Dose-averaged linear energy transfer per se does not correlate with late rectal complications in carbon-ion radiotherapy

Dose Constraints in Carbon-Ion Radiation Therapy to Minimize the Risk of Pectoral Myositis

Purpose

Carbon-ion radiation therapy (C-ion RT) is an emerging nonsurgical treatment for early stage breast cancer, offering biological advantages such as high linear energy transfer (LET) and precise dose distribution. However, the risk of radiation-induced toxicity, particularly pectoral myositis, remains unclear. This study evaluates the relationship between RBE-weighted dose, LET, and pectoral myositis in patients receiving C-ion RT.

Patients and Methods

Fourteen patients with cT0-1N0M0 breast cancer were treated with C-ion RT (46-50 Gy [RBE]) in the prone position. Magnetic resonance imaging was performed pretreatment and at 3-month intervals post treatment to assess pectoral myositis. RBE-weighted dose-volume histograms and LET distributions were analyzed. Statistical comparisons between patients with and without pectoral myositis were conducted using chi-square and t-tests.

Results

Four of 14 patients (29%) developed pectoral myositis, all within 3 months of treatment. Higher RBE-weighted doses in small volumes of pectoralis major muscle were significantly associated with myositis (D2 cm3 >10 Gy [RBE], P = .014). The D0.1 cm3 to the pectoralis major muscle in patients without pectoral myositis was consistently below 33 Gy (RBE). However, LET distributions showed no significant correlation with myositis development.

Conclusion

This study is the first to evaluate pectoral myositis after C-ion RT for breast cancer. The findings suggest that high RBE-weighted doses in small muscle volumes increase the risk of myositis, while LET is not a significant factor. Establishing dose constraints for the pectoralis major muscle is crucial to minimize radiation-induced toxicity. Further studies with larger cohorts are needed to validate these results.

Design of multi-ion therapy for head and neck cancers using carbon-, oxygen-, and neon-ion beams: potential efficacy against tumor hypoxia

Objective. In order to initiate multi-ion therapy for head and neck cancers, it is necessary to predetermine the target dose-averaged linear energy transfer (LETd) prescription to the gross tumor volume (GTV). This study investigated LETdoptimized treatment plans with carbon-, oxygen-, and neon-ion beams and demonstrated their potential efficacy against tumor hypoxia.Approach. Sixteen head and neck cancer patients with GTV sizes ranging from 5.5 to 143.1 cm3were selected for this retrospective planning study. Carbon, oxygen, and neon ions were used alone or in combination with two ion species. The treatment plans were optimized to increase LETdwithin the GTV and to make the LETddistribution uniform while maintaining the relative biological effectiveness weighted dose distributions of conventional intensity modulated carbon-ion therapy (IMIT). The effective dose improvement rate against IMIT was then estimated by changing oxygen partial pressure within the GTV to 0 mmHg because a substantial number of anoxic cancer cells is predicted to exist in a hypoxic tumor microenvironment.Main results. The target LETdof 90 keVμm-1was prescribable without deteriorating the dose distributions when: for example, carbon- and oxygen-ion beams were used for small tumors (around 20 cm3); oxygen-ion beams alone were used for medium tumors (around 50 cm3); and carbon- and neon-ion beams were used for large tumors (around 100 cm3). The uniformity of the LETddistributions within the GTV was about 10%. With the LETdprescription, the improvement rate of the effective dose covering 98% (i.e.D98%) of the GTV against anoxic cancer cells was about 30%.Significance. For the application to the first multi-ion therapy program, the target LETdprescription to the GTV was determined to be 90 keVμm-1. If tumor hypoxia contributes to the cause of recurrence, the proposed treatment may offer better local tumor control without compromising normal tissue sparing.

Linear energy transfer optimized proton therapy for rectal cancer

PURPOSE

To evaluate the feasibility and utility of an LET-optimized proton treatment planning algorithm in locally advanced rectal cancer and to assess whether the degree of LET-optimization achieved in clinical plans improves efficacy and toxicity in preclinical models.

MATERIALS AND METHODS

A series of five rectal cancer patients treated with standard 25 fraction clinical proton plans were re-planned using an LET-optimization treatment planning algorithm and evaluated for dosimetric endpoints. LET-optimized plans were generated using an algorithm which iteratively increases the weights of higher LET spots in GTV and lower LET in OARs. Murine and in vitro preclinical models of tumor efficacy and normal tissue toxicity were evaluated using comparable LETd range to that achieved in clinical LET-optimized plans.

RESULTS

LET-optimized proton plans increased dose-averaged LET (LETd) in the GTV and LET-weighted dose in the GTV, and CTV5625cGy V100% coverage. At the same time, LET-optimization also decreased mean LET-weighted dose to bladder and small bowel, as well as small bowel V30Gy(cc) compared to standard proton plans. Optimizing the LETd to a volume of GTV-3 mm further increased LETd compared to total GTV. LET-optimization in preclinical models increased tumor efficacy in colorectal cancer cell lines in vitro and decreased small bowel radiation enteropathy in murine models of normal tissue toxicity.

CONCLUSIONS

LET-optimized proton plans increased LETd in gross tumor while maintaining or improving target coverage and OAR sparing, with acceptable plan robustness. Preclinical models demonstrated that comparable LET-optimization may increase tumor efficacy and decrease normal tissue toxicity in rectal cancer.

Optimizing the dose-averaged linear energy transfer for the dominant intraprostatic lesions in high-risk localized prostate cancer patients

Background and purpose

Radiotherapy for localized prostate cancer often targets the entire prostate with a uniform dose despite the presence of high-risk dominant intraprostatic lesions (DILs). This study investigated the feasibility of focal dose-averaged linear energy transfer (LETd) boost for prostate carbon-ion radiotherapy to deposit higher LETd to DILs while ensuring desired relative biological effectiveness weighted dose coverage to targets and sparing organs at risk (OARs).

Materials and methods

A retrospective planning study was conducted on 15 localized prostate cancer cases. The DILs were identified on multiparametric MRI and used to define the boost target (PTVboost). Two treatment plans were designed for each patient: 1) conventional plan optimized by the single-field uniform dose technique, and 2) boost plan optimized by the multifield optimization and LET painting technique, to achieve LETd boost within the PTVboost. Dose and LETd metrics of the targets and OARs were compared between the two plans.

Results

Compared to the conventional plans, the boost plans delivered clinically acceptable dose coverage (D90% and D50%) to the target (PTV2) within 1% differences while significantly increasing the minimum LETd by 16 ∼ 24 keV/μm for the PTVboost (63.9 ± 2.8 vs. 44.0 ± 1.3 keV/μm, p < 0.001). Furthermore, these improvements were consistent across all cases, irrespective of their anatomical features, including the boost volume's size, location, and shape.

Conclusion

Focal LETd boost was a feasible strategy for prostate carbon-ion radiotherapy. This investigation demonstrated its superiority in delivering LETd boost without depending on tumor location and volume across different cases.

Dose-averaged linear energy transfer within the gross tumor volume of non-small-cell lung cancer affects the local control in carbon-ion radiotherapy

BACKGROUND AND PURPOSE

High linear energy transfer (LET) radiation exhibits stronger tumor-killing effect. However, the correlation between LET and the therapeutic efficacy in Carbon-ion radiotherapy (CIRT) for locally advanced non-small-cell lung cancer (LA-NSCLC) is currently not clear. This study aimed to investigate the relationship between the dose-averaged LET (LETd) distribution within tumor and local recurrence for LA-NSCLC treated with CIRT.

METHODS AND MATERIALS

An analysis of 62 consecutive patients with LA-NSCLC who underwent CIRT from 2018 to 2022 was conducted. The LETd distribution was calculated based on their treated plans, and the correlation between local recurrence and LETd, relative biological effectiveness (RBE)-weighted doses (DRBE) and clinical factors was investigated. Receiver operating characteristic (ROC) curve, log-rank test, and Cox regression analysis were performed based on that.

RESULTS

16 patients were defined as local recurrence. Overall survival (OS) and local control (LC) at 24 months were 76.9 % and 73.2 %, respectively. The mean LETd in internal gross tumor volume (iGTV) in the local recurrence group was 48.7 keV/µm, significantly lower than the mean LETd of 53.2 keV/µm in the local control group (p = 0.016). No significant difference was observed in DRBE between the local recurrence and local control groups. ROC curve analysis indicated that a percentage of 88 % of volume in iGTV receiving at least 40 keV/µm (V40keV/μm) is the optimal threshold for predicting local recurrence (Area under curve (AUC) = 0.7636). The log-rank test and Cox regression analysis revealed that the LETd value covering 98 % volume of iGTV (LETd98%) was a significant risk factor for LC (p = 0.020).

CONCLUSIONS

Our study revealed an association between LETd distribution and local recurrence in patients with LA-NSCLC. These findings suggest that lower LETd may increase the probability of local recurrence. We suggest that LETd distribution within iGTV should be routinely assessed in CIRT for lung cancer.

Monte carlo simulation study on the dose and dose-averaged linear energy transfer distributions in carbon ion radiotherapy

Dose-averaged linear energy transfer (LETd) is conventionally evaluated from the relative biological effectiveness (RBE)-LETd fitted function used in the treatment planning system. In this study, we calculated the physical doses and their linear energy transfer (LET) distributions for patterns of typical CIRT beams using Monte Carlo (MC) simulation. The LETd was then deduced from the MC simulation and compared with that obtained from the conventional method. The two types of LETd agreed well with each other, except around the distal end of the spread-out Bragg peak. Furthermore, an MC simulation was conducted with the material composition of water and realistic materials. The profiles of physical dose and LETd were in good agreement for both techniques. These results indicate that the previous studies to analyze the minimum LETd in CIRT cases are valid for practical situations, and the material composition conversion to water little affects the dose distribution in the irradiation field.

Dose-averaged LET optimized carbon-ion radiotherapy for head and neck cancers

This feasibility study confirmed the initial safety and efficacy of a novel carbon-ion radiotherapy (CIRT) using linear energy transfer (LET) painting for head and neck cancer. This study is the first step toward establishing CIRT with LET painting in clinical practice and making it a standard practice in the future.

Sacral-Nerve-Sparing Planning Strategy in Pelvic Sarcomas/Chordomas Treated with Carbon-Ion Radiotherapy

To minimize radiation-induced lumbosacral neuropathy (RILSN), we employed sacral-nerve-sparing optimized carbon-ion therapy strategy (SNSo-CIRT) in treating 35 patients with pelvic sarcomas/chordomas. Plans were optimized using Local Effect Model-I (LEM-I), prescribed DRBE|LEM-I|D50% (median dose to HD-PTV) = 73.6 (70.4-76.8) Gy (RBE)/16 fractions. Sacral nerves were contoured between L5-S3 levels. DRBE|LEM-I to 5% of sacral nerves-to-spare (outside HD-CTV) (DRBE|LEM-I|D5%) were restricted to <69 Gy (RBE). The median follow-up was 25 months (range of 2-53). Three patients (9%) developed late RILSN (≥G3) after an average period of 8 months post-CIRT. The RILSN-free survival at 2 years was 91% (CI, 81-100). With SNSo-CIRT, DRBE|LEM-I|D5% for sacral nerves-to-spare = 66.9 ± 1.9 Gy (RBE), maintaining DRBE|LEM-I to 98% of HD-CTV (DRBE|LEM-I|D98%) = 70 ± 3.6 Gy (RBE). Two-year OS and LC were 100% and 93% (CI, 84-100), respectively. LETd and DRBE with modified-microdosimetric kinetic model (mMKM) were recomputed retrospectively. DRBE|LEM-I and DRBE|mMKM were similar, but DRBE-filtered-LETd was higher in sacral nerves-to-spare in patients with RILSN than those without. At DRBE|LEM-I cutoff = 64 Gy (RBE), 2-year RILSN-free survival was 100% in patients with <12% of sacral nerves-to-spare voxels receiving LETd > 55 keV/µm than 75% (CI, 54-100) in those with ≥12% of voxels (p < 0.05). DRBE-filtered-LETd holds promise for the SNSo-CIRT strategy but requires longer follow-up for validation.

High-Linear Energy Transfer Irradiation in Clinical Carbon-Ion Beam With the Linear Energy Transfer Painting Technique for Patients With Head and Neck Cancer

Purpose

Dose-averaged linear energy transfer (LETd) is one of the important factors in determining clinical outcomes for carbon-ion radiation therapy. Innovative LET painting (LP) has been developed as an advanced form of conventional intensity modulated carbon-ion radiation therapy (IMIT) at the QST Hospital. The study had 2 motivations: to increase the minimum LETd (LETdmin) and to improve uniformity of the LETd distribution within the gross tumor volume (GTV) by using LP treatment plans for patients with head and neck cancer while maintaining the relative biologic effectiveness (RBE)-weighted dose coverage within the planning tumor volume (PTV) the same as in the conventional IMIT plan.

Methods and Materials

The LP treatment plans were designed with the in-house treatment planning system. For the plans, LETd constraints and LETdmin, goal-LETd, and maximum-LETd (LETdmax) constraints for the GTV were added to the conventional dose constraints in the IMIT prescription. For 13 patients with head and neck cancer, the RBE-weighted dose to 90% (D90) and 50% (D50) of the PTV and the LETdmin, mean (LETdmean), and LETdmax values within the GTV in the LP plans were evaluated by comparing them with those in the conventional IMIT plans.

Results

The LP for 13 patients with head and neck cancer could keep D90s and D50s for the PTV within 1.0% of those by the conventional IMIT. Among the 13 patients, the mean LETdmin of the LP plans for the GTV was 59.2 ± 7.9 keV/μm, whereas that of the IMIT plans was 45.9 ± 6.0 keV/μm. The LP increased the LETdmin to 8 to 24 keV/μm for the GTV compared with IMIT.

Conclusions

While maintaining the dose coverage to the PTV as comparable to that for IMIT, the LP increased the mean LETdmin to 13.2 keV/μm for the GTV. For a GTV up to 170 cm3, LETd > 44 keV/μm could be achieved using LP, which according to previous studies was associated with lower recurrence. In addition, the LP method delivered more uniform LETd distributions compared with IMIT.

Investigation on the physical dose filtered by linear energy transfer for treatment plan evaluation in carbon ion therapy

BACKGROUND

Large tumor size has been reported as a predicting factor for inferior clinical outcome in carbon ion radiotherapy (CIRT). Besides the clinical factors accompanied with such tumors, larger tumors receive typically more low linear energy transfer (LET) contributions than small ones which may be the underlying physical cause. Although dose averaged LET is often used as a single parameter descriptor to quantify the beam quality, there is no evidence that this parameter is the optimal clinical predictor for the complex mixed radiation fields in CIRT.

PURPOSE

Purpose of this study was to investigate on a novel dosimetric quantity, namely high-LET-dose (D > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ , the physical dose filtered based on an LET threshold) as a single parameter estimator to differentiate between carbon ion treatment plans (cTP) with a small and large tumor volume.

METHODS

Ten cTPs with a planning target volume,PTV ≥ 500 cm 3 $\mathrm{PTV}\ge {500}\,{{\rm cm}^{3}}$ (large) and nine with aPTV < 500 cm 3 $\mathrm{PTV}<{500}\,{{\rm cm}^{3}}$ (small) were selected for this study. To find a reasonable LET threshold (L thr $\textrm {L}_{\textrm {thr}}$ ) that results in a significant difference in terms ofD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ , the voxel based normalized high-LET-dose (D ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) distribution in the clinical target volume (CTV) was studied on a subset (12 out of 19 cTPs) for 18 LET thresholds, using standard distribution descriptors (mean, variance and skewness). The classical dose volume histogram concept was used to evaluate theD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ distributions within the target of all 19 cTPs at the before determinedL thr $\textrm {L}_{\textrm {thr}}$ . Statistical significance of the difference between the two groups in terms of meanD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ volume histogram parameters was evaluated by means of (two-sided) t-test or Mann-Whitney-U-test. In addition, the minimum target coverage at the above determinedL thr $\textrm {L}_{\textrm {thr}}$ was compared and validated against three other thresholds to verify its potential in differentiation between small and large volume tumors.

RESULTS

AnL thr $\textrm {L}_{\textrm {thr}}$ of approximately30 keV / μ m ${30}\,{\rm keV/}\umu {\rm m}$ was found to be a reasonable threshold to classify the two groups. At this threshold, theD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ were significantly larger (p < 0.05 $p<0.05$ ) in small CTVs. For the small tumor group, the near-minimum and medianD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ (andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) in the CTV were in average9.3 ± 1.5 Gy $9.3\pm {1.5}\,{\rm Gy}$ (0.31 ± 0.08) and13.6 ± 1.6 Gy $13.6\pm {1.6}\,{\rm Gy}$ (0.46 ± 0.06), respectively. For the large tumors, these parameters were6.6 ± 0.2 Gy $6.6\pm {0.2}\,{\rm Gy}$ (0.20 ± 0.01) and8.6 ± 0.4 Gy $8.6\pm {0.4}\,{\rm Gy}$ (0.28 ± 0.02). The difference between the two groups in terms of mean near-minimum and medianD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ (D ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) was 2.7 Gy (11%) and 5.0 Gy (18%), respectively.

CONCLUSIONS

The feasibility of high-LET-dose based evaluation was shown in this study where a lowerD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ was found in cTPs with a large tumor size. Further investigation is needed to draw clinical conclusions. The proposed methodology in this work can be utilized for future high-LET-dose based studies.

Carbon Ions for Hypoxic Tumors: Are We Making the Most of Them?

Hypoxia, which is associated with abnormal vessel growth, is a characteristic feature of many solid tumors that increases their metastatic potential and resistance to radiotherapy. Carbon-ion radiation therapy, either alone or in combination with other treatments, is one of the most promising treatments for hypoxic tumors because the oxygen enhancement ratio decreases with increasing particle LET. Nevertheless, current clinical practice does not yet fully benefit from the use of carbon ions to tackle hypoxia. Here, we provide an overview of the existing experimental and clinical evidence supporting the efficacy of C-ion radiotherapy in overcoming hypoxia-induced radioresistance, followed by a discussion of the strategies proposed to enhance it, including different approaches to maximize LET in the tumors.

Calculating dose-averaged linear energy transfer in an analytical treatment planning system for carbon-ion radiotherapy

BACKGROUND

Compelling evidence shows the association between the relative biological effectiveness (RBE) of carbon-ion radiotherapy (CIRT) and the dose averaged linear energy transfer (LETd). However, the ability to calculate the LETd in commercially available treatment planning systems (TPS) is lacking.

PURPOSE

This study aims to develop a method of calculating the LETd of CIRT plans that could be robustly carried out in RayStation (V10B, Raysearch, Sweden).

METHODS

The calculation used the fragment spectra in RayStation for the CIRT treatment planning. The dose-weighted averaging procedure was supported by the microdosimetric kinetic model (MKM). The MKM-based pencil beam dose engine (PBA, v4.2) for calculating RBE-weighted doses was reformulated to become a LET-weighted calculating engine. A separate module was then configured to inversely calculate the LETd from the absorbed dose of a plan and the associated fragment spectra. In this study, the ion and energy-specific LET table in the LETd module was further matched with the values decoded from the baseline data of the Syngo TPS (V13C, Siemens, Germany). The LETd distributions of several monoenergetic and modulated beams were calculated and validated against the values derived from the Syngo TPS and the published data.

RESULTS

The differences in LETds of the monoenergetic beams between the new method and the traditional method were within 3% in the entrance and Bragg-peak regions. However, a larger difference was observed in the distal region. The results of the modulated beams were in good agreement with the works from the published literature.

CONCLUSIONS

The method presented herein reformulates the MKM dose engine in the RayStation TPS to inversely calculate LETds. The robustness and accuracy were demonstrated.

Effects of dose and dose-averaged linear energy transfer on pelvic insufficiency fractures after carbon-ion radiotherapy for uterine carcinoma

BACKGROUND AND PURPOSE

The correlation between dose-averaged linear energy transfer (LETd) and its therapeutic or adverse effects, especially in carbon-ion radiotherapy (CIRT), remains controversial. This study aimed to investigate the effects of LETd and dose on pelvic insufficiency fractures after CIRT.

MATERIAL AND METHODS

Among patients who underwent CIRT for uterine carcinoma, 101 who were followed up for > 6 months without any other therapy were retrospectively analyzed. The sacrum insufficiency fractures (SIFs) were graded according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer toxicity criteria. The correlations between the relative biological effectiveness (RBE)-weighted dose, LETd, physical dose, clinical factors, and SIFs were evaluated. In addition, we analyzed the association of SIF with LETd, physical dose, and clinical factors in cases where the sacrum D50% RBE-weighted dose was above the median dose.

RESULTS

At the last follow-up, 19 patients developed SIFs. Receiver operating characteristic curve analysis revealed that the sacrum D50% RBE-weighted dose was a valuable predictor of SIF. Univariate analyses suggested that LETd V10 keV/µm, physical dose V5 Gy, and smoking status were associated with SIF. Cox regression analysis in patients over 50 years of age validated that current smoking habit was the sole risk factor for SIF. Therefore, LETd or physical dose parameters were not associated with SIF prediction.

CONCLUSION

The sacrum D50% RBE-weighted dose was identified as a risk factor for SIF. Additionally, neither LETd nor physical dose parameters were associated with SIF prediction.

Roadmap: helium ion therapy

Helium ion beam therapy for the treatment of cancer was one of several developed and studied particle treatments in the 1950s, leading to clinical trials beginning in 1975 at the Lawrence Berkeley National Laboratory. The trial shutdown was followed by decades of research and clinical silence on the topic while proton and carbon ion therapy made debuts at research facilities and academic hospitals worldwide. The lack of progression in understanding the principle facets of helium ion beam therapy in terms of physics, biological and clinical findings persists today, mainly attributable to its highly limited availability. Despite this major setback, there is an increasing focus on evaluating and establishing clinical and research programs using helium ion beams, with both therapy and imaging initiatives to supplement the clinical palette of radiotherapy in the treatment of aggressive disease and sensitive clinical cases. Moreover, due its intermediate physical and radio-biological properties between proton and carbon ion beams, helium ions may provide a streamlined economic steppingstone towards an era of widespread use of different particle species in light and heavy ion therapy. With respect to the clinical proton beams, helium ions exhibit superior physical properties such as reduced lateral scattering and range straggling with higher relative biological effectiveness (RBE) and dose-weighted linear energy transfer (LET_d) ranging from ∼4 keVμm^-1to ∼40 keVμm^-1. In the frame of heavy ion therapy using carbon, oxygen or neon ions, where LET_dincreases beyond 100 keVμm^-1, helium ions exhibit similar physical attributes such as a sharp lateral penumbra, however, with reduced radio-biological uncertainties and without potentially spoiling dose distributions due to excess fragmentation of heavier ion beams, particularly for higher penetration depths. This roadmap presents an overview of the current state-of-the-art and future directions of helium ion therapy: understanding physics and improving modeling, understanding biology and improving modeling, imaging techniques using helium ions and refining and establishing clinical approaches and aims from learned experience with protons. These topics are organized and presented into three main sections, outlining current and future tasks in establishing clinical and research programs using helium ion beams-A. Physics B. Biological and C. Clinical Perspectives.

Development of the DICOM-based Monte Carlo dose reconstruction system for a retrospective study on the secondary cancer risk in carbon ion radiotherapy

Objective. A retrospective study on secondary cancer risk on carbon ion radiotherapy (CIRT) is ongoing at the Heavy Ion Medical Accelerator in Chiba (HIMAC). The reconstruction of the whole-body patient dose distribution is the key issue in the study because dose distribution only around the planning target volume was evaluated in the treatment planning system. Approach. We therefore developed a new dose reconstruction system based on the Particle and Heavy Ion Transport code System (PHITS) coupled with the treatment plan DICOM data set by extending the functionalities of RadioTherapy package based on PHITS (RT-PHITS). In the system, the geometry of patient-specific beam devices such as the range shifter, range compensator, and collimators as well as the individual patient’s body are automatically reconstructed. Various functions useful for retrospective analysis on the CIRT are implemented in the system, such as those for separately deducing dose contributions from different secondary particles and their origins. Main results. The accuracy of the developed system was validated by comparing the dose distribution to the experimental data measured in a water tank and using a treatment plan on an anthropomorphic phantom. Significance. The extended RT-PHITS will be used in epidemiological studies based on clinical data from HIMAC.

Carbon Ion Radiotherapy for Locally Recurrent Rectal Cancer of Patients with Prior Pelvic Irradiation

BACKGROUND

This study aimed to assess the safety and efficacy of carbon-ion radiotherapy (CIRT) for salvage of previously X-ray-irradiated (XRT) locally recurrent rectal cancer (LRRC).

METHODS

Between September 2005 and December 2017, 77 patients with LRRC were treated with CIRT re-irradiation. All the patients had received prior XRT with a median dose of 50.0 Gy (range 20-74 Gy), principally for neoadjuvant or adjuvant recurrence prophylaxis in 34 patients and for recurrence in 43 patients. The total CIRT dose of 70.4 Gy (RBE) (gray relative biologic effectiveness) was administered in 16 fixed fractions during 4 weeks (4.4 Gy [RBE] per fraction).

RESULTS

All the patients completed the scheduled treatment course. None of the patients received resection after CIRT. Acute grade 3 toxicities occurred for eight patients (10 %), including five grade 3 pelvic infections (2 involving pain and 1 involving neuropathy). Late grade 3 toxicities occurred for 16 patients (21 %): 13 with late grade 3 pelvic infections, 9 with gastrointestinal toxicity, 1 with skin toxicity, 2 with pain, and 4 with neuropathy. No grade 4+ toxicity was noted. The overall local control rates (infield + out-of-field recurrence) were 69 % at 3 years and 62 % at 5 years. In the planning target volume (PTV), the infield recurrence rates were 90 % and 87 % respectively. The control rates for regional recurrence were 85 % at 3 years and 81 % at 5 years. The median overall survival time was 47 months. The survival rates were 61 % at 3 years and 38 % at 5 years.

CONCLUSION

Carbon-ion re-irradiation of previously X-ray-irradiated locally recurrent rectal cancer appears to be safe and effective, providing good local control and survival advantage without unacceptable morbidity.

A systematic review on the usage of averaged LET in radiation biology for particle therapy

Linear Energy Transfer (LET) is widely used to express the radiation quality of ion beams, when characterizing the biological effectiveness. However, averaged LET may be defined in multiple ways, and the chosen definition may impact the resulting reported value. We review averaged LET definitions found in the literature, and quantify which impact using these various definitions have for different reference setups. We recorded the averaged LET definitions used in 354 publications quantifying the relative biological effectiveness (RBE) of hadronic beams, and investigated how these various definitions impact the reported averaged LET using a Monte Carlo particle transport code. We find that the kind of averaged LET being applied is, generally, poorly defined. Some definitions of averaged LET may influence the reported averaged LET values up to an order of magnitude. For publications involving protons, most applied dose averaged LET when reporting RBE. The absence of what target medium is used and what secondary particles are included further contributes to an ill-defined averaged LET. We also found evidence of inconsistent usage of averaged LET definitions when deriving LET-based RBE models. To conclude, due to commonly ill-defined averaged LET and to the inherent problems of LET-based RBE models, averaged LET may only be used as a coarse indicator of radiation quality. We propose a more rigorous way of reporting LET values, and suggest that ideally the entire particle fluence spectra should be recorded and provided for future RBE studies, from which any type of averaged LET (or other quantities) may be inferred.

The Emerging Potential of Multi-Ion Radiotherapy

Research into high linear energy transfer (LET) radiotherapy now spans over half a century, beginning with helium and deuteron treatment in 1952 and today ranging from fast neutrons to carbon-ions. Owing to pioneering work initially in the United States and thereafter in Germany and Japan, increasing focus is on the carbon-ion beam: 12 centers are in operation, with five under construction and three in planning. While the carbon-ion beam has demonstrated unique and promising suitability in laboratory and clinical trials toward the hypofractionated treatment of hypoxic and/or radioresistant cancer, substantial developmental potential remains. Perhaps most notable is the ability to paint LET in a tumor, theoretically better focusing damage delivery within the most resistant areas. However, the technique may be limited in practice by the physical properties of the beams themselves. A heavy-ion synchrotron may provide irradiation with multiple heavy-ions: carbon, helium, and oxygen are prime candidates. Each ion varies in LET distribution, and so a methodology combining the use of multiple ions into a uniform LET distribution within a tumor may allow for even greater treatment potential in radioresistant cancer.