Evaluation of the relative biological effectiveness of spot-scanning proton irradiation in vitro

A Single-Institution Prospective Study To Evaluate the Safety and Efficacy of Real- Time Image-Gated Spot-Scanning Proton Therapy (RGPT) for Prostate Cancer

Purpose

In real-time image-gated spot-scanning proton therapy (RGPT), the dose distribution is distorted by gold fiducial markers placed in the prostate. Distortion can be suppressed by using small markers and more than 2 fields, but additional fields may increase the dose to organs at risk. Therefore, we conducted a prospective study to evaluate the safety and short-term clinical outcome of RGPT for prostate cancer.

Methods and Materials

Based on the previously reported frequency of early adverse events (AE) and the noninferiority margin of 10%, the required number of cases was calculated to be 43 using the one-sample binomial test by the Southwest Oncology Group statistical tools with the one-sided significance level of 2.5% and the power 80%. Patients with localized prostate cancer were enrolled and 3 to 4 pure gold fiducial markers of 1.5-mm diameter were inserted in the prostate. The prescribed dose was 70 Gy(relative biologic effectiveness) in 30 fractions, and treatment was performed with 3 fields from the left, right, and the back, or 4 fields from either side of slightly anterior and posterior oblique fields. The primary endpoint was the frequency of early AE (≥grade 2) and the secondary endpoint was the biochemical relapse-free survival rate and the frequency of late AE.

Results

Forty-five cases were enrolled between 2015 and 2017, and all patients completed the treatment protocol. The median follow-up period was 63.0 months. The frequency of early AE (≥grade 2) was observed in 4 cases (8.9%), therefore the noninferiority was verified. The overall 5-year biochemical relapse-free survival rate was 88.9%. As late AE, grade 2 rectal bleeding was observed in 8 cases (17.8%).

Conclusions

The RGPT for prostate cancer with 1.5-mm markers and 3- or 4- fields was as safe as conventional proton therapy in early AE, and its efficacy was comparable with previous studies.

A Consistent Protocol Reveals a Large Heterogeneity in the Biological Effectiveness of Proton and Carbon-Ion Beams for Various Sarcoma and Normal-Tissue-Derived Cell Lines

Simple Summary

Using a consistent experimental protocol, we found a large heterogeneity in the relative biological effectiveness (RBE) values of both proton and carbon-ion beams in various sarcomas and normal-tissue-derived cell lines. Our data suggest that proton beam therapy may be more beneficial for some types of tumors. In carbon-ion therapy, for some types of tumors, large heterogeneity in RBE should prompt consideration of dose reduction or an increased dose per fraction. In particular, a higher RBE value in normal tissues requires caution. Specific dose evaluations for tumor and normal tissues are needed for both proton and carbon-ion therapies.

Abstract

This study investigated variations in the relative biological effectiveness (RBE) values among various sarcoma and normal-tissue-derived cell lines (normal cell line) in proton beam and carbon-ion irradiations. We used a consistent protocol that specified the timing of irradiation after plating cells and detailed the colony formation assay. We examined the cell type dependence of RBE for proton beam and carbon-ion irradiations using four human sarcoma cell lines (MG63 osteosarcoma, HT1080 fibrosarcoma, SW872 liposarcoma, and SW1353 chondrosarcoma) and three normal cell lines (HDF human dermal fibroblast, hTERT-HME1 mammary gland, and NuLi-1 bronchus epithelium). The cells were irradiated with gamma rays, proton beams at the center of the spread-out Bragg peak, or carbon-ion beams at 54.4 keV/μm linear energy transfer. In all sarcoma and normal cell lines, the average RBE values in proton beam and carbon-ion irradiations were 1.08 ± 0.11 and 2.08 ± 0.36, which were consistent with the values of 1.1 and 2.13 used in current treatment planning systems, respectively. Up to 34% difference in the RBE of the proton beam was observed between MG63 and HT1080. Similarly, a 32% difference in the RBE of the carbon-ion beam was observed between SW872 and the other sarcoma cell lines. In proton beam irradiation, normal cell lines had less variation in RBE values (within 10%), whereas in carbon-ion irradiation, RBE values differed by up to 48% between hTERT-HME1 and NuLi-1. Our results suggest that specific dose evaluations for tumor and normal tissues are necessary for treatment planning in both proton and carbon-ion therapies.

Proton Therapy for Prostate Cancer: Challenges and Opportunities

Simple Summary

Reported clinical outcomes of proton therapy (PT) for localized prostate cancer are similar to photon-based external beam radiotherapy. Apparently, the dosimetric advantages of PT have yet to be translated to clinical benefits. The suboptimal clinical outcomes of PT might be attributable to inadequate dose prescription, as indicated by the ASCENDE-RT trial. Moreover, uncertainties involved in the treatment planning and delivery processes, as well as technological limitations in PT treatment systems, may lead to discrepancies between planned doses and actual doses delivered to patients. In this article, we reviewed the current status of PT for prostate cancer and discussed different clinical implementations that could potentially improve the clinical outcome of PT for prostate cancer. Various technological advancements under which uncertainties in dose calculations can be minimized, including MRI-guided PT, dual-energy photon-counting CT and high-resolution Monte Carlo-based treatment planning systems, are highlighted.

Abstract

The dosimetric advantages of proton therapy (PT) treatment plans are demonstrably superior to photon-based external beam radiotherapy (EBRT) for localized prostate cancer, but the reported clinical outcomes are similar. This may be due to inadequate dose prescription, especially in high-risk disease, as indicated by the ASCENDE-RT trial. Alternatively, the lack of clinical benefits with PT may be attributable to improper dose delivery, mainly due to geometric and dosimetric uncertainties during treatment planning, as well as delivery procedures that compromise the dose conformity of treatments. Advanced high-precision PT technologies, and treatment planning and beam delivery techniques are being developed to address these uncertainties. For instance, external magnetic resonance imaging (MRI)-guided patient setup rooms are being developed to improve the accuracy of patient positioning for treatment. In-room MRI-guided patient positioning systems are also being investigated to improve the geometric accuracy of PT. Soon, high-dose rate beam delivery systems will shorten beam delivery time to within one breath hold, minimizing the effects of organ motion and patient movements. Dual-energy photon-counting computed tomography and high-resolution Monte Carlo-based treatment planning systems are available to minimize uncertainties in dose planning calculations. Advanced in-room treatment verification tools such as prompt gamma detector systems will be used to verify the depth of PT. Clinical implementation of these new technologies is expected to improve the accuracy and dose conformity of PT in the treatment of localized prostate cancers, and lead to better clinical outcomes. Improvement in dose conformity may also facilitate dose escalation, improving local control and implementation of hypofractionation treatment schemes to improve patient throughput and make PT more cost effective.

Impact of a spatially dependent dose delivery time structure on the biological effectiveness of scanning proton therapy

PURPOSE

In the scanning beam delivery of protons, different portions of the target are irradiated with different linear energy transfer protons with various time intervals and irradiation times. This research aimed to evaluate the spatially dependent biological effectiveness of protracted irradiation in scanning proton therapy.

METHODS

One and two parallel opposed fields plans were created in water phantom with the prescribed dose of 2 Gy. Three scenarios (instantaneous, continuous, and layered scans) were used with the corresponding beam delivery models. The biological dose (physical dose × relative biological effectiveness) was calculated using the linear quadratic model and the theory of dual radiation action to quantitatively evaluate the dose delivery time effect. In addition, simulations using clinical plans (postoperative seminoma and prostate tumor cases) were conducted to assess the impact of the effects on the dose volume histogram parameters and homogeneity coefficient (HC) in targets.

RESULTS

In a single-field plan of water phantom, when the treatment time was 19 min, the layered-scan scenario showed a decrease of <0.2% (almost 3.3%) in the biological dose from the plan on the distal (proximal) side because of the high (low) dose rate. This is in contrast to the continuous scenario, where the biological dose was almost uniformly decreased over the target by approximately 3.3%. The simulation with clinical geometry showed that the decrease rates in D_99% were 0.9% and 1.5% for every 10 min of treatment time prolongation for postoperative seminoma and prostate tumor cases, respectively, whereas the increase rates in HC were 0.7% and 0.2%.

CONCLUSIONS

In protracted irradiation in scanning proton therapy, the spatially dependent dose delivery time structure in scanning beam delivery can be an important factor for accurate evaluation of biological effectiveness.

Models for Translational Proton Radiobiology-From Bench to Bedside and Back

The number of proton therapy centers worldwide are increasing steadily, with more than two million cancer patients treated so far. Despite this development, pending questions on proton radiobiology still call for basic and translational preclinical research. Open issues are the on-going discussion on an energy-dependent varying proton RBE (relative biological effectiveness), a better characterization of normal tissue side effects and combination treatments with drugs originally developed for photon therapy. At the same time, novel possibilities arise, such as radioimmunotherapy, and new proton therapy schemata, such as FLASH irradiation and proton mini-beams. The study of those aspects demands for radiobiological models at different stages along the translational chain, allowing the investigation of mechanisms from the molecular level to whole organisms. Focusing on the challenges and specifics of proton research, this review summarizes the different available models, ranging from in vitro systems to animal studies of increasing complexity as well as complementing in silico approaches.

Technical Note: First report on an in vivo range probing quality control procedure for scanned proton beam therapy in head and neck cancer patients

PURPOSE

The capability of proton therapy to provide highly conformal dose distributions is impaired by range uncertainties. The aim of this work is to apply range probing (RP), a form of a proton radiography-based quality control (QC) procedure for range accuracy assessment in head and neck cancer (HNC) patients in a clinical setting.

METHODS AND MATERIALS

This study included seven HNC patients. RP acquisition was performed using a multi-layer ionization chamber (MLIC). Per patient, two RP frames were acquired within the first two weeks of treatment, on days when a repeated CT scan was obtained. Per RP frame, integral depth dose (IDD) curves of 81 spots around the treatment isocenter were acquired. Range errors are determined as a discrepancy between calculated IDDs in the treatment planning system and measured residual ranges by the MLIC. Range errors are presented relative to the water equivalent path length of individual proton spots. In addition to reporting results for complete measurement frames, an analysis, excluding range error contributions due to anatomical changes, is presented.

RESULTS

Discrepancies between measured and calculated ranges are smaller when performing RP calculations on the day-specific patient anatomy rather than the planning CT. The patient-specific range evaluation shows an agreement between calculated and measured ranges for spots in anatomically consistent areas within 3% (1.5 standard deviation).

CONCLUSIONS

The results of an RP-based QC procedure implemented in the clinical practice for HNC patients have been demonstrated. The agreement of measured and simulated proton ranges confirms the 3% uncertainty margin for robust optimization. Anatomical variations show a predominant effect on range accuracy, motivating efforts towards the implementation of adaptive radiotherapy.

Are simple verbal instructions sufficient to ensure that bladder volume does not deteriorate prostate position reproducibility during spot scanning proton therapy?

Objectives:

The purpose of this study is to investigate whether verbal instructions are sufficient for bladder volume (BV) control not to deteriorate prostate position reproducibility in image-guided spot scanning proton therapy (SSPT) for localized prostate cancer.

Methods:

A total of 268 treatment sessions in 12 consecutive prostate cancer patients who were treated with image-guided SSPT with fiducial markers were retrospectively analyzed. In addition to strict rectal volume control procedures, simple verbal instructions to void urine one hour before the treatment were used here. The BV was measured by a Bladder Scan just before the treatment, and the prostate motion was measured by intraprostatic fiducial markers and two sets of X-ray fluoroscopy images. The correlation between the BV change and prostate motion was assessed by linear mixed-effects models and systematic and random errors according to the reproducibility of the BV.

Results:

The mean absolute BV change during treatment was from −98.7 to 86.3 ml (median 7.1 ml). The mean absolute prostate motion of the patients in the left-right direction was −1.46 to 1.85 mm; in the cranial-caudal direction it was −6.10 to 3.65 mm, and in the anteroposterior direction −1.90 to 5.23 mm. There was no significant relationship between the BV change and prostate motion during SSPT. The early and late genitourinary and gastrointestinal toxicity was minimal with a minimum follow up of 4.57 years.

Conclusions:

Simple verbal instructions about urination was suggested to be sufficient to control the BV not to impact on the prostate motion and clinical outcomes in image-guided SSPT. Careful attention to BV change is still needed when the seminal vesicle is to be treated.

Advances in knowledge:

Our data demonstrated that there was no apparent relationship between BV changes and prostate position reproducibility and simple verbal instruction about urination could be sufficient for image-guided SSPT.

A High-Precision Method for In Vitro Proton Irradiation

Purpose

Although proton therapy has become a well-established radiation modality, continued efforts are needed to improve our understanding of the molecular and cellular mechanisms occurring during treatment. Such studies are challenging, requiring many resources. The purpose of this study was to create a phantom that would allow multiple in vitro experiments to be irradiated simultaneously with a spot-scanning proton beam.

Materials and Methods

The setup included a modified patient-couch top coupled with a high-precision robotic arm for positioning. An acrylic phantom was created to hold 4 6-well cell-culture plates at 2 different positions along the Bragg curve in a reproducible manner. The proton treatment plan consisted of 1 large field encompassing all 4 plates with a monoenergetic 76.8-MeV posterior beam. For robust delivery, a mini pyramid filter was used to broaden the Bragg peak (BP) in the depth direction. Both a Markus ionization chamber and EBT3 radiochromic film measurements were used to verify absolute dose.

Results

A treatment plan for the simultaneous irradiation of 2 plates irradiated with high linear energy transfer protons (BP, 7 keV/μm) and 2 plates irradiated with low linear energy transfer protons (entrance, 2.2 keV/μm) was created. Dose uncertainty was larger across the setup for cell plates positioned at the BP because of beam divergence and, subsequently, variable proton-path lengths. Markus chamber measurements resulted in uncertainty values of ±1.8% from the mean dose. Negligible differences were seen in the entrance region (<0.3%).

Conclusion

The proposed proton irradiation setup allows 4 plates to be simultaneously irradiated with 2 different portions (entrance and BP) of a 76.8-MeV beam. Dosimetric uncertainties across the setup are within ±1.8% of the mean dose.

Particle therapy in the future of precision therapy

The first hospital-based treatment facilities for particle therapy started operation about thirty years ago. Since then, the clinical experience with protons and carbon ions has grown continuously and more than 200,000 patients have been treated to date. The promising clinical results led to a rapidly increasing number of treatment facilities and many new facilities are planned or under construction all over the world. An inverted depth-dose profile combined with potential radiobiological advantages make charged particles a precious tool for the treatment of tumours that are particularly radioresistant or located nearby sensitive structures. A rising number of trials have already confirmed the benefits of particle therapy in selected clinical situations and further improvements in beam delivery, image guidance and treatment planning are expected. This review summarises some physical and biological characteristics of accelerated charged particles and gives some examples of their clinical application. Furthermore, challenges and future perspectives of particle therapy will be discussed.

Investigating the impact of alpha/beta and LETd on relative biological effectiveness in scanned proton beams: An in vitro study based on human cell lines

PURPOSE

A relative biological effectiveness (RBE) of 1.1 is commonly used in clinical proton therapy, irrespective of tissue type and depth. This in vitro study was conducted to quantify the RBE of scanned protons as a function of the dose-averaged linear energy transfer (LET_d ) and the sensitivity factor (α/ß)_X . Additionally, three phenomenological models (McNamara, Rørvik, and Jones) and one mechanistic model (repair-misrepair-fixation, RMF) were applied to the experimentally derived data.

METHODS

Four human cell lines (FaDu, HaCat, Du145, SKMel) with differential (α/ß)_X ratios were irradiated in a custom-designed irradiation setup with doses between 0 and 6 Gy at proximal, central, and distal positions of a 80 mm spread-out Bragg peak (SOBP) centered at 80 mm (setup A: proton energies 66.5-135.6 MeV) and 155 mm (setup B: proton energies 127.2-185.9 MeV) depth, respectively. LET_d values at the respective cell positions were derived from Monte Carlo simulations performed with the treatment planning system (TPS, RayStation). Dosimetric measurements were conducted to verify dose homogeneity and dose delivery accuracy. RBE values were derived for doses that resulted in 90 % (RBE₉₀ ) and 10 % (RBE₁₀ ) of cell survival, and survival after a 0.5 Gy dose (RBE_0.5Gy ), 2 Gy dose (RBE_2Gy ), and 6 Gy dose (RBE_6Gy ).

RESULTS

LET_d values at sample positions were 1.9, 2.1, 2.5, 2.8, 4.1, and 4.5 keV/µm. For the cell lines with high (α/ß)_X ratios (FaDu, HaCat), the LET_d did not impact on the RBE. For low (α/ß)_X cell lines (Du145, SKMel), LQ-derived survival curves indicated a clear correlation of LET_d and RBE. RBE₉₀ values up to 2.9 and RBE₁₀ values between 1.4 and 1.8 were obtained. Model-derived RBE predictions slightly overestimated the RBE for the high (α/ß)_X cell lines, although all models except the Jones model provided RBE values within the experimental uncertainty. For low (α/ß)_X cell lines, no agreement was found between experiments and model predictions, that is, all models underestimated the measured RBE.

CONCLUSIONS

The sensitivity parameter (α/ß)_X was observed to be a major influencing factor for the RBE of protons and its sensitivity toward LET_d changes. RBE prediction models are applicable for high (α/ß)_X cell lines but do not estimate RBE values with sufficient accuracy in low (α/ß)_X cell lines.

Radiation biology considerations of proton therapy for gastrointestinal cancers

Clinical enthusiasm for proton therapy (PT) is high, with an exponential increase in the number of centers offering treatment. Attraction for this charged particle therapy modality stems from the favorable proton dose distribution, with low radiation dose absorption on entry and maximum radiation deposition at the Bragg peak. The current clinical convention is to use a fixed relative biological effectiveness (RBE) value of 1.1 in order to correct the physical dose relative to photon therapy (i.e., proton radiation is 10% more biologically effective then photon radiation). In recent years, concerns about the potential side effects of PT have emerged. Various studies and review articles have sought to better quantify the RBE of PT and shine some light on the complexity of this problem. Reduction in biologic hot spots of non-target tissue is paramount in proton radiation therapy (RT) planning as the primary benefit of proton RT is a reduction in organ at risk (OAR) irradiation. New and emerging clinical data is in support of variable proton biological effectiveness and demonstrate late toxicity, presumably associated with high biological dose, to OAR. Overall, PT has promise to treat many cancer sites with similar efficacy as conventional RT but with fewer acute and late toxicities. However, further knowledge of biologic effective dose and its impact on both cancer and adjacent OAR is paramount for effective and safe treatment of patients with PT.

Nucleoside analogs as a radiosensitizer modulating DNA repair, cell cycle checkpoints, and apoptosis

The combination of low dose of radiation and an anticancer drug is a potent strategy for cancer therapy. Nucleoside analogs are known to have a radiosensitizing effects via the inhibition of DNA damage repair after irradiation. Certain types of nucleoside analogs have the inhibitory effects on RNA synthesis, but not DNA synthesis, with multiple functions in cell cycle modulation and apoptosis. In this review, the most up-to-date findings regarding radiosensitizing nucleoside analogs will be discussed, focusing especially on the mechanisms of action.

Biologic Dose and Imaging Changes in Pediatric Brain Tumor Patients Receiving Spot Scanning Proton Therapy

PURPOSE

To evaluate the incidence of imaging changes in our pediatric brain tumor population treated with spot-scanning proton therapy and analyze the spatial correlation of imaging changes with a novel biologic dose model.

METHODS AND MATERIALS

All pediatric patients treated during the first year of our institution's experience who received a minimum treatment planning dose (TPD) of 5040 cGyE with available follow-up magnetic resonance imaging scans were selected for analysis. Posttreatment magnetic resonance imaging scans were fused with the treatment planning computed tomography. All T1 post-gadolinium enhancement, T2 fluid attenuated inversion recovery changes, TPD, and biologic dose (BD) volumes outside of the original gross tumor volume were contoured for analysis.

RESULTS

Thirty patients were included in the analysis, 7 of whom developed posttreatment radiologic changes. The volumetric overlap of the T2 fluid attenuated inversion recovery changes and BD volumes was significantly greater than the overlap with the TPD volumes. Median volumetric overlaps of 85%, 18%, and 0% were observed with the BD105%, BD110%, and TPD105%, respectively. A nonsignificant increase in the volumetric overlap of the T1C+ changes and BD volumes was also observed. No correlation was observed between the total volume of BD110%, BD105%, or physical dose 105% and the development of imaging changes.

CONCLUSIONS

Within our pediatric brain tumor population treated with spot-scanning proton therapy, our BD model demonstrated superior volumetric overlap with posttreatment T2 changes compared with the TPD model. Using a BD model in treatment planning for spot-scanning proton therapy may help avoid delivery of excessive BD to critical structures and may help minimize the risk of radiation-related late effects.

Clinical experience of craniospinal intensity-modulated spot-scanning proton therapy using large fields for central nervous system medulloblastomas and germ cell tumors in children, adolescents, and young adults

The outcomes of intensity-modulated proton craniospinal irradiation (ipCSI) are unclear. We evaluated the clinical benefit of our newly developed ipCSI system that incorporates two gantry-mounted orthogonal online X-ray imagers with a robotic six-degrees-of-freedom patient table. Nine patients (7-19 years old) were treated with ipCSI. The prescribed dose for CSI ranged from 23.4 to 36.0 Gy (relative biological effectiveness) in 13-20 fractions. Four adolescent and young adult (AYA) patients (15 years or older) were treated with vertebral-body-sparing ipCSI (VBSipCSI). Myelosuppression following VBSipCSI was compared with that of eight AYA patients treated with photon CSI at the same institution previously. The mean homogeneity index (HI) in the nine patients was 0.056 (95% confidence interval: 0.044-0.068). The mean time from the start to the end of all beam delivery was 37 min 39 s ± 2 min 24 s (minimum to maximum: 22 min 49 s - 42 min 51 s). The nadir white blood cell, hemoglobin, and platelet levels during the 4 weeks following the end of the CSI were significantly higher in the VBSipCSI group than in the photon CSI group (P = 0.0071, 0.0453, 0.0024, respectively). The levels at 4 weeks after the end of CSI were significantly higher in the VBSipCSI group than in the photon CSI group (P = 0.0023, 0.0414, 0.0061). Image-guided ipCSI was deliverable in a reasonable time with sufficient HI. Using VBSipCSI, AYA patients experienced a lower incidence of serious acute hematological toxicity than AYA patients treated with photon CSI.

Proton beam therapy for cancer in the era of precision medicine

Precision radiotherapy, which accurately delivers the dose on a tumor and confers little or no irradiation to the surrounding normal tissue and organs, results in maximum tumor control and decreases the toxicity to the utmost extent. Proton beam therapy (PBT) provides superior dose distributions and has a dosimetric advantage over photon beam therapy. Initially, the clinical practice and study of proton beam therapy focused on ocular tumor, skull base, paraspinal tumors (chondrosarcoma and chordoma), and unresectable sarcomas, which responded poorly when treated with photon radiotherapy. Then, it is widely regarded as an ideal mode for reirradiation and pediatrics due to reducing unwanted side effects by lessening the dose to normal tissue. During the past decade, the application of PBT has been rapidly increasing worldwide and gradually expanding for the treatment of various malignancies. However, to date, the role of PBT in clinical settings is still controversial, and there are considerable challenges in its application. We systematically review the latest advances of PBT and the challenges for patient treatment in the era of precision medicine.

Investigating Dependencies of Relative Biological Effectiveness for Proton Therapy in Cancer Cells

Purpose

Relative biological effectiveness (RBE) accounts for the differences in biological effect from different radiation types. The RBE for proton therapy remains uncertain, as it has been shown to vary from the clinically used value of 1.1. In this work we investigated the RBE of protons and correlated the biological differences with the underlying physical quantities.

Materials and Methods

Three cell lines were irradiated (CHO, Chinese hamster ovary; A549, human lung adenocarcinoma; and T98, human glioma) and assessed for cell survival by using clonogenic assay. Cells were irradiated with 71- and 160-MeV protons at depths along the Bragg curve and 6-MV photons to various doses. The dose-averaged lineal energy ( y‒D ) was measured under similar conditions as the cells by using a microdosimeter. Dose-averaged linear energy transfer (LET_d) was also calculated by using Monte Carlo (MC) simulations. Survival data were fit by using the linear quadratic model. The RBE values were calculated by comparing the physical dose (D_6MV/D_p) that results in 50% (RBE_0.5) and 10% (RBE_0.1) cell survival, and survival after 2 Gy (RBE_2Gy).

Results

Proton RBE values ranged from 0.89 to 2.40. The RBE for all 3 cell lines increased with decreasing proton energy and was higher at 50% survival than at 10% survival. Additionally, both A549 and T98 cells generally had higher RBE values relative to the CHO cells, indicating a greater biological response to protons. An increase in RBE corresponded with an increase in y‒D and LET_d.

Conclusion

Proton RBE was found to depend on mean proton energy, survival end point, and cell type. Changes in both y‒D and LET_d were also found to impact proton RBE values, but consideration of the energy spectrum may provide additional information. The RBE values in this study vary greatly, indicating the clinical value of 1.1 may not be suitable in all cases.

Prospective study to evaluate the safety of the world-first spot-scanning dedicated, small 360-degree gantry, synchrotron-based proton beam therapy system

This is a report of a single-institution prospective study evaluating the safety of a spot-scanning dedicated, small 360-degree gantry, synchrotron-based proton beam therapy (PBT) system. Data collection was performed for 56 patients with 59 treatment sites who received proton beam therapy at Hokkaido University Hospital between March 2014 and July 2015. Forty-one patients were male and 15 were female. The median age was 66 years. The primary lesion sites were prostate (n = 17), bone/soft tissue (n = 10), liver (n = 7), lung (n = 6), central nervous system (n = 5), colon (n = 2), pancreas (n = 2), kidney (n = 2) and others (n = 5). Chemotherapy was administered in 11 patients. The prescribed total dose was from 20 to 76 GyE (Radiobiological equivalent dose, RBE = 1.1), with the median dose of 65 GyE in 4 to 35 fractions. No PBT-related Common Terminology Criteria for Adverse Events Grade 4 or 5 toxicities were observed; the incidence of early PBT-related Grade 4 adverse events was 0% (95% confidence interval 0 to 6.38%). The most common Grade 3 toxicities were hematologic toxicity (12.5%) unlikely to be related to the PBT. One patient developed a left femoral neck fracture (Grade 3) at 14.5 months after PBT for chondrosarcoma of the left pelvis. The pathological findings showed no other malignancies, suggesting that it was possibly related to the PBT. In conclusion, the spot-scanning dedicated, synchrotron-based PBT system is feasible, but further studies on its long-term safety and efficacy are warranted.

Selection of external beam radiotherapy approaches for precise and accurate cancer treatment

Physically precise external-beam radiotherapy (EBRT) technologies may not translate to the best outcome in individual patients. On the other hand, clinical considerations alone are often insufficient to guide the selection of a specific EBRT approach in patients. We examine the ways in which to compare different EBRT approaches based on physical, biological and clinical considerations, and how they can be enhanced with the addition of biophysical models and machine-learning strategies. The process of selecting an EBRT modality is expected to improve in tandem with knowledge-based treatment planning.

Investigating Dependencies of Relative Biological Effectiveness for Proton Therapy in Cancer Cells

PURPOSE

Relative biological effectiveness (RBE) accounts for the differences in biological effect from different radiation types. The RBE for proton therapy remains uncertain, as it has been shown to vary from the clinically used value of 1.1. In this work we investigated the RBE of protons and correlated the biological differences with the underlying physical quantities.

MATERIALS AND METHODS

Three cell lines were irradiated (CHO, Chinese hamster ovary; A549, human lung adenocarcinoma; and T98, human glioma) and assessed for cell survival by using clonogenic assay. Cells were irradiated with 71- and 160-MeV protons at depths along the Bragg curve and 6-MV photons to various doses. The dose-averaged lineal energy ( y‒D ) was measured under similar conditions as the cells by using a microdosimeter. Dose-averaged linear energy transfer (LET_d) was also calculated by using Monte Carlo (MC) simulations. Survival data were fit by using the linear quadratic model. The RBE values were calculated by comparing the physical dose (D_6MV/D_p) that results in 50% (RBE_0.5) and 10% (RBE_0.1) cell survival, and survival after 2 Gy (RBE_2Gy).

RESULTS

Proton RBE values ranged from 0.89 to 2.40. The RBE for all 3 cell lines increased with decreasing proton energy and was higher at 50% survival than at 10% survival. Additionally, both A549 and T98 cells generally had higher RBE values relative to the CHO cells, indicating a greater biological response to protons. An increase in RBE corresponded with an increase in y‒D and LET_d.

CONCLUSION

Proton RBE was found to depend on mean proton energy, survival end point, and cell type. Changes in both y‒D and LET_d were also found to impact proton RBE values, but consideration of the energy spectrum may provide additional information. The RBE values in this study vary greatly, indicating the clinical value of 1.1 may not be suitable in all cases.

Relative biological effectiveness in a proton spread-out Bragg peak formed by pencil beam scanning mode

In recent years, there is an increased interest in using scanning modes in proton therapy, due to the more conformal dose distributions, thanks to the spot-weighted dose delivery. The dose rate in each spot is however much higher than the dose rate when using passive irradiation modes, which could affect the cell response. The purpose of this work was to investigate how the relative biological effectiveness changes along the spread-out Bragg peak created by protons delivered by the pencil beam scanning mode. Cell survival and micronuclei formation were investigated in four positions along the spread-out Bragg peak for various doses. Monte Carlo simulations were used to estimate the dose-averaged linear energy transfer values in the irradiation positions. The cell survival was found to decrease the deeper the sample was placed in the spread-out Bragg peak, which corresponds to the higher linear energy transfer values found using Monte Carlo simulations. The micronuclei frequencies indicate more complex cell injuries at that distal position compared to the proximal part of the spread-out Bragg peak. The relative biological effectiveness determined in this study varies significantly and systematically from 1.1, which is recommended value by the International Commission on Radiation Units, in all the studied positions. In the distal position of spread-out Bragg peak the relative biological effectiveness values were found to be 2.05 ± 0.44, 1.85 ± 0.42, 1.53 ± 0.38 for survival levels 90, 50 and 10%, respectively.

Systematics of relative biological effectiveness measurements for proton radiation along the spread out Bragg peak: experimental validation of the local effect model

The purpose of this study is to compare the predictions of the local effect model (LEM) in an extensive analysis to proton relative biological effectiveness (RBE) experiments found in the literature, and demonstrate the capabilities of the model as well as to discuss potential limitations. 19 publications with in vitro experiments and 10 publications with in vivo experiments focusing on proton RBE along the spread out Bragg peak (SOBP) were considered. In total the RBE values of over 100 depth positions were compared to LEM predictions. The treatment planning software TRiP98 was used to reconstruct the proton depth dose profile, and, together with the physical dose distribution, the RBE prediction was conducted based on the LEM. Only parameters from photon dose response curves are used as input for the LEM, and no free parameters are introduced, thus allowing us to demonstrate the predictive power of the LEM for protons. The LEM describes the RBE adequately well within the SOBP region with a relative deviation of typically less than 10% up to 10 keV µm^-1. In accordance with previous publications a clear dependence of RBE on the dose-averaged linear energy transfer (LET_D) was observed. The RBE in the experiments tends to increase above 1.1 for LET_D values above 2 keV µm^-1 and above 1.5 for LET_D values higher than 10 keV µm^-1 (distal part of the SOBP). The dose dependence is most pronounced for doses lower than 3 Gy (RBE). However, both the LEM predictions and experimental data show only a weak dependence of RBE on the tissue type, as characterized by the α/β ratio, which is considered insignificant with regard to the general uncertainties of RBE. The RBE predicted by the LEM shows overall very good agreement with the experimental data within the SOBP region and is in better agreement with the experimental data than the constant RBE of 1.1 that is currently applied in the clinics. All RBE trends deduced from the experiments were also reflected by the LEM predictions, which are purely based on input parameters derived from low-LET photon radiation.

A Nucleoside Anticancer Drug, 1-(3-C-Ethynyl-β-D-Ribo-Pentofuranosyl)Cytosine, Induces Depth-Dependent Enhancement of Tumor Cell Death in Spread-Out Bragg Peak (SOBP) of Proton Beam

The effect of 1-(3-C-ethynyl-β-D-ribo-pentofuranosyl)cytosine (ECyd) on proton-induced cell death was evaluated in human lung carcinoma cell line A549 and Chinese hamster fibroblast cell line V79 to enhance relative biological effectiveness (RBE) within the spread-out Bragg peak (SOBP) of proton beams. Treatment with ECyd significantly enhanced the proton-induced loss of clonogenicity and increased senescence at the center, but not at the distal edge of SOBP. The p53-binding protein 1 foci formation assay showed that ECyd decelerated the rate of DNA double-strand break (DSB) repair at the center, but not the distal region of SOBP, suggesting that the ECyd-induced enhancement of proton-induced cell death is partially associated with the inhibition of DSB repair. This study demonstrated that ECyd enhances proton-induced cell killing at all positions of SOBP, except for the distal region and minimizes the site-dependent differences in RBE within SOBP. Thus, ECyd is a unique radiosensitizer for proton therapy that may be useful because it levels the biological dose within SOBP, which improves tumor control and reduces the risk of adverse effects at the distal edge of SOBP.