Proceedings to the 58th Annual Conference of the Particle Therapy Cooperative Group (PTCOG58)

A potential revolution in cancer treatment: A topical review of FLASH radiotherapy

FLASH radiotherapy (RT) is a novel technique in which the ultrahigh dose rate (UHDR) (≥40 Gy/s) is delivered to the entire treatment volume. Recent outcomes of in vivo studies show that the UHDR RT has the potential to spare normal tissue without sacrificing tumor control. There is a growing interest in the application of FLASH RT, and the ultrahigh dose irradiation delivery has been achieved by a few experimental and modified linear accelerators. The underlying mechanism of FLASH effect is yet to be fully understood, but the oxygen depletion in normal tissue providing extra protection during FLASH irradiation is a hypothesis that attracts most attention currently. Monte Carlo simulation is playing an important role in FLASH, enabling the understanding of its dosimetry calculations and hardware design. More advanced Monte Carlo simulation tools are under development to fulfill the challenge of reproducing the radiolysis and radiobiology processes in FLASH irradiation. FLASH RT may become one of standard treatment modalities for tumor treatment in the future. This paper presents the history and status of FLASH RT studies with a focus on FLASH irradiation delivery modalities, underlying mechanism of FLASH effect, in vivo and vitro experiments, and simulation studies. Existing challenges and prospects of this novel technique are discussed in this manuscript.

Reduction of clinical safety margins in proton therapy enabled by the clinical implementation of dual-energy CT for direct stopping-power prediction

PURPOSE

To quantifiy the range uncertainty in proton treatment planning using dual-energy computed tomography (DECT) for a direct stopping-power prediction (DirectSPR) algorithm and its clinical implementation.

METHODS AND MATERIALS

To assess the overall uncertainty in stopping-power ratio (SPR) prediction of a DirectSPR implementation calibrated for different patient geometries, the influencing factors were categorized in imaging, modeling as well as others. The respective SPR uncertainty was quantified for lung, soft tissue and bone and translated into range uncertainty for several tumor types. The amount of healthy tissue spared was quantified for 250 patients treated with DirectSPR and the dosimetric impact was evaluated exemplarily for a representative brain-tumor patient.

RESULTS

For bone, soft tissue and lung, an SPR uncertainty (1σ) of 1.6%, 1.3% and 1.3% was determined for DirectSPR, respectively. This allowed for a reduction of the clinically applied range uncertainty from currently (3.5% + 2 mm) to (1.7% + 2 mm) for brain-tumor and (2.0% + 2 mm) for prostate-cancer patients. The 150 brain-tumor and 100 prostate-cancer patients treated using DirectSPR benefitted from sparing on average 2.6 mm and 4.4 mm of healthy tissue in beam direction, respectively. In the representative patient case, dose reduction in organs at risk close to the target volume was achieved, with a mean dose reduction of up to 16% in the brainstem. Patient-specific DECT-based treatment planning with reduced safety margins was successfully introduced into clinical routine.

CONCLUSIONS

A substantial increase in range prediction accuracy in clinical proton treatment planning was achieved by patient-specific DECT-based SPR prediction. For the first time, a relevant imaging-based reduction of range prediction uncertainty on a 2% level has been achieved.

Patient and Public Involvement Refines the Design of ProtOeus: A Proposed Phase II Trial of Proton Beam Therapy in Oesophageal Cancer

BACKGROUND

Neoadjuvant chemoradiotherapy for oesophageal cancer significantly improves overall survival but is associated with severe post-operative complications. Proton beam therapy may reduce these toxicities by sparing normal tissues compared with standard radiotherapy. ProtOeus is a proposed randomised phase II study of neoadjuvant chemoradiotherapy in oesophageal cancer that compares proton beam therapy to standard radiotherapy techniques. As proton beam therapy services are often centralised in academic centres in major cities, proton beam therapy trials raise distinct challenges including patient acceptance of travelling for proton beam therapy, coordination of treatments with local centres and ensuring equity of access for patients.

METHODS

Focus groups were held early in the trial development process to establish patients' views on the trial proposal. Topics discussed include perception of proton beam therapy, patient acceptability of the trial pathway and design, patient-facing materials, and common clinical scenarios. Focus groups were led by the investigators and facilitated by patient involvement teams from the institutions who are involved in this research. Responses for each topic were analysed, and fed back to the trial's development group.

RESULTS

Three focus groups were held in separate locations in the UK (Manchester, Cardiff, Wigan). Proton beam therapy was perceived as superior to standard radiotherapy making the trial attractive. Patients felt strongly that travel costs should be reimbursed to ensure equity of access to proton beam therapy. They were very supportive of a shorter treatment schedule and felt that toxicity reduction was the most important endpoint.

DISCUSSION AND CONCLUSIONS

Incorporating patient views early in the trial development process resulted in significant trial design refinements including travel/accommodation provisions, choice of primary endpoint, randomisation ratio and fractionation schedule. Focus groups are a reproducible and efficient method of incorporating the patient and public voice into research.

Preliminary Experience of Treating Children and Young Adults With Image-Guided Proton Beam Therapy in India

PURPOSE

Proton beam therapy (PBT) has been a preferred modality in pediatric malignancies requiring radiotherapy. We report our preliminary experience of treating consecutive patients younger than 25 years with image-guided pencil beam scanning PBT from the first and only center on the Indian subcontinent.

METHODS

Patients were selected for PBT on the basis of a multidisciplinary tumor board decision. Patient demographic data, as well as tumor and treatment-related characteristics of the cohort, were captured. Patient and treatment-related factors and their association with acute toxicities were analyzed using univariable and multivariable analyses.

RESULTS

Forty-seven patients (27 with CNS and 20 with non-CNS tumors) with a median age of 9 years (range, 2-25 years) were evaluated. Most common diagnoses were ependymoma, rhabdomyosarcoma, and glioma. Seventy-seven percent of patients traveled more than 500 km, and 70% of them lived in metropolitan cities. Forty-nine percent of patients had recurrent disease at presentation, and 15% had received a previous course of radiation. The median dose delivered was 54.8 cobalt gray equivalents (range, 40.0-70.4 cobalt gray equivalents) to a median clinical target volume of 175 mL (range, 18.7-3,083.0 mL), with 34% of patients requiring concurrent chemotherapy (CCT). Acute grade 2 and grade 3 dermatitis, mucositis, and hematologic toxicity was noted in 45% and 2%, 34% and 0%, and 38% and 30% of patients, respectively. Grade 2 fatigue was noted in 26% of patients. On multivariable analysis, for CNS tumors, both CCT and craniospinal irradiation were independently associated with ≥ 2 grade hematologic toxicity, whereas among non-CNS tumors, a clinical target volume > 150 mL was associated with ≥ 2 grade fatigue, head and neck irradiation was associated with ≥ 2 grade mucositis, and CCT was associated with grade ≥ 2 hematologic toxicity.

CONCLUSION

This study demonstrates safe implementation of a PBT program for children and young adults on the Indian subcontinent. Image-guided pencil beam scanning PBT in judiciously selected patients is feasible and can be delivered with acceptable acute toxicities.