Abstract Body

Preclinical studies have shown that radiotherapy treatment at ultra-high dose rates (FLASH) has the potential to significantly increase the therapeutic window in radiotherapy. As a step towards introducing FLASH radiotherapy in the clinic for the treatment of human patients, we are using a 10 MeV electron beam from a clinical linear accelerator (Elekta Precise, Elekta AB, Stockholm, Sweden), which has been modified to deliver FLASH dose rates, for treatment of canine patients suffering from various spontaneous superficial solid tumors or microscopic residual disease. Our studies are performed at Skåne University Hospital (Lund, Sweden) in collaboration with veterinarians from the University of Copenhagen. Our aim is to give our canine patients the best treatment option available, in order to reduce their suffering, prolong their life and hopefully eradicate their tumors, while learning how to deliver the treatment with improved dosimetric accuracy and improve our radiobiological knowledge on how tumors and normal healthy tissue respond to FLASH radiotherapy.

Our setup consist of a short electron applicator with various Cerrobend inserts to shape the beam aperture and a 5 cm air gap between the end of the applicator and the patient, resulting in a source-to-surface distance of 70 cm. Currently, our dosimetry is reliant on radiochromic film measurement in preparation for treatment and as in vivo dosimeters during treatment, as well as a farmer-type ionization chamber positioned at a fixed position in the applicator for “live” dose measurements. However, the precision of the “live” dose measurements from the ionization chamber is severely hampered by the poor ion collection efficiency at the high dose-per-pulse values (≈2 Gy).

Our first veterinarian study was a dose escalation study aimed to evaluate the feasibility and safety of FLASH radiotherapy in a relevant clinical setting. Ten canine cancer patients were included in this initial study; seven patients with nine solid superficial tumors and three patients with microscopic disease. The treatment was administered in a single fraction, with a treatment dose starting at 15 Gy, which was then escalated in 5 Gy steps to 35 Gy. Treatments resulted in partial response, complete response or stable disease in 11 of the 13 irradiated tumors. Adverse events observed at follow-up, ranging from 3-6 months, were mild and consisted of local alopecia, leukotricia, dry desquamation, mild erythema or swelling. One patient receiving a 35 Gy dose to the nasal planum, had a grade 3 skin adverse event. From this study and subsequent treatments, we found that the maximum tolerated dose to the skin surface was 30 Gy in a single fraction which could be increased to at least 35-40 Gy at depths beyond the skin surface.

The experience from this initial study will be used as a basis for a veterinary phase II clinical trial with more specific patient inclusion selection, and subsequently for human trials. In future studies, we aim to improve our dosimetric accuracy, investigate hypofractionated treatments vs. single fraction treatments, and improved conformality of our treatments with the use of dedicated treatment planning and bolus electron conformal therapy (BECT).

Abstract Body

Preclinical studies have shown that radiotherapy treatment at ultra-high dose rates (FLASH) has the potential to significantly increase the therapeutic window in radiotherapy. As a step towards introducing FLASH radiotherapy in the clinic for the treatment of human patients, we are using a 10 MeV electron beam from a clinical linear accelerator (Elekta Precise, Elekta AB, Stockholm, Sweden), which has been modified to deliver FLASH dose rates, for treatment of canine patients suffering from various spontaneous superficial solid tumors or microscopic residual disease. Our studies are performed at Skåne University Hospital (Lund, Sweden) in collaboration with veterinarians from the University of Copenhagen. Our aim is to give our canine patients the best treatment option available, in order to reduce their suffering, prolong their life and hopefully eradicate their tumors, while learning how to deliver the treatment with improved dosimetric accuracy and improve our radiobiological knowledge on how tumors and normal healthy tissue respond to FLASH radiotherapy.

Our setup consist of a short electron applicator with various Cerrobend inserts to shape the beam aperture and a 5 cm air gap between the end of the applicator and the patient, resulting in a source-to-surface distance of 70 cm. Currently, our dosimetry is reliant on radiochromic film measurement in preparation for treatment and as in vivo dosimeters during treatment, as well as a farmer-type ionization chamber positioned at a fixed position in the applicator for “live” dose measurements. However, the precision of the “live” dose measurements from the ionization chamber is severely hampered by the poor ion collection efficiency at the high dose-per-pulse values (≈2 Gy).

Our first veterinarian study was a dose escalation study aimed to evaluate the feasibility and safety of FLASH radiotherapy in a relevant clinical setting. Ten canine cancer patients were included in this initial study; seven patients with nine solid superficial tumors and three patients with microscopic disease. The treatment was administered in a single fraction, with a treatment dose starting at 15 Gy, which was then escalated in 5 Gy steps to 35 Gy. Treatments resulted in partial response, complete response or stable disease in 11 of the 13 irradiated tumors. Adverse events observed at follow-up, ranging from 3-6 months, were mild and consisted of local alopecia, leukotricia, dry desquamation, mild erythema or swelling. One patient receiving a 35 Gy dose to the nasal planum, had a grade 3 skin adverse event. From this study and subsequent treatments, we found that the maximum tolerated dose to the skin surface was 30 Gy in a single fraction which could be increased to at least 35-40 Gy at depths beyond the skin surface.

The experience from this initial study will be used as a basis for a veterinary phase II clinical trial with more specific patient inclusion selection, and subsequently for human trials. In future studies, we aim to improve our dosimetric accuracy, investigate hypofractionated treatments vs. single fraction treatments, and improved conformality of our treatments with the use of dedicated treatment planning and bolus electron conformal therapy (BECT).

Background and Aims

We have previously found that FLASH-irradiation with a pulsed electron beam (average doserate ≥600Gy/s) was less efficient to sterilize cancer cells in-vitro compared with conventional doserate irradiation (CONV, 0.2Gy/s). In the current work we aimed at investigating the effect for a malignant cell line both in vitro and in vivo.

Methods

Radiation response of melanoma cell line B16_F10 was determined in-vitro by clonogenic assays for an absorbed dose in the range 0-9 Gy comparing FLASH to CONV. In-vivo-response was studied in a syngeneic mice model (C57BL/6J) with subcutaneously injected B16_F10-tumors, irradiated to an absorbed dose of 15, 20 or 25Gy (FLASH and CONV). The tumor growth was quantified by using the relative tumor volume, normalized to unity at the time of irradiation, TV_rel.

Results

The in-vitro results showed a significantly increased survival after FLASH compared with CONV (F-test: p=0.02). Tumor growth curves in-vivo were similar for CONV and FLASH at 15 and 20Gy, but FLASH was relatively less efficient at 25Gy. Four weeks after irradiation with 25Gy, a relative tumor volume of TV_rel<1 was seen in 2/9 mice in the CONV group but in 0/8 mice in the FLASH group. A relative tumor volume of TV_rel<4 was seen in 5/9 mice in the CONV group but 0/8 mice in the FLASH group. Severe skin toxicity was observed in 5/9 vs 0/8.

Conclusions

FLASH may be less efficient than CONV to sterilize malignant cells in-vitro as well as in-vivo. Future work will address the differential response between normal tissue and tumors at higher doses.

Background and Aims

FLASH proton therapy (FLASH-PT) is beginning the transition into clinical practice. The beam delivery, dose rates, and fractionation schemes for FLASH-PT differ from standard radiotherapy. Hence, no guidelines exist regarding the development of treatment plans for this novel technique. This study aims to determine if FLASH-PT treatment plans can be developed and if the in silico results are comparable to the treatment plans produced for standard radiotherapy.

Methods

FLASH-PT and IMPT treatment plans were created using a novel research version of the MIROpt TPS, developed by Ion Beam Applications SA from the open source version of UCLouvain, for nine patient cases of bone (3), brain (3), and lung metastases (3), previously clinically treated with 3DCRT or VMAT. A FLASH proton dose rate of ≥ 40 Gy/s was included as an optimisation criterion for each patient case. Treatment plans were compared using dose volume histograms (DVHs), boxplots, and the Wilcoxon Rank Sum Test with a 5% significance level and using DVH parameters V100%, V95%, V50%, D99%, D95%, and D2% for target and body structures.

Results

No significant differences were found between the optimised FLASH-PT plans and the clinical 3DCRT/VMAT and optimised IMPT plans.

Conclusions

The FLASH-PT treatment plans created in this study produced in silico results comparable to those of clinically competitive treatment plans. Future work involves the verification of the calculated dose against delivered dose and dose rate, to ensure that the produced treatment plans can be delivered safely and accurately, confirming the feasibility of the clinical implementation of conformal FLASH-PT.

Background and Aims

For a clinical translation of FLASH radiotherapy, evidence of an enhanced therapeutic index within the same biological system is needed. In this study we aim to simultaneously investigate the tumor response (TCP) and skin toxicity (NTCP) of hypofractionated FLASH compared to conventional radiotherapy (CONV) in a fully immunocompetent rat glioma model.

Methods

Fisher 344 rats with subcutaneously inoculated NS1 glioma cells were treated with FLASH (450-550 Gy/s) or CONV in three fractions of either 8 Gy, 12.5 Gy or 15 Gy (n=9-10) using a 10 MeV electron beam. Animals were followed for 100 days. The tumor control probability was determined as the ratio of animals with no tumor progression at the end of the study period. Local dermal side effects were evaluated weekly.

Results

There was a statistically significant difference in overall survival between controls and all treatment groups, but no significant difference between FLASH and CONV for any of the dose levels (log-rank test). The tumor control probability for 3x8 Gy, 3x12.5 Gy and 3x15 Gy, respectively, were 3/9, 5/10 and 9/10 for CONV and 3/10, 5/9 and 9/10 for FLASH. Local dermal side effects were generally mild, consisting of hair loss, erythema, and dry desquamation. The ratio of animals with erythema or more severe effects during the study period was 78%/60%/90% for CONV and 50%/78%/70% for FLASH.

Conclusions

In this study we demonstrate that hypofractionated FLASH is equally effective as CONV in terms of controlling glioma in rats. No difference in acute effects could be resolved. Late effects will be histologically evaluated.

Background and Aims

Radiation-induced gastrointestinal toxicity is a dose-limiting factor in radiotherapy. Recent preclinical studies have shown in multiple models that irradiation at ultra-high dose rates (FLASH) reduces normal tissue toxicity compared to irradiation at conventional dose rates. Here we used a 6 MeV electron LINAC to study acute normal tissue toxicity in C3H mice, where the whole abdomen was irradiated to various radiation doses with FLASH irradiation (mean dose rate = 2-6 MGy/s) or conventional dose rate irradiation (mean = 15 Gy/min). Subsequently, the importance of the temporal pulse structure for the FLASH sparing effect was investigated by varying the number of pulses and the different time interval between two pulses.

Methods

Mice were weighed daily and culled 3.75 days after irradiation and the gastrointestinal systems collected for histological assessment. Stools were collected one day before mice culling to assess the gastrointestinal functionality. Normal tissue damage was quantified using a modified Swiss roll-based crypt assay. Whole blood and plasma were also collected for immunological assessment.

Results

Compared to conventional irradiation, FLASH irradiation showed a dose modifying factor of ≈1.1(a 10% higher dose was required for FLASH irradiation to cause the same level of acute toxicity as conventional dose rate irradiation). Mice irradiated with FLASH also showed reduced weight loss compared to mice that received conventional irradiation. Overall, the normal tissue sparing effect seen in our study seemed to be primarily governed by the mean dose rate.

Conclusions

This study further demonstrates the clinical potential of FLASH radiotherapy, with its capacity of sparing gastrointestinal normal tissue.

Background and Aims

In vivo studies report the normal tissue sparing effects of FLASH radiotherapy compared to conventional dose rate therapy (CONV). Towards this, it is proposed that FLASH causes a transient/local hypoxia so reducing the level of induced DNA damage for FLASH vs. CONV exposure. Therefore, aim of this study is to assess the levels of DNA damage in whole blood PBLs induced by FLASH vs. CONV irradiation under different oxygen concentrations ex vivo.

Methods

Samples of whole blood were irradiated at different oxygen concentrations (ranging between 0.25 and 21%) with a 6 MeV electron beam at a dose rate of either 2 kGy/s (FLASH) or 0.1 Gy/s (CONV). Induced DNA damage levels in whole blood PBLs were evaluated by alkaline Comet assay after 20 Gy irradiation.

Results

<10% difference was noted between the DNA damage levels induced following FLASH and CONV irradiation under normoxic conditions (21% O₂). However, following irradiation over 0.25-1% oxygen, lower levels of DNA damage were clearly induced following FLASH exposure, the difference being significant at 0.5% O₂. Furthermore, dose-response studies show the FLASH effect to be lost at ≤10Gy.

Conclusions

This study extends our earlier preliminary findings and substantiates that lower levels of DNA damage are induced following FLASH exposure under low oxygen tension, supporting transient radiation-induced hypoxia/ transient oxygen depletion as a mechanism possibly contributing to the tissue sparing effect of FLASH radiotherapy. Further studies to characterise the induced-damage manifest by FLASH and to dissect/differentiate the exact mechanisms involved (radical-radical recombination vs. transient oxygen depletion) will be presented/discussed.