sacral chordoma contains beams with comparatively high energy ranges (70.18 MeV to 226.2 MeV). Similarly, brain plans had lesser number of layers per beam (ranges from 11 to 25) in comparison with chordoma plans (ranging from 20 to 61). Also, the number of spots in any single layer of a plan and the same in an entire plan itself were significantly lesser in brain plans in comparison with Chordoma plans. The maximum number of spots per plan in brain plans was 2337, whereas the number of spots per plan in chordoma plans started at 3939 and went up to 1,1431. In summary, treatment plans for the brain patients were using lower energy beams, uses lesser MUs and contains lesser number of spots and layers per plan in comparison with chordoma plans.

Table 1. The treatment plan parameters of all 10 nominal plans used in the study

Patient 1Patient 2Patient 3Patient 4Patient 5Patient 6Patient 7Patient 8Patient 9Patient 10
Planning techniqueSFOSFOSFOSFOSFOSFOSFOMFOSFOMFO
Tumor size (cm3)2.7710.6632.6310819744855867210691740
No of beams2333434333
No of layers11–1515–1814–1719–2818–2422–2531–4131–4027–6120–40
Energy range (MeV)70.18–113.470.18–13582–13670.18–137115–202113–190100–226100–22670.18–226.2100–226
Max no of spots in a layer6456970110176351392654854
No of spots in plan31–43350–394479–678574–615876–14061408–23377144–89068618–95314270–71003939–11431
No of spots (MU >1)11–15319–178–580–129–73100–33348–14061–447
No of spots (MU >2)2–3000–4100–30–121–300–46
No of spots (MU >3)10000000–300–1

Dosimetric impact in brain plans

The variation in dose to PTV and critical structures in SPE introduced plans in comparison with nominal plans are summarized in Table 2 for both brain and chordoma treatment plans. Figure 1 details the variation in PTV dose (in D95% as well as in D2%) for both brain and chordoma plans. The maximum dose variation in PTV D95% (1.88 %) was observed in the plan in which the target volume was the smallest (2.7 cm3) amongst all 10 plans included in the study. This variation in D95% dropped down to 0.4% for the plan in which the PTV volume increased to 197 cm3. The maximum difference in the hot dose (PTV D2%) was observed in a plan with PTV volume of 108 cm3. The maximum difference in OARs in terms of absolute dose (D0.03 cm3) was found in left optic nerve (9.81%) and the minimum difference was observed in brainstem (2.48%) (Figure 2).

Figure 1.
Figure 1.

Dosimetric impact of SPEs in dose to PTV in brain and chordoma plans.

Figure 2.
Figure 2.

Dosimetric impact of SPEs in dose to OARs in the brain plans.

Table 2. The variation in dose to PTV and critical structures in SPE introduced plans in comparison with nominal plans

Brain plansChordoma plans
Maximum differenceStd DevMaximum DifferenceStd.Dev
PTVD95% (%)1.88±1.72PTVD95% (%)0.50±0.24
D2% (%)1.36D2% (%)0.69±0.46
BrainstemD0.03cm3 (%)5.99±1.40RectumV65Gy(RBE) (%)4.16±1.40
V40Gy(RBE) (%)3.08±1.27
Optic chiasmD0.03cm3 (%)6.33±1.69SigmoidV65Gy(RBE) (cm3)0.53±0.16
V63Gy(RBE) (cm3)1.34±0.41
Right optic nerveD0.03cm3 (%)7.04±2.67BowelV50Gy(RBE) (cm3)1.67±0.44
V15Gy(RBE) (cm3)1.96±0.73
Left optic nerveD0.03cm3 (%)9.81±4.28

Dosimetric impact in chordoma plans

The variation in hot dose PTV D2% follows a systematic pattern of decrease with respect to increase in PTV volume. Thus, the minimum deviation in the PTV D2% among all the 10 plans included in the study was observed in the plan having the largest PTV volume of 1740 cm3. The largest difference in the PTV D95% among the chordoma plans was observed in the plan with comparatively lesser PTV volume of 558 cm3. Similar to the variation in hot volume dose PTV D2%, the data for PTV D95% also follow a trend of decrease in dose difference to increase in PTV volume. Among the OARs, the maximum impact was observed in rectum in which the variation found in V65Gy(RBE) and in V40Gy(RBE) were 4.18 and 3.06%, respectively (Figure 3). Sigmoid was the OAR in the chordoma plans that was affected minimally due to SPEs in comparison with rectum and bowel.

Figure 3.
Figure 3.

Dosimetric impact of SPEs in dose to OARs in the chordoma plans.

Analysis of different SPE scenario

To study the role of various possible scenarios of random SPEs, data from all error simulated plans are clubbed and displayed in Figures 4 and 5 depicting results analyzed from brain and chordoma plans, respectively. In each nominal plan, three different possibilities of displacement errors were applied (1 mm, −1 mm and ±1 mm) and for each displacement error, there were four different combination of spots selected (20%, 40%, 60%, and 80% of randomly chosen spots from each layer) to simulate the positioning error. Figure 6 contains two set of images that displays the dosimetric difference between two different error scenario in the first set and the same between the nominal plan and 80% SPE+1mm scenario in the second set. In PTVs and in few OARs, the dosimetric impact was lesser in the scenario in which only 20% of the spots have undergone the error in comparison to other error scenarios. However, it is evident from Figure 4 and Figure 5 that there is not any significant correlation between the number of spots that underwent the positioning error with the magnitude of dosimetric impact on the PTV and on OARs.

Figure 4.
Figure 4.

Role of different SPE scenario in the dosimetric impact on brain plans.

Figure 5.
Figure 5.

Role of different SPE scenario in the dosimetric impact on chordoma plans.

Figure 6.
Figure 6.

Visual difference in dose distribution due to SPE.

Discussion

Long-term stability of spot characteristics defined during commissioning is an essential requirement to achieve accurate treatment delivery using technically advanced PBS proton therapy systems. Periodical quality assurance checks are performed in regular interval to ensure that key spot-related parameters are consistent with the commissioning data within an accepted tolerance window. It is also important to have an understanding of the impact of any variations in these beam parameters on the treatment plan dosimetry. Spot size and spot positioning accuracy are two such crucial parameters in PBS proton therapy. Various authors have already reported the dosimetric consequences of spot size variations.4,7 Also a log file-based study has attempted to model various inaccuracies in PBS proton therapy and analyzed the resultant changes in the clinical dosimetry of the treatment plans.10

The impact of SPE on plan quality and patient safety has been reported earlier for limited error scenario.8 In the study by Yu et al, the SPEs were introduced in the nominal plan in a systematic manner. Also, in their study, the spot positioning error was implemented mostly on phantom plans and the only clinical site investigated was limited to intracranial lesions of smaller volume. PBS proton therapy is indicated for small-to-large volume tumors including treatment sites in the brain, pelvic and thoracic regions. Various studies have impressed upon the efficacy of proton beam therapy for treatment of primary as well as recurrent sacral chordoma with the curative and palliative intent.11,12 Lily Bark et al in their study have followed up sacral chordoma patients for more than 10 years indicating the efficacy of proton therapy in improving the long term survival.11 In our clinic, a significant proportion of patients treated in the first year in to commissioning of the facility includes patients diagnosed with sacral chordoma with large size tumor volumes ranging from 558 to 1740 cm3. Hence, a study on dosimetric impact of SPEs in small-to-large volume tumors is important to gain confidence in efficient treatment with the set tolerance limits for the spot characteristics of our system.

The tolerance value accepted for spot positioning accuracy during commissioning and in periodical QA checks of the PBS proton therapy system in our center is ±1 mm. In the Proteus®PLUS-235 PBT system, positioning accuracy of spots is verified by monitor chambers installed in the dedicated PBS nozzle and is communicated to the beam control systems in real time during treatment delivery. Treatment interlocks defined in the Proteus®PLUS-235 PBT system will stop the beam delivery if there is a SPE of magnitude greater than the tolerance value (±1 mm). Hence, the study was designed to investigate the dosimetric impact of SPE for the worst case scenario of 1 mm in any lateral direction.

From the results of the dosimetric comparison between nominal treatment plans and SPE introduced plans, it was observed that the dosimetric impact on the dose to PTV (D95% and D2%) was systematically decreasing with the increase in tumor volume. Among the OARs, left optic nerve in brain plans and rectum in sacral chordoma plans have shown the maximum difference dosimetrically. Close proximity of these OARs to the PTV in treatment plans included in this study could be the cause behind this observation. All treatment plans employed in this study were robustly optimized and evaluated to deliver sufficient dose coverage to the clinical tumor volume against all possible uncertainties in dose delivery. However, OAR structures were not part of this robust optimization. Findings from this study can be correlated with this clinical practice that the dosimetric impact of spot positioning error was significant in OARs compared to that in target structures.

It is possible that other beam-related parameters including number of beams, spot size and MU weightage of SPE introduced spots might influence the dosimetric impact of SPE. In this study, the number of beams used in the treatment plan varied from 2 to 4. Although the maximum difference in PTV was observed in the plan with two beams, a correlation between number of beams and the dosimetric impact could not be established as the dose difference in OARs were observed in other plans using 3 and 4 beams as well. The spot position-related dose error solely depending on the positioning error alone but on many parameters including the spot size and MUs. Investigating the influence of spot size on the dosimetric impact of SPE is beyond the scope of this study. However, a detailed analysis on this would give the advantage of a better understanding on whether the occurrence of SPE in any particular layer is more detrimental to the plan quality than the occurrence of SPE in other layers. The data extracted and worked on each energy-specific layers and on the spots in each layer of the treatment plans included in the study, makes it feasible for an analysis on number of spots per layer as a function of energy range. It was observed from the plots in Figure 7 that all IMPT plans generated from the TPS consists more number of spots in the medium energy ranges than the proximal and distal energy ranges. This can be correlated with the shape of the target structures as tumor dimensions are generally largest in the middle.

Figure 7.
Figure 7.

Number of spots present in each layer as a function of energy range.

Another point of consideration is to check the role of spot weight (SW) on the dosimetric impact of SPE. To investigate this, the relative MU weightage of the SPE introduced spots (SWSPE-S) to the total MUs in the plan was analyzed for all error introduced plans in the study (Figure 8). It was observed that, within each error scenario, the SWSPE-S was almost uniform across all ten plans. That is, in none of the error scenario, the SWSPE-S of any plan was significantly higher in comparison with that in another plan. This indicates that the significant dosimetric impact observed in plans chosen for this study with smaller volume targets was mostly due to the target size alone and had no noticeable influence by the SW. A further study in which spots are made to undergo the SPE process systematically based on their MU weightage (unlike the SPE being introduced on randomly chosen spots in this study) would elaborate on the added role of SW in addition to SPE on the dosimetric impact.

Figure 8.
Figure 8.

Weightage (in MU) proportion of SPE introduced spots to total MU in the plan.

Conclusion

A detailed study on dosimetric impact of different scenarios of spot positioning errors is carried out in this work for small-to-large volume tumors. A combination of different possible error scenario is investigated to check the role of number of spots that are subjected to SPE on the plan quality. The number of spots per layer as a function of energy range was also analyzed in the study. It was found through the study that the SPE has a greater dosimetric impact on the plans with small volume tumors than on the plans with large volume tumors. It was observed that beyond 20% of spots, there is no correlation between the number of spots with SPE and it’s dosimetric impact on the treatment plan.

REFERENCES

  • 1. Mishra MV, , Aggarwal S, , Bentzen SM, , Knight N, , Mehta MP, , Regine WF. Establishing evidence-based indications for proton therapy: an overview of current clinical trials. Int J Radiat Oncol Biol Phys 2017; 97: 228–35. doi: https://doi.org/10.1016/j.ijrobp.2016.10.045 http://www.ncbi.nlm.nih.gov/pubmed/28068231 Crossref Medline ISIGoogle Scholar

  • 2. Newhauser WD, , Zhang R. The physics of proton therapy. Phys Med Biol 2015; 60: R155–209. doi: https://doi.org/10.1088/0031-9155/60/8/R155 http://www.ncbi.nlm.nih.gov/pubmed/25803097 Crossref Medline ISIGoogle Scholar

  • 3. Smith AR. Vision 20/20: proton therapy. Med Phys 2009; 36: 556–68. doi: https://doi.org/10.1118/1.3058485 http://www.ncbi.nlm.nih.gov/pubmed/19291995 Crossref Medline ISIGoogle Scholar

  • 4. Kraan AC, , Depauw N, , Clasie B, , Madden T, , Kooy HM. Impact of spot size variations on dose in scanned proton beam therapy. Phys Med 2019; 57: 58–64. doi: https://doi.org/10.1016/j.ejmp.2018.12.011 http://www.ncbi.nlm.nih.gov/pubmed/30738532 Crossref Medline ISIGoogle Scholar

  • 5. Liu C, , Schild SE, , Chang JY, , Liao Z, , Korte S, , Shen J, , Chenbin Liu P, , Chang PhD JY, , Zhongxing Liao MD, , Shawn Korte CMD, , Jiajian Shen P, , et al.. Impact of spot size and spacing on the quality of robustly optimized intensity modulated proton therapy plans for lung cancer. Int J Radiat Oncol Biol Phys 2018; 101: 479-489. doi: https://doi.org/10.1016/j.ijrobp.2018.02.009 http://www.ncbi.nlm.nih.gov/pubmed/29550033 Crossref Medline ISIGoogle Scholar

  • 6. Archambault L, , Poenisch F, , Sahoo N, , Robertson D, , Lee A, , Gillin MT, , et al.. “Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system”, Med.. Phys 2012; 39. Google Scholar

  • 7. Chanrion MA, , Ammazzalorso F, , Wittig A, , Engenhart-Cabillic R, , Jelen U. Dosimetric consequences of pencil beam width variations in scanned beam particle therapy. Phys Med Biol 2013; 58: 3979–93. doi: https://doi.org/10.1088/0031-9155/58/12/3979 http://www.ncbi.nlm.nih.gov/pubmed/23685746 Crossref Medline ISIGoogle Scholar

  • 8. Yu J, , Beltran CJ, , Herman MG. Implication of spot position error on plan quality and patient safety in pencil-beam-scanning proton therapy. Med Phys 2014; 41: 081706. doi: https://doi.org/10.1118/1.4885956 http://www.ncbi.nlm.nih.gov/pubmed/25086516 Crossref Medline ISIGoogle Scholar

  • 9. Newhauser W. International Commission on Radiation Units and Measurements Report 78: Prescribing, Recording and Reporting Proton-beam Therapy: Oxford University Press; 2009. Google Scholar

  • 10. Belosi MF, , Meer Rvander. Paz Garcia de Acilu Laa, Alessandra Bolsi, Damien C. Weber, Antony J Lomax, “Treatment log files as a tool to identify treatment plan sensitivity to inaccuracies in scanned proton beam delivery”, Radiother Oncol 2017; 125: 514–9. Google Scholar

  • 11. Aibe N, , Demizu Y, , Sulaiman NS, , Matsuo Y, , Mima M, , Nagano F, , et al.. Outcomes of patients with primary sacral chordoma treated with definitive proton beam therapy. Int J Radiat Oncol Biol Phys 2018; 100: 972–9. doi: https://doi.org/10.1016/j.ijrobp.2017.12.263 http://www.ncbi.nlm.nih.gov/pubmed/29485077 Crossref Medline ISIGoogle Scholar

  • 12. Park L, , Delaney TF, , Liebsch NJ, , Hornicek FJ, , Goldberg S, , Mankin H, , et al.. Sacral chordomas: impact of high-dose proton/photon-beam radiation therapy combined with or without surgery for primary versus recurrent tumor. Int J Radiat Oncol Biol Phys 2006; 65: 1514–21. doi: https://doi.org/10.1016/j.ijrobp.2006.02.059 http://www.ncbi.nlm.nih.gov/pubmed/16757128 Crossref Medline ISIGoogle Scholar

Volume 94, Issue 1119March 2021

© 2021 The Authors. Published by the British Institute of Radiology


History

  • ReceivedAugust 25,2020
  • RevisedJanuary 18,2021
  • AcceptedJanuary 20,2021
  • Published onlineFebruary 02,2021

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