In studies of electron and proton radiotherapy, ultrahigh dose rates of FLASH radiotherapy appear to produce fewer toxicities than standard dose rates while maintaining local tumor control. FLASH-proton radiotherapy (F-PRT) brings the spatial advantages of PRT to FLASH dose rates (>40 Gy/second), making it important to understand if and how F-PRT spares normal tissues while providing antitumor efficacy that is equivalent to standard-proton radiotherapy (S-PRT). Here we studied PRT damage to skin and mesenchymal tissues of muscle and bone and found that F-PRT of the C57BL/6 murine hind leg produced fewer severe toxicities leading to death or requiring euthanasia than S-PRT of the same dose. RNA-seq analyses of murine skin and bone revealed pathways upregulated by S-PRT yet unaltered by F-PRT, such as apoptosis signaling and keratinocyte differentiation in skin, as well as osteoclast differentiation and chondrocyte development in bone. Corroborating these findings, F-PRT reduced skin injury, stem cell depletion, and inflammation, mitigated late effects including lymphedema, and decreased histopathologically detected myofiber atrophy, bone resorption, hair follicle atrophy, and epidermal hyperplasia. F-PRT was equipotent to S-PRT in control of two murine sarcoma models, including at an orthotopic intramuscular site, thereby establishing its relevance to mesenchymal cancers. Finally, S-PRT produced greater increases in TGFβ1 in murine skin and the skin of canines enrolled in a phase I study of F-PRT versus S-PRT. Collectively, these data provide novel insights into F-PRT-mediated tissue sparing and support its ongoing investigation in applications that would benefit from this sparing of skin and mesenchymal tissues.Significance: These findings will spur investigation of FLASH radiotherapy in sarcoma and additional cancers where mesenchymal tissues are at risk, including head and neck cancer, breast cancer, and pelvic malignancies.

Ultra-high dose rate FLASH proton radiotherapy (F-PRT) has been shown to reduce normal tissue toxicity compared to standard dose rate proton radiotherapy (S-PRT) in experiments using the entrance portion of the proton depth dose profile, while proton therapy uses a spread-out Bragg peak (SOBP) with unknown effects on FLASH toxicity sparing. To investigate, the biological effects of F-PRT using an SOBP and the entrance region were compared to S-PRT in mouse intestine. In this study, 8–10-week-old C57BL/6J mice underwent 15 Gy (absorbed dose) whole abdomen irradiation in four groups: (1) SOBP F-PRT, (2) SOBP S-PRT, (3) entrance F-PRT, and (4) entrance S-PRT. Mice were injected with EdU 3.5 days after irradiation, and jejunum segments were harvested and preserved. EdU-positive proliferating cells and regenerated intestinal crypts were quantified. The SOBP had a modulation (width) of 2.5 cm from the proximal to distal 90%. Dose rates with a SOBP for F-PRT or S-PRT were 108.2 ± 8.3 Gy/s or 0.82 ± 0.14 Gy/s, respectively. In the entrance region, dose rates were 107.1 ± 15.2 Gy/s and 0.83 ± 0.19 Gy/s, respectively. Both entrance and SOBP F-PRT preserved a significantly higher number of EdU + /crypt cells and percentage of regenerated crypts compared to S-PRT. Moreover, tumor growth studies showed no difference between SOBP and entrance for either of the treatment modalities.

Purpose: The time course of serum prostate-specific antigen (PSA) response to photodynamic therapy (PDT) of prostate cancer was measured. Experimental Design: Seventeen patients were treated in a phase I trial of motexafin lutetium-PDT. PDT dose was calculated in each patient as the product of the ex vivo measured pre-PDT photosensitizer level and the in situ measured light dose. Serum PSA level was measured within 2 months before PDT (baseline), and at day 1; weeks 1 to 3; months 1, 2, and 3; months 4 to 6; and months 7 to 11 after PDT. Results: At 24 hours after PDT, serum PSA increased by 98% ± 36% (mean ± SE) relative to baseline levels (P = 0.007). When patients were dichotomized based on median PDT dose, those who received high PDT dose showed a 119% ± 52% increase in PSA compared with a 54% ± 27% increase in patients treated at low PDT dose. Patients treated with high versus low PDT dose showed a median biochemical delay of 82 versus 43 days (P = 0.024), with biochemical delay defined as the length of time between PDT and a nonreversible increase in PSA to a value greater than or equal to baseline. Conclusions: Results show PDT to induce large, transient increases in serum PSA levels. Patients who experienced high PDT dose showed greater short-term increase in PSA and a significantly more durable PSA response (biochemical delay). These data strongly promote the need for individualized delivery of PDT dose and assessment of treatment effect in PDT of prostate cancer. Information gained from such patient-specific measurements could facilitate the introduction of multiple PDT sessions in patients who would benefit.

The efficacy of photodynamic therapy (PDT) depends upon the delivery of both photosensitizing drug and oxygen. In this study, we hypothesized that local vascular microenvironment is a determinant of tumor response to PDT. Tumor vascularization and its basement membrane (collagen) were studied as a function of supplementation with basement membrane matrix (Matrigel) at the time of tumor cell inoculation. Effects on vascular composition with consequences to tumor hypoxia, photosensitizer uptake, and PDT response were measured. Matrigel-supplemented tumors developed more normalized vasculature, composed of smaller and more uniformly spaced blood vessels than their unsupplemented counterparts, but these changes did not affect tumor oxygenation or PDT-mediated direct cytotoxicity. However, PDT-induced vascular damage increased in Matrigel-supplemented tumors, following an affinity of the photosensitizer Photofrin for collagen-containing vascular basement membrane coupled with increased collagen content in these tumors. The more highly collagenated tumors showed more vascular congestion and ischemia after PDT, along with a higher probability of curative outcome that was collagen dependent. In the presence of photosensitizer-collagen localization, PDT effects on collagen were evidenced by a decrease in its association with vessels. Together, our findings show that photosensitizer localization to collagen increases vascular damage and improves treatment efficacy in tumors with greater collagen content. The vascular basement membrane is thus identified to be a determinant of therapeutic outcome in PDT of tumors. Cancer Res; 72(8); 2079–88. ©2012 AACR.

In 1978, a Cancer Research article by Dougherty and colleagues reported the first large-scale clinical trial of photodynamic therapy (PDT) for treatment of 113 cutaneous or subcutaneous lesions associated with ten different kinds of malignancies. In classic applications, PDT depends on excitation of a tissue-localized photosensitizer with wavelengths of visible light to damage malignant or otherwise diseased tissues. Thus, in this landmark article, photosensitizer (hematoporphyrin derivative) dose, drug–light interval, and fractionation scheme were evaluated for their therapeutic efficacy and normal tissue damage. From their observations came early evidence of the mechanisms of PDT's antitumor action, and in the decades since this work, our knowledge of these mechanisms has grown to build an understanding of the multifaceted nature of PDT. These facets are comprised of multiple cell death pathways, together with antivascular and immune stimulatory actions that constitute a PDT reaction. Mechanism-informed PDT protocols support the contribution of PDT to multimodality treatment approaches. Moreover, guided by an understanding of its mechanisms, PDT can be applied to clinical needs in fields beyond oncology. Undoubtedly, there still remains more to learn; new modes of cell death continue to be elucidated with relevance to PDT, and factors that drive PDT innate and adaptive immune responses are not yet fully understood. As research continues to forge a path forward for PDT in the clinic, direction is provided by anchoring new applications in mechanistically grounded protocol design, as was first exemplified in the landmark work conducted by Dougherty and colleagues. See related article by Dougherty and colleagues, Cancer Res 1978;38:2628–35